Lean Manufacturing Practices and Environmental Performance

*Ruhet Genç*

### **Abstract**

Lean manufacturing is considered a rewarding production strategy due to its positive effects on organizational and economic efficiency in various industries. Given the growing ecological consciousness, environmental achievements of lean manufacturing also incorporate a strong economic relevance. The main objective of this chapter is, therefore, to investigate the impact of lean manufacturing practices on environmental performance and the existing coherences between Lean and ecologically oriented variables such as resource usage, energy consumption, and air pollution. The methodology is literature review evaluating the findings of research in this sphere. Besides the discussion of its principles and methods, current trends and challenges regarding lean production as a business model that supports eco-efficiency are presented. The implications of this study will allow executives to better recognize and simultaneously solve both the economic and environmental problems posed by their companies.

**Keywords:** lean manufacturing, environmental impacts, eco-efficiency, lean, environmental performance

#### **1. Introduction**

Lean Manufacturing, also known as Lean Production [1], has long been considered an advantageous manufacturing method in various companies and industries on a global level as it aims for continually improving quality and efficiency in production process [2, 3]. It is essentially about eliminating the activities that do not add value in the production and seven types of waste, also referred as muda in lean settings [4] and includes several techniques such as Total Quality Management, Just-in-Time [5], Kanban, and Jidoka [6]. It has become essential with the increased price competition that companies experience in face of changing customer demands for goods and services by helping them produce same amounts at the lower costs.

In response to growing concerns for environmental issues since the early 1990s, stemming from the climate change and exhaustion of natural resources, the relationship between Lean and environmental performance has been a critical topic in business and manufacturing literature especially from 2008 onwards [7]. Moreover, numerous companies adopted lean measures to advance their manufacturing methods for decreasing their environmental burdens since Lean Manufacturing focuses on increasing production efficiency and decreasing waste.

Considering the higher efficiency provided by lean systems, which is the reduction in energy and material resources needed per unit output, the waste of production such as the emissions to air and water, generation of solid or hazardous waste, and chemical hazards abated along with the high costs of production. Thus, lean methods may alleviate environment by means of its inherent emphasis on waste reduction even though they do not automatically involve environmentally responsible intentions [8]. Research shows that although lean systems may not directly aim for reducing environmental effect, changing the production system to lean resulted in higher efficiency in energy consumption and in less waste [9].

The fact that incorporating lean manufacturing techniques may address environmental waste problem even without prioritizing it led companies to adopt lean activities. By integrating environmental considerations, lean manufacturing may provide businesses with long-term benefits such as environmental sustainability and recognition as a responsible corporate citizen. In this regard, companies embraced lean practices to reduce their ecological footprint for maintaining a better public opinion. Furthermore, many of them considered the "triple bottom line" of economic, environmental, and social concerns into consideration and took precautions on a bigger scale than those necessitated by the regulatory bodies [10].

The main objective of this study is therefore to provide insights into the contribution of lean manufacturing practices and measures for achieving a better environmental performance of manufacturing systems and the existing relationship of these two concepts. This is accomplished through a literature review along with a critical discussion of the findings. Focusing on the meaning of the green concept waste, lean manufacturing is described as a rising business model for endorsing environmental efficiency. The most effective lean practices and their tools in relation to supporting greener production are underlined. The improvement of the environmental measures of the firms that adopt the lean manufacturing principles, the reduction in ecological impacts stemming from their industrial operations are highlighted. The results of this study are of interest to scholars as well as to business managers to better understand and overcome environmental problems encountered by their business organizations.


*Article* **Study of the Wellbore Instability Mechanism of Shale in the Jidong Oilfield under the Action of Fluid**

**Xiaofeng Xu 1, Chunlai Chen 1, Yan Zhou 1, Junying Pan 1, Wei Song 1, Kuanliang Zhu 1, Changhao Wang 2,\* and Shibin Li 2**

**Abstract:** Wellbore instability is the primary technical problem that restricts the low-cost drilling of long-interval horizontal wells in the shale formation of the Jidong Oilfield. Based on the evaluation of the mineral composition, structure and physicochemical properties of shale, this paper investigates the mechanical behavior and instability characteristics of shale under fluid action by combining theoretical analysis, experimental evaluation and numerical simulation. Due to the existence of shale bedding and microcracks, the strength of shale deteriorates after soaking in drilling fluid. The conductivity of the weak surface of shale is much higher than that of the rock matrix. The penetration of drilling fluid into the formation along the weak surface directly reduces the strength of the structural surface of shale, which is prone to wellbore collapse. The collapse pressure of the shale formation in the Nanpu block of the Jidong oilfield was calculated. The well inclination angle, azimuth angle and drilling fluid soaking time were substituted in the deterioration model of rock mechanics parameters, and the safe drilling fluid density of the target layer was given. This work has important guiding significance for realizing wellbore stability and safe drilling of hard brittle shale in the Jidong Oilfield.

**Keywords:** wellbore stability; rock strength deterioration; shale formation; oil-based drilling fluid

#### **1. Introduction**

The wellbore instability of shale has always been a complex problem in drilling engineering worldwide. Borehole wall instability causes great difficulties in drilling engineering, mainly manifested as diameter reduction and collapse sticking. These problems not only prolong the drilling cycle but also increase the drilling cost [1–3]. Borehole instability is due to the bedding development and strong water sensitivity of shale. The invasion of drilling fluid causes fractures to continuously extend and expand along the formation, eventually forming a complex fracture network. This greatly reduces rock strength [4,5]. The integrity of near-wellbore water content and cementation changes the formation strength and stress field around a borehole, causing stress concentration and failure to establish a new equilibrium in the borehole, resulting in borehole instability [6,7].

Current research on the wellbore stability of shale focuses on the mechanical–chemical–thermal coupling of multiple fields and is basically in the preliminary research stage [8,9]. However, whether the strength of layered shale under the action of fluid changes with the action time and the influence of strength deterioration on the collapse pressure of well walls remains ambiguous [10,11]. The outstanding feature of the shale formation in the Jidong Oilfield is that it has approximately parallel bedding planes, that is, the dividing planes between rock layers. Bedding is different from microcrack with no obvious displacement, and the strength of the rock affected by the bedding plane has stronger anisotropy. In this paper, laboratory experiments are carried out on the bedded shale in the Nanpu block of Jidong

**Citation:** Xu, X.; Chen, C.; Zhou, Y.; Pan, J.; Song, W.; Zhu, K.; Wang, C.; Li, S. Study of the Wellbore Instability Mechanism of Shale in the Jidong Oilfield under the Action of Fluid. *Energies* **2022**, *16*, 2989. https://doi.org/10.3390/ en16072989

Academic Editors: Reza Rezaee, Jalel Azaiez and Paul Stewart

Received: 28 August 2022 Accepted: 25 November 2022 Published: 24 March 2023

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Oilfield. The hydration of shale is analyzed by the finite difference method and numerical simulation, and the degradation mechanism of the shale in the lower layer is revealed. This research has guiding significance in the design of drilling fluids for shale formations and for realizing high-quality, safe, efficient and low-cost drilling.

#### **2. Experiment on Mineral Composition and Mechanical Parameters of Bedded Shale**

In order to study the degradation law of shale under the action of different fluids, cores from Well NP2-46 in the Jidong Oilfield were selected for laboratory experiments. The core was taken by pressure maintaining coring on site and sealed with heat-shrinkable tube to prevent it from drying. Because the bedding and fracture of the shale are obviously developed, the wire-cutting method was adopted to conduct secondary sampling for the core, which ensures that the rock sample does not contact any fluid during processing. This ensures that the experimental results accurately reflect the real situation in the well.

#### *2.1. Mineral Composition and Fracture Development Characteristics*

X-ray diffraction (X-RD) and scanning electron microscopy experiments were carried out on the shale in the Nanpu block of Jidong Oilfield. Figure 1 shows the X-RD experimental test results. It can be seen from the figure that the main mineral components of the shale are clay and quartz minerals. Minerals with extremely poor expansiveness, such as quartz, are attached to the surrounding of the clay. After the clay is hydrated and expanded under the action of the drilling fluid, a pressure difference is generated, which leads to the instability of the shale. In addition, hard and brittle minerals such as plagioclase, calcite and dolomite are developed to different degrees. The average content of clay minerals is 34.85%. The clay minerals mainly consist of illite and illite-mixed layers, followed by kaolinite and chlorite without montmorillonite. The illite content is higher in the illite-mixed layer, which also shows the typical brittleness characteristics of deep shale in the structure. This kind of lithology is not easy to expand, but the illite in it also undergoes rapid microexpansion. This is due to the adsorption of water molecules on illite, which leads to the surface hydration of rocks, and the main driving force is the surface hydration energy. This reduces the strength of the shale and causes its spalling.

**Figure 1.** Mineral composition of shale in the Nanpu block.

The shale in the Nanpu block is mainly argillaceous, with a compact and hard structure. Figure 2 shows the observation results of scanning electron microscopy. It can be found that the pores in the shale are not developed, and the micro pores are scattered, but the microfractures and micropores are relatively developed. There is no typical occurrence of minerals and clay minerals in the core. The fracture width in the SEM image is 2.48 μm. When shale is placed in water, a large number of small air bubbles are attached to the surface

of the shale. Over time, obvious cracks appeared in the rock sample, which gradually opened and finally collapsed.

**Figure 2.** SEM image of deep shale in Nanpu block.

#### *2.2. Triaxial Compressive Strength Test of Shale*

Due to the significant anisotropy of bedded shale, coring was carried out along different bedding angles. The samples were then machined into standard core columns with a diameter of 25 mm and a length of 50 mm for triaxial compressive strength testing of the shale in an unimmersed state. Figure 3 shows the fracture patterns of the core after loading in the parallel and vertical bedding directions.

(**a**) Loading in the parallel direction. (**b**) Loading in the vertical direction.

**Figure 3.** The failure form of shale under different loading directions.

According to the photos of rock breakage, the fracture morphology of parallel and vertical stratified rocks is different after loading. When loading is parallel to the direction of stratification, multiple openings occur along the bedding cracks, forming multiple groups of fragments along the direction of the bedding plane. However, shear cracks are generated when the vertical bedding direction is loaded, and large volume fractures are formed after the shear cracks connect with the microcracks along the bedding. Rock loaded in the parallel bedding direction produces tensile failure along the structural plane, and rock loaded in the vertical bedding direction needs to overcome the shear strength of the shale body.

Table 1 shows the experimental test results. The strength of the shale under parallel bedding and vertical bedding loading is very different, and the anisotropy is strong. When the confining pressure is set to 60 MPa, the elastic modulus of the shale loaded by vertical bedding is high and the Poisson ratio is low, indicating that the shale in the Nanpu block is relatively brittle.

**Table 1.** Triaxial stress test results.

Due to the well-developed bedding and significant anisotropy of the shale, the compressive strength experiments under loading at angles of 30◦, 45◦ and 60◦ with the bedding were continued. Combined with the above triaxial compressive strength test results loaded in the parallel and vertical bedding directions, the cohesive force and internal friction angle of the rock were calculated as shown in Table 2.

**Table 2.** Experimental results of rock mechanics parameters in different loading directions.

According to the analysis of the triaxial stress experimental results in different loading directions, the failure of the shale can be divided into three regions, as shown in Figure 4. When the loading direction is perpendicular to the shale bedding, the strength is the highest, and with the increase in the included angle, the strength gradually decreases. According to Mohr–Coulomb criterion, when the angle is 45◦ + ϕ/2 (ϕ is the angle of internal friction), the intensity reaches the lowest value and then gradually increases.

**Figure 4.** Core anisotropic failure law.

According to the different loading directions, shale failure forms can be expressed as follows:

#### *2.3. Triaxial Compressive Strength Test of Shale under Different Fluid Immersions*

The vertical coring shale and the horizontal coring shale were immersed in pure water and diesel oil, respectively. After different times, the triaxial compressive strength test was conducted again, and the cohesion and internal friction angle were calculated. Experimental photos are shown in Figures 5 and 6.

**Figure 5.** Soaking in water for 48 h.

**Figure 6.** Soaking in diesel oil for 48 h.

When the shale was soaked in water, three cores were directly broken within 1 h, and two cores had obvious cracks. After soaking in oil for 48 h, the surface of one core peeled off, and the other cores were not significantly broken. A comparison of the amount and time of core damage in water and in oil revealed that the damage rate of water to shale cores is much higher than that of oil to shale cores [12].

After soaking for 48 h, the unbroken cores were subjected to triaxial testing. According to the experimental results in Table 3, it can be seen that the shale cored parallel to the bedding direction was completely broken after soaking in water. The triaxial strength was reduced to 0, and the strength was reduced by 100%. The average triaxial strength of the shale after being soaked in oil was 45.728 MPa, and the strength decreased by 31.9%. The average triaxial strength of the shale in the vertical bedding direction was 38.907 MPa after soaking in water, and the strength decreased by 61%. The average triaxial strength after oil soaking was 63.874 MPa, which was almost unchanged compared with the original rock sample.

**Table 3.** 48 h triaxial stress test results of shale soaking.

#### *2.4. Tensile Strength Test of Shale*

The tensile strength of the shale was tested by the Brazilian splitting method. It can be seen from the test results in Table 4 that the tensile strength of the shale varies greatly in the vertical and horizontal directions.

**Table 4.** Shale tensile strength test results in vertical/parallel bedding directions.

According to the fracture morphology of the rock in Figure 7, tensile failure is prone to occur along the bedding plane of the rock, which is an important factor that affects the stability of the well wall [13].

(**a**) Vertical bedding loading. (**b**) Parallel bedding loading.

**Figure 7.** Fragmentation morphology of loaded shale in different bedding directions.

#### **3. Study on the Deterioration Law of Shale under the Action of Fluid**

Shale is a rock that is composed of water-sensitive clay minerals whose interaction with drilling fluids is inevitable [14]. Due to the characteristics of shale's structure and composition, the effect of different drilling fluid systems is also very different. Most shale is strong enough to withstand borehole stress for some time as the bit cuts through the shale formation [15]. However, over time, the strength of the shale decreases significantly, regardless of the type of drilling fluid used. This is due to hydration, which occurs even with very little water uptake by the shale. This creates a zone of softening around the well, leading to the delayed failure of the shale [16].

The main components of shale are clay minerals. The composition, content and microstructure of the clay minerals determine the basic physical and chemical properties and hydration mechanism of the shale. Understanding the hydration mechanism of shale is the basis of analyzing the stability of shale wellbore. When the shale is in contact with the drilling fluid, the transport of water and ions is induced, driven by hydraulic and chemical potential gradients. These include the Darcy flow, which is driven by the pressure difference between the drilling fluid column pressure and the pore pressure, and ion diffusion, which is driven by the chemical potential difference between the drilling fluid and the shale [17].

According to C.H. Yew [18], hydration of the shale formation around the well can be described by a diffusion equation of water molecules. The water absorption diffusion equation of shale wellbore can be established by the law of conservation of mass. Let *q* be the mass flow of water adsorption, and let *W(r, t)* be the weight percentage of adsorbed water at point *r* away from the well axis and time *t*. In the conservation of mass, the following expression is needed:

$$
\nabla q = \frac{\partial W}{\partial t} \tag{1}
$$

Assumptions:

$$q = \mathbb{C}\_f \nabla \mathcal{W} \tag{2}$$

where ∇ is the gradient operator, and *Cf* represents the adsorption constants of materials, which are related to the properties of the shale and the drilling fluid and can be measured with the water adsorption test.

Substitute Equation (2) into Equation (1). In the cylindrical coordinate system, the basic equation of water adsorption can be obtained as follows:

$$C\_f \frac{1}{r} \frac{\partial}{\partial r} \left( r \frac{\partial W}{\partial r} \right) = \frac{\partial W}{\partial t} \tag{3}$$

Under boundary conditions at infinity, the formation water content is the original formation water content. At the well wall, the formation water content is considered saturated and does not change with time:

$$\left.W\right|\_{r=r\_w} = \left.W\_{\mathbf{s}\prime} \cdot \left.W\right|\_{r\to\infty} = \left.W\_0\right|\_{\prime} 0 \prec \mathbf{t} \prec \infty \tag{4}$$

Initial conditions:

$$\mathcal{W}|\_{t=0} = \mathcal{W}\_0 \tag{5}$$

Using the finite difference method to solve the above equation, the water content of the formation around the borehole at different locations at different times can be obtained. Let the time step be Δ*t* and the radial distance step be Δ*h*. Therefore, the radial distance is *ri* = *i*Δ*h*, and the time is *ti* = *j*Δ*t*(*i* = (0, 1, 2, ... *n*); *j* = (0, 1, 2, ... *m*)). *W*(*ri*, *tj*) is recorded as *W*(*i*,*j*)(*i* = (0, 1, 2, ... *n*); *j* = (0, 1, 2, ... *m*)). Approximately, replace *∂W ∂t* , *∂W ∂r* with the forward difference quotient and *∂*2*W ∂r*2 with the second central difference quotient, namely:

$$\frac{\partial W}{\partial t} = \frac{W\_{(i,j+1)} - W\_{(i,j)}}{\Delta t} \tag{6}$$

$$\frac{\partial W}{\partial r} = \frac{W\_{(i+1,j)} - W\_{(i,j)}}{\Delta h} \tag{7}$$

$$\frac{\partial^2 \mathcal{W}}{\partial r^2} = \frac{\mathcal{W}\_{(i+1,j)} - 2\mathcal{W}\_{(i,j)} + \mathcal{W}\_{(i-1,j)}}{\Delta h^2} \tag{8}$$

Substituting (6)~(8) into Equation (3), the difference equation of Equation (3) is:

$$\frac{\mathcal{W}\_{(i+1,j)} - 2\mathcal{W}\_{(i,j)} + \mathcal{W}\_{(i-1,j)}}{\Delta h^2} + \frac{1}{r\_i} \frac{\mathcal{W}\_{(i+1,j)} - \mathcal{W}\_{(i,j)}}{\Delta h} = \frac{1}{\mathcal{C}\_f} \frac{\mathcal{W}\_{(i,j+1)} - \mathcal{W}\_{(i,j)}}{\Delta t} \tag{9}$$

The difference format for boundary conditions and initial conditions is:

$$\mathcal{W}(0,j) = \mathcal{W}\_5, \mathcal{W}(n,j) = \mathcal{W}\_0, \mathcal{W}(i,0) = \mathcal{W}\_0 \tag{10}$$

According to the above experimental results, the shale water absorption rate equation is modified, and the water content of the shale formation around the borehole at different times and positions can be obtained using a numerical simulation method.

The formation collapse pressure can be calculated by the following formula [19]:

$$P\_b = \frac{1}{2} (3\sigma\_H - \sigma\_h)(1 - \sin\varphi\_0) + \mathfrak{a}P\_P \sin\varphi\_0 - \mathfrak{C}\_0 \cos\varphi\_0 \tag{11}$$

where *Pb* is collapse pressure (MPa); *σH* is the maximum horizontal stress (MPa); *σh* is the minimum horizontal formation stress (MPa); *Pp* is the formation pore pressure (MPa); *C*0 is the cohesion (MPa); *ϕ*0 is the angle of internal friction (◦); and *α* is the effective stress coefficient.

In order to facilitate the design of the drilling fluid density window, the collapse pressure is converted into the form of equivalent density, that is, the drilling fluid density required to just balance the formation collapse pressure, g/cm3.

According to the test results of the rock mechanical parameters soaked in different fluids, the shale deteriorates after drilling fluid soaking due to the existence of cleavage and microfractures [20]. As the conductivity of the structural plane is much higher than that of raw rock, the penetration of drilling fluid along the structural plane will directly cause the weakening of the structural plane strength [21]. As shown in Figure 8, the stress distribution of the vertical well section, the deflection section and the horizontal section of the wellbore in a bedded shale formation is established. The mechanical parameters of the surrounding rock of the wellbore are obtained from the above triaxial compressive strength test results. The change in the cohesion force and the internal friction angle after the core soaking experiment are used to characterize the weakening of the shale's strength, and the force-chemical coupling finite element numerical simulation model of borehole collapse pressure is obtained. It can be clearly seen that stress concentration occurs at the intersection of the well wall and the bedding plane. Under the action of the liquid column pressure in the wellbore, the drilling fluid can easily enter the bedding and increase the collapse pressure around the wellbore.

**Figure 8.** The von Mises stress around the borehole wall for different well types and water contents.

In the drilling process of shale formations, the rock strength decreases, and its bearing capacity weakens, and the increased stress leads to an enhanced trend of rock collapse around the well, which is quantitatively reflected in the increase in collapse pressure, leading to wellbore instability [22]. Therefore, finite element numerical simulation is employed to calculate the collapse pressure equivalent density of the shale around the well at different water content times, which can more intuitively analyze the influence of water absorption on the deterioration degree of the shale and the borehole wall stability. Changing the action time of the drilling fluid in the wellbore and the shale bedding plane under a different well inclination and azimuth in Figure 8 until the liquid content of the surrounding rock reaches 5%, 10% and 15%, the derived values were utilized to calculate the maximum collapse pressure and the equivalent density of the collapse pressure of different schemes.

Figure 9 shows the distribution chart of the equivalent density of collapse pressure when the liquid content around the well reaches 5%, 10% and 15% under different well inclination and azimuth conditions. When the shale liquid content around the well is 5%, the variation range of the equivalent density of the collapse pressure under different wellbore trajectories is 1.02~1.53 g/cm3. When the liquid content of the shale reservoir increases, the shale strength changes, and the equivalent density of the borehole collapse pressure decreases rapidly. The collapse density increases from 0.15 to 0.22 g/cm3 and from 0.31 to 0.45 g/cm3 when the liquid content is 10% and 15%, respectively. At a certain time, the water absorption of the shale decreases with the increasing distance from the borehole wall. A hydration zone is formed in the shale formation around the borehole; however, its liquid content is similar to the original liquid content after a certain distance. At a certain distance, the longer the time, the more water absorption of the shale occurs, but at a certain time, saturation and stability will occur.

**Figure 9.** Collapse pressure distribution under different well types and water contents.

After the ground layer is drilled, driven by the pressure difference and chemical potential difference between the drilling fluid and the formation pore fluid, the drilling fluid filtrate enters the formation, causing a change in the mechanical properties of the shale [23]. The loss of filtrate from the drilling fluid increases the water content of the formation around the borehole, which leads to a series of changes in the mechanical properties of the formation. For example, the elastic modulus sharply decreases with increasing formation water content, and Poisson's ratio increases with increasing formation water content, whereas the cohesion and internal friction angle of the formation strength decrease with increasing formation water content [24]. These changes are particularly pronounced in near-wellbore formations. The fluid loss caused a softening zone around the borehole wall, which greatly affected the stability of the borehole wall. Under a certain pressure and temperature, the interaction between the shale and the drilling fluid produces a hydration zone around the borehole [25]. The size of the hydration zone depends on the formation and drilling fluid characteristics, drilling fluid column pressure, interaction time between the drilling fluid and formation, interaction time between formation and the drilling fluid temperature, etc. Because of the formation hydration zone around the borehole, the rock around the borehole becomes a complex rock mass medium with a variable water content, modulus and strength that vary with radius and time. As a result, the strength of the shale in certain areas around the well is reduced, and a plastic zone appears or further expands to cause wellhole collapse.

#### **4. Field Case Analysis**

A horizontal well in the Nanpu block of the Jidong Oilfield was unstable during drilling, which caused serious downhole accidents, such as well collapse and stuck drilling. The shale in this area has obvious bedding, so if the conventional model is selected to analyze the borehole wall collapse problem, the result is very different from the actual situation. Therefore, the calculation model of the equivalent density of collapse pressure with different water contents is used to correct the results, and a reasonable safe density window of drilling fluid is obtained.

The well depth of the well to be designed is 4996 m, and the length of the horizontal section is 1200 m. Adjacent wells were drilled with a maximum drilling fluid density of 1.45 g/cm3. Due to the low density of the drilling fluid, five wellbore collapse and sticking accidents occurred. It took 32 days to deal with complex downhole accidents, and 700.77 m3 of drilling fluid was lost. An analysis of the reasons for the incidents showed that shale microfissures in the Dongying Formation and the Shahejie Formation developed. Most shale formations are strong enough to withstand borehole stress for a certain period as the bit cuts through them. Over time, however, due to the hydration of the shale, even though the water uptake may be small, regardless of the type of drilling mud used, the strength of the shale will significantly decrease, creating a softening zone around the well and causing delayed rock failure. The collapse pressure is corrected according to the well trajectory and drilling construction period, and the safe drilling fluid density window is obtained as shown in Figure 10.

**Figure 10.** Corrected safe drilling fluid density window.

The well was drilled to the Shahejie Formation with water-based drilling fluid in the second drilling. The depth of the well is 2615 m. The estimated construction period is 12 days. The liquid density is 1.26 g/cm3. In the third drilling, oil-based drilling fluid was used to drill to a well depth of 4996 m, and the construction period was 34 days. After calculating the rock deterioration, the maximum collapse pressure appeared in the well inclination angle of 55◦~65◦ and the horizontal reservoir. The maximum collapse pressure of this well section is 1.51 g/cm3, the minimum rupture pressure is 1.79 g/cm3, and the density window is narrow. The recommended drilling fluid density is 1.51 g/cm3. Drilling under the density of the drilling fluid, the well wall did not collapse, and the drilling was successfully completed.

It can be seen that the weakening strength of the shale bedding surface (increased water content) has a significant effect on the wellbore stability of horizontal wells. For any bedding surface occurrence, the collapse pressure increases with increasing water content, and the collapse pressure caused by the weakening of the bedding surface increases by approximately 0.32 g/cm3 compared with that without weakening. The weakening of shale planes is the main controlling factor of shaft wall collapse, and its influence cannot be disregarded. The drilling fluid intrudes into the stratum along the bedding under the action of osmosis, which reduces the cohesion and the internal friction angle of the bedding plane and increases the risk of shear slip of the borehole wall rock along the bedding, thus aggravating the risk of borehole wall collapse and instability. For drilling sites, if the influence of the shale strength deterioration in the drilling fluid on collapse pressure is not

taken into account, the drilling fluid density may then be low, causing the collapse of the borehole wall at an early stage of drilling.

#### **5. Conclusions**

**Author Contributions:** Conceptualization, X.X. and K.Z.; methodology, C.C.; software, C.W.; validation, Y.Z., J.P. and W.S.; formal analysis, C.W.; investigation, X.X. and C.C.; resources, C.C.; data curation, S.L.; writing—original draft preparation, C.W.; writing—review and editing, C.W.; visualization, C.W.; supervision, X.X.; project administration, S.L.; funding acquisition, S.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by [Natural Science Foundation of China], grant number [51874098].

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

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**9. Preventive measures**

There are many issues, including a lack of adequate epidemiological data on infection, control, and elimination. Suspected cases can be avoided through community health and education efforts, as well as a comprehensive healthcare approach involving [41–44].


**8. Conclusions**

As such, the features of prostate cancer, including long latency, high prevalence, significant mortality and morbidity, combined with the already accepted use of CAM, in particular n-3 PUFA presents an excellent opportunity for nutritional intervention. However, until we unravel this controversial enigma of how n-3 PUFA affects prostate cancer, much remains to be explored and examined. More research is required to support or refute the value of n-3 PUFA in prostate cancer. And thus the search continues as to how one should proceed with fatty acid intake, including supplementation, as a prophylactic or therapeutic. Currently there is not enough evidence to discount the noted beneficial effects of n-3 PUFA intake to prevent heart disease as they seem to outweigh any harm related to prostate cancer risk [40]. However, further research is required to clarify the precise mechanisms of n-3 PUFA and its role in human disease, in particular cancer. Without such, we cannot develop novel strategies for n-3 PUFA to target molecular pathways (inflammatory or other involved processes) in the prevention, development or treatment of prostate cancer or other cancers. Expansion of research in this area is warranted.


**2. Printed stereotactic atlases**

The history of human brain atlases begins in 1947, when the surgeons noticed that it was possible to adapt mechanical devices used in animals to navigate precisely the human brain. Therefore, it was mandatory to generate maps with coordinates of the uncertain territory in order to plan the best routes to avoid even minimal collateral damage and to focus the targets with high precision. Printed stereotactic atlases were considered to supply blueprints to neurosurgeons for intracerebral navigation.

#### **2.1 Stereoencephalotomy ( thalamotomy and related procedures), part I: methods and stereotaxic atlas of the human brain (Spiegel & Wycsis, 1952)**

The first stereotactic human brain atlas was entitled *Stereoencephalotomy.* It was developed to solve the lack of accuracy of referenced systems based on cranial references and to allow the stereotaxic surgery to be performed in humans. It should be noted that since 1973 the term "stereotactic" is used for surgery in humans, whereas "stereotaxic" is used exclusively for neurosurgery in animals.

In 1947, Spiegel and Wycsis described the first human stereotactic instrument used routinely in subcortical surgery. It is based on the use of intraoperative radiographs, allowing the visualization of cerebral references. After the emergence of the technique of interventricular instillation of air (ventriculography), anatomical structures including the foramen of Monro (FM) and the calcification of the pineal gland could be used for localization of intracranial targets. With the advent of new contrast media it was possible to use new landmarks in the brain, such as the anterior commissure (AC), the posterior commissure (PC) and the intercommissural line (ICL).These new references are much more reliable than the former ones, as verified by Talairach (Talairach et al., 1957).With these new reference points at hand, the indication for neurosurgical interventions could be extended, since new targets appear within the coordinates of the stereotactic apparatus. Spiegel and Wycsis' (1952) atlas consisted of photographs of a series of coronal brain, sliced at regular intervals in relation to the posterior commissure and the midline, with a reference graph located at the edges of each section. Using this parameter, the surgeon was able to assess distances in millimeters in depth and laterality of subcortical targets with a known distance of the posterior commissure. Coordinates for many targets were derived from this concept, and neuroablative procedures became reality. These studies were the basic guidelines for targets used in pallidotomy for treatment of abnormal movements and mesencephalotomy for treatment of refractory chronic pain, published in 1950 by Spiegel and Wycis.

Prefrontal lobotomies were frequently performed in the days before the emergence of appropriate psychotropic medication. At the time of the development of stereotactic surgery, Spiegel hoped to refine this procedure to avoid the unwanted complications and deficits frequently associated with these procedures. With this in mind, the first use of the stereotactic apparatus was the coagulation of the dorsal median nucleus of the thalamus in patients with severe psychiatric disorders, seeking a less traumatic intervention than a lobotomy. Around the same time the use of stereotaxy for interruption of pain pathways, surgical treatment of abnormal movements, and drainage of fluid from pathological cavities, - , for instance, cystic tumors -, had also been proposed.

Initially, their reference coordinate system was the Cp-PO line (posterior commissure-pons line). This coordinate system was not simple and was not widely used. In their next work in 1962, Spiegel and Wycsis (Spiegel & Wycsis, 1962) assumed the intercommissural line as

The history of human brain atlases begins in 1947, when the surgeons noticed that it was possible to adapt mechanical devices used in animals to navigate precisely the human brain. Therefore, it was mandatory to generate maps with coordinates of the uncertain territory in order to plan the best routes to avoid even minimal collateral damage and to focus the targets with high precision. Printed stereotactic atlases were considered to supply blueprints

**2.1 Stereoencephalotomy ( thalamotomy and related procedures), part I: methods and**

The first stereotactic human brain atlas was entitled *Stereoencephalotomy.* It was developed to solve the lack of accuracy of referenced systems based on cranial references and to allow the stereotaxic surgery to be performed in humans. It should be noted that since 1973 the term "stereotactic" is used for surgery in humans, whereas "stereotaxic" is used exclusively for

In 1947, Spiegel and Wycsis described the first human stereotactic instrument used routinely in subcortical surgery. It is based on the use of intraoperative radiographs, allowing the visualization of cerebral references. After the emergence of the technique of interventricular instillation of air (ventriculography), anatomical structures including the foramen of Monro (FM) and the calcification of the pineal gland could be used for localization of intracranial targets. With the advent of new contrast media it was possible to use new landmarks in the brain, such as the anterior commissure (AC), the posterior commissure (PC) and the intercommissural line (ICL).These new references are much more reliable than the former ones, as verified by Talairach (Talairach et al., 1957).With these new reference points at hand, the indication for neurosurgical interventions could be extended, since new targets appear within the coordinates of the stereotactic apparatus. Spiegel and Wycsis' (1952) atlas consisted of photographs of a series of coronal brain, sliced at regular intervals in relation to the posterior commissure and the midline, with a reference graph located at the edges of each section. Using this parameter, the surgeon was able to assess distances in millimeters in depth and laterality of subcortical targets with a known distance of the posterior commissure. Coordinates for many targets were derived from this concept, and neuroablative procedures became reality. These studies were the basic guidelines for targets used in pallidotomy for treatment of abnormal movements and mesencephalotomy for

treatment of refractory chronic pain, published in 1950 by Spiegel and Wycis.

Prefrontal lobotomies were frequently performed in the days before the emergence of appropriate psychotropic medication. At the time of the development of stereotactic surgery, Spiegel hoped to refine this procedure to avoid the unwanted complications and deficits frequently associated with these procedures. With this in mind, the first use of the stereotactic apparatus was the coagulation of the dorsal median nucleus of the thalamus in patients with severe psychiatric disorders, seeking a less traumatic intervention than a lobotomy. Around the same time the use of stereotaxy for interruption of pain pathways, surgical treatment of abnormal movements, and drainage of fluid from pathological cavities,

Initially, their reference coordinate system was the Cp-PO line (posterior commissure-pons line). This coordinate system was not simple and was not widely used. In their next work in 1962, Spiegel and Wycsis (Spiegel & Wycsis, 1962) assumed the intercommissural line as

**2. Printed stereotactic atlases**

neurosurgery in animals.

to neurosurgeons for intracerebral navigation.

**stereotaxic atlas of the human brain (Spiegel & Wycsis, 1952)**

standard reference system. The second part was a textbook and revised atlas updated from the first version. This atlas is currently out of print (Coffey, 2009).

*Main lines for spatial orientation are the mediosagittal Cp-Po line that connects the posterior commissure (Cp) with the bulbopontine sulcus. The h 0, a horizontal line (perpendicular to the Cp-Po-line) which crosses the Cp-Po-line at the level of the posterior commissure. The h1 line emerges in an acute angle at the crossing point of the Ch and the Cp and runs in a rostral direction. Cran.1 and cran.2 can be considered as ancillary lines. Cran.1 is perpendicular to h1 and forms an acute angle of 4° (-i) with the CP-PO-line, cran.2 like cran.1 leaves the Cp-Poline with an inclination of 4°, however, posterior to the Cp-Po-line (+i).*

Fig. 1. First intracerebral reference system proposed by Spiegel & Wycsis in 1952.

#### **2.2 Atlas d'anatomie stéréotaxique: Repérage radiologique indirect des noyaux gris centraux des regions mésencéphalo-sous-optique et hypothalamique de l'homme (Talairach et al., 1957)**

The most important contribution of this publication is the use of AC and PC as reliable intracerebral stereotactic markers and their stable relationship to deep brain structures. From this work, came the concept of Talairach's space. The Talairach's space is a coordinate system based on AC and PC as pivotal landmarks. Using the distance between them and the orthogonal plans erected, it is possible to compare the location of brain structures in two different brains, independent from individual differences. Because of the individual variations in the three dimensions of human brains, the distances measured in millimeters are applicable only to one individual. This becomes increasingly true with greater distance from the basal lines. Talairach concluded that dimensions given in millimeters can apply only in a general population to the gray central nuclei, whose dimensional variations remain moderate. For this reason they presented later the three-dimensional proportional grid system ( Talairach & Tournoux, 1988).

Review of Printed and Electronic Stereotactic Atlases of the Human Brain 149

Since the beginning of stereotactic surgery, the main concern of the neurosurgeons was precision. The authors noted that all the previously presented atlases were based on very few specimens and even in the Schaltenbrand's atlas, despite the high total number of brains, only 7 out 111 were comprehensively studied. It was concluded that the stereotactic coordinates of subcortical nuclei were inadequately established, and that the variations would be so great that the procedures could not be reliably done with these

The authors thus presented a variability study, aiming to compensate for this lack of precision. They studied nineteen brains, and focused on thalamic nuclei variability. Besides the thalamus, the atlas includes adjacent basal ganglia, and medial temporal lobe structures. It consists of statistical analysis of the data, with probability tables and distances to important ventricular reference points such as the foramen of Monro (FM), PC and midcommisural plane. It was surely an important work to define variability patterns in pre-

They used the FM-PC line and the total thalamic length (TthL) as reference system instead of

**2.5 Variations and connections of the human thalamus (Van Buren & Borke,1972)**  These authors, together with Schaltenbrand and Talairach, published one of the most important atlases of that time. The atlas is the result of a meticulous work on this challenging structure of the human brain, and it comprises a detailed cytoarchitectonic description of the individual thalamic nuclei. The literature on the connections of each nucleus is also reviewed. A special section of the book is dedicated to present the primary lesions (intervention) and secondary thalamic degeneration in fifty-four patients due to stereotactic procedures. The authors care about questions such as correcting differential shrinkage with computational tools, and variation of the position of thalamic nuclei in

**2.6 The human somesthetic thalamus with maps for physiological target localization**

In this work Tasker and Emmers have presented a detailed map for electrical stimulation of the thalamus, with 2mm interval stimulation. During stereotactic procedures performed with awake patients, the distribution of somesthetic responses elicited by electrical stimuli was projected into the templates and used to build a three-dimensional homunculus of the somesthetic thalamus. Tasker emphasized that physiologically defined anatomy rather than blind obedience to atlas coordinates should determine how to conduct functional

**2.7 Atlas for stereotaxy of the human brain with accompanying guide (Schaltenbrand**

According to the preface of the second edition of their atlas the first one was "an exploration of a new field of clinical anatomy." The latest edition concentrates on clinically relevant issues. The authors emphasized the importance of the myelin sections and reduced the

the Talairach's space, although they have measured the AC-PC distance as well.

relation to midline, anterior, and posterior commissures.

**during stereotactic neurosurgery (Emmers & Tasker, 1975)**

**2.4 A stereotaxic atlas of the human thalamus and adjacent structures: A variability**

**study (Andrew et al.,1969)**

coordinates.

CT/MRI era.

stereotactic operations.

**& Wahren,1977)**

Several brains were studied, but the atlas was based on a single specimen. Talairach used his double-grid stereotactic instrument to create perforations in the craniocerebral specimen filled with air in the ventricular system. It was mounted on a stereotactic apparatus and metal probes were introduced. Radiographs were taken in profile and the brain was then removed and sectioned. The paths of the probes in the brain were used to establish the directional planes. Accurate coordinate measurements and profiles were derived for deep cerebral nuclei, subnuclei, and tracts.The stereotactically marked brains were cut in either parasagittal or frontal sections along Talairach's standard planes.

*The principal line for intracerebral orientation is the AC-PC line (also called the intercommissural line or ICL) that connects the superior edge of the anterior commissure with the inferior edge of the posterior commissure. Two perpendicular lines cut the AC-PC-line, the VAC, that runs through the center of the anterior commissure and the VPC, that runs through the center of the posterior commissure. Transformation of the ICL, VAC and VPC lines into planes yield the intercommissural plane (ICP), the anterior verticofrontal plane (VACp) and the posterior the verticofrontal plane, (VPCp) respectively.*

Fig. 2. Talairach's stereotactic references.

The three-dimensional profiles of the thalamic nuclei and other structures were mapped on millimeter-ruled diagrams. The reference line was the intercommissural line. This line is widely used even to the present day. This atlas is currently out of print.

#### **2.3 Introduction to stereotaxis with an atlas of the human brain (Schaltenbrand & Bailey, 1959 )**

Schaltenbrand and Bailey published the most comprehensive and detailed stereotactic atlas. They studied 111 brains that were sectioned in the coronal, sagittal, and horizontal planes. Variability diagrams were based on seven specimens. Hassler and Wahren completed profound studies of nuclear structures in coronal and parasagittal sections. Akert, Bucy, Walker, Snider, and Hassler contributed with detailed chapters on the physiology and pathophysiology of deep structures of the human brain. Nearly forty years later, this atlas also contributes immensely to functional neurosurgeons, and it could be the most used atlas in the pre-CT era. Even today, the expanded 1977 edition with the most useful features of this work is used world-wide. Their coordinate system appears to be derived from Talairach's space, but shows slight differences.

Several brains were studied, but the atlas was based on a single specimen. Talairach used his double-grid stereotactic instrument to create perforations in the craniocerebral specimen filled with air in the ventricular system. It was mounted on a stereotactic apparatus and metal probes were introduced. Radiographs were taken in profile and the brain was then removed and sectioned. The paths of the probes in the brain were used to establish the directional planes. Accurate coordinate measurements and profiles were derived for deep cerebral nuclei, subnuclei, and tracts.The stereotactically marked brains were cut in either

*The principal line for intracerebral orientation is the AC-PC line (also called the intercommissural line or ICL) that connects the superior edge of the anterior commissure with the inferior edge of the posterior commissure. Two perpendicular lines cut the AC-PC-line, the VAC, that runs through the center of the anterior commissure and the VPC, that runs through the center of the posterior commissure. Transformation of the ICL, VAC and VPC lines into planes yield the intercommissural plane (ICP), the anterior verticofrontal plane (VACp) and the posterior the*

The three-dimensional profiles of the thalamic nuclei and other structures were mapped on millimeter-ruled diagrams. The reference line was the intercommissural line. This line is

Schaltenbrand and Bailey published the most comprehensive and detailed stereotactic atlas. They studied 111 brains that were sectioned in the coronal, sagittal, and horizontal planes. Variability diagrams were based on seven specimens. Hassler and Wahren completed profound studies of nuclear structures in coronal and parasagittal sections. Akert, Bucy, Walker, Snider, and Hassler contributed with detailed chapters on the physiology and pathophysiology of deep structures of the human brain. Nearly forty years later, this atlas also contributes immensely to functional neurosurgeons, and it could be the most used atlas in the pre-CT era. Even today, the expanded 1977 edition with the most useful features of this work is used world-wide. Their coordinate system appears to be derived from

**2.3 Introduction to stereotaxis with an atlas of the human brain (Schaltenbrand &**

widely used even to the present day. This atlas is currently out of print.

parasagittal or frontal sections along Talairach's standard planes.

*verticofrontal plane, (VPCp) respectively.*

**Bailey, 1959 )**

Fig. 2. Talairach's stereotactic references.

Talairach's space, but shows slight differences.

#### **2.4 A stereotaxic atlas of the human thalamus and adjacent structures: A variability study (Andrew et al.,1969)**

Since the beginning of stereotactic surgery, the main concern of the neurosurgeons was precision. The authors noted that all the previously presented atlases were based on very few specimens and even in the Schaltenbrand's atlas, despite the high total number of brains, only 7 out 111 were comprehensively studied. It was concluded that the stereotactic coordinates of subcortical nuclei were inadequately established, and that the variations would be so great that the procedures could not be reliably done with these coordinates.

The authors thus presented a variability study, aiming to compensate for this lack of precision. They studied nineteen brains, and focused on thalamic nuclei variability. Besides the thalamus, the atlas includes adjacent basal ganglia, and medial temporal lobe structures. It consists of statistical analysis of the data, with probability tables and distances to important ventricular reference points such as the foramen of Monro (FM), PC and midcommisural plane. It was surely an important work to define variability patterns in pre-CT/MRI era.

They used the FM-PC line and the total thalamic length (TthL) as reference system instead of the Talairach's space, although they have measured the AC-PC distance as well.

### **2.5 Variations and connections of the human thalamus (Van Buren & Borke,1972)**

These authors, together with Schaltenbrand and Talairach, published one of the most important atlases of that time. The atlas is the result of a meticulous work on this challenging structure of the human brain, and it comprises a detailed cytoarchitectonic description of the individual thalamic nuclei. The literature on the connections of each nucleus is also reviewed. A special section of the book is dedicated to present the primary lesions (intervention) and secondary thalamic degeneration in fifty-four patients due to stereotactic procedures. The authors care about questions such as correcting differential shrinkage with computational tools, and variation of the position of thalamic nuclei in relation to midline, anterior, and posterior commissures.

#### **2.6 The human somesthetic thalamus with maps for physiological target localization during stereotactic neurosurgery (Emmers & Tasker, 1975)**

In this work Tasker and Emmers have presented a detailed map for electrical stimulation of the thalamus, with 2mm interval stimulation. During stereotactic procedures performed with awake patients, the distribution of somesthetic responses elicited by electrical stimuli was projected into the templates and used to build a three-dimensional homunculus of the somesthetic thalamus. Tasker emphasized that physiologically defined anatomy rather than blind obedience to atlas coordinates should determine how to conduct functional stereotactic operations.

#### **2.7 Atlas for stereotaxy of the human brain with accompanying guide (Schaltenbrand & Wahren,1977)**

According to the preface of the second edition of their atlas the first one was "an exploration of a new field of clinical anatomy." The latest edition concentrates on clinically relevant issues. The authors emphasized the importance of the myelin sections and reduced the

Review of Printed and Electronic Stereotactic Atlases of the Human Brain 151

*The reference lines of Afshar et al. are confined to brainstem and cerebellar structures. The ventral HBG line runs in a rostro-caudal direction tangentially to the floor of the IVth ventricle. The parallel dorsal YFX line passes the tip of the fastigium. A line perpendicular to YX and HG goes likewise through the tip of the fastigium. Its*

However this variation is proportional for each brain, and they propose to divide the brain

Each hemisphere is divided in twelve parts (1-12) in its supero-inferior (z) axis, in nine parts (A-I) in its anteroposterior (y) axis and in four parts (a-d) in the laterolateral (x) axis. Each voxel has the following dimensions: x: one-fourth of the distance between midline and the most lateral point of the parietotemporal cortex, named a-d ; y: The voxels located anterior to AC have ¼ from the distance from AC to the frontal pole(named A-D). The voxels located posterior to PC have ¼ of the distance from PC to the occipital pole (named F-I), and finally the AC/PC distance is the 9th voxel (named E) and can be divided in thirds, called "minivoxels"; z: the voxels located above the IC line have 1/8 of the distance between IC line and the highest point of the parietal cortex (called 1-8) and the voxels located below the IC line have ¼ of the distance between the IC line and the most inferior point of the temporal cortex (named 9-12). This voxels are fixed for every brain, allowing the normalization between two different subjects having in mind these proportions and not absolute distances in millimeters. The proportional grid system is the greatest contribution given by this work.It allows us to warp our atlases to the patient's MRI using this proportionality. The term "warping" is used in neuroimaging to describe the process of distorting an image (e.g.,

*intersection with YX defines F ; the one with HG defines B.*

Fig. 3. Coordinate system proposed by Afshar et. al., 1978.

in proportional voxels or "orthogonal parallelograms."

number of macroscopical sections that proved to be of less interest. Thus the material could be condensed into a single and more practical volume. Some of the modifications took into account promising procedures at that time, such as operations on the small nuclei of the hypothalamus to treat deviant sexual behavior and vegetative disorders. They also added electroanatomical observations on the localization of important trigger points and radioanatomical observations in more than 300 patients.

The 111 brains, ranging in age from neonate to 86 years used in the preparation of this atlas were collected at the University of Würzburg, University of Lund (Germany) and the Sodersjuhuset in Stockholm (Sweden). They have considered to use Reid's plane (the plane that extends from the lower margin of the orbit to the center of the external acustic foramen) as reference, however the examination of the macroscopic series did not reveal consistent spatial relations between points on the outer surface of the skull and the subcortical structures, and they chose the AC-PC axis and the perpendicular line erected on the middle point of the two commissures as the basis for their system of reference.

The extremely careful work with great histological sections through the most clinically relevant structures inserted in a consensual reference system made this atlas one of the most consulted until the present day. This atlas is still in print, due to its practical value and use in functional neurosurgical procedures. Many other authors used this work to construct a computational tool for stereotactic surgery.

#### **2.8 Stereotaxic atlas of the human brainstem and cerebellar nuclei: A variability study (Afshar et al., 1978)**

In the early '70s, strategies for the treatment of spasticity due to cerebral palsy and other disturbances of muscle tone and posture by ablation of the dentate nucleus of the cerebellum were developed. In this context, Afshar focused on the cerebellar nuclei and brainstem structures involved in muscle tone disturbances and pain.

Thirty brains were studied using positive-contrast ventriculography and stereotactic marking of the specimens *in situ*. Their atlas comprises a variability and probabilistic study comparable to their previous study in 1969. They present a coordinate system based on the fastigium-floor line (FFL), and orthogonal to the ventricular-floor plane.

#### **2.9 Co-planar stereotaxic atlas of the human brain: Three-dimensional proportional system: An approach to cerebral imaging (Talairach & Tournoux, 1988)**

This work can be considered one of the most important in the field of brain mapping. Its focus is less on histology and architectonic and more on presenting the concept of proportionality and a new coordinate system, the so called Talairach space.

The Talairach's proportional grid system is based on the three dimensions (length, height, and width) of the human brain. The reference planes are defined as: the midline, defining the sagittal plane; the intercommissural plane that is obtained from the line that passes through the superior edge of the AC and inferior edge of PC, defining the horizontal plane; and two verticofrontal planes that intersect the anterior and posterior commissures (named VACp and VPCp). The authors state that direct distance coordinates vary widely from one brain to another and the variation is greater considering points far from the midline.

number of macroscopical sections that proved to be of less interest. Thus the material could be condensed into a single and more practical volume. Some of the modifications took into account promising procedures at that time, such as operations on the small nuclei of the hypothalamus to treat deviant sexual behavior and vegetative disorders. They also added electroanatomical observations on the localization of important trigger points and

The 111 brains, ranging in age from neonate to 86 years used in the preparation of this atlas were collected at the University of Würzburg, University of Lund (Germany) and the Sodersjuhuset in Stockholm (Sweden). They have considered to use Reid's plane (the plane that extends from the lower margin of the orbit to the center of the external acustic foramen) as reference, however the examination of the macroscopic series did not reveal consistent spatial relations between points on the outer surface of the skull and the subcortical structures, and they chose the AC-PC axis and the perpendicular line erected on the middle point of the two commissures as the basis for their system of

The extremely careful work with great histological sections through the most clinically relevant structures inserted in a consensual reference system made this atlas one of the most consulted until the present day. This atlas is still in print, due to its practical value and use in functional neurosurgical procedures. Many other authors used this work to construct a

**2.8 Stereotaxic atlas of the human brainstem and cerebellar nuclei: A variability**

brainstem structures involved in muscle tone disturbances and pain.

fastigium-floor line (FFL), and orthogonal to the ventricular-floor plane.

**system: An approach to cerebral imaging (Talairach & Tournoux, 1988)**

proportionality and a new coordinate system, the so called Talairach space.

In the early '70s, strategies for the treatment of spasticity due to cerebral palsy and other disturbances of muscle tone and posture by ablation of the dentate nucleus of the cerebellum were developed. In this context, Afshar focused on the cerebellar nuclei and

Thirty brains were studied using positive-contrast ventriculography and stereotactic marking of the specimens *in situ*. Their atlas comprises a variability and probabilistic study comparable to their previous study in 1969. They present a coordinate system based on the

**2.9 Co-planar stereotaxic atlas of the human brain: Three-dimensional proportional**

This work can be considered one of the most important in the field of brain mapping. Its focus is less on histology and architectonic and more on presenting the concept of

The Talairach's proportional grid system is based on the three dimensions (length, height, and width) of the human brain. The reference planes are defined as: the midline, defining the sagittal plane; the intercommissural plane that is obtained from the line that passes through the superior edge of the AC and inferior edge of PC, defining the horizontal plane; and two verticofrontal planes that intersect the anterior and posterior commissures (named VACp and VPCp). The authors state that direct distance coordinates vary widely from one brain to another and the variation is greater considering points far from the

radioanatomical observations in more than 300 patients.

computational tool for stereotactic surgery.

**study (Afshar et al., 1978)**

reference.

midline.

*The reference lines of Afshar et al. are confined to brainstem and cerebellar structures. The ventral HBG line runs in a rostro-caudal direction tangentially to the floor of the IVth ventricle. The parallel dorsal YFX line passes the tip of the fastigium. A line perpendicular to YX and HG goes likewise through the tip of the fastigium. Its intersection with YX defines F ; the one with HG defines B.*

Fig. 3. Coordinate system proposed by Afshar et. al., 1978.

However this variation is proportional for each brain, and they propose to divide the brain in proportional voxels or "orthogonal parallelograms."

Each hemisphere is divided in twelve parts (1-12) in its supero-inferior (z) axis, in nine parts (A-I) in its anteroposterior (y) axis and in four parts (a-d) in the laterolateral (x) axis. Each voxel has the following dimensions: x: one-fourth of the distance between midline and the most lateral point of the parietotemporal cortex, named a-d ; y: The voxels located anterior to AC have ¼ from the distance from AC to the frontal pole(named A-D). The voxels located posterior to PC have ¼ of the distance from PC to the occipital pole (named F-I), and finally the AC/PC distance is the 9th voxel (named E) and can be divided in thirds, called "minivoxels"; z: the voxels located above the IC line have 1/8 of the distance between IC line and the highest point of the parietal cortex (called 1-8) and the voxels located below the IC line have ¼ of the distance between the IC line and the most inferior point of the temporal cortex (named 9-12). This voxels are fixed for every brain, allowing the normalization between two different subjects having in mind these proportions and not absolute distances in millimeters. The proportional grid system is the greatest contribution given by this work.It allows us to warp our atlases to the patient's MRI using this proportionality. The term "warping" is used in neuroimaging to describe the process of distorting an image (e.g.,

Review of Printed and Electronic Stereotactic Atlases of the Human Brain 153

They could observe the distribution of immunostaining among the thalamic groups and

One interesting technical aspect of this atlas is the correction for shrinkage factors. It was done by means of preoperative MRI from two patients who underwent medial thalamotomy for cancer-related neurogenic pain and who died later from their disease. Intercommissural distances were measured post-mortem at the end of histological processing, and an additional distortion factor was taken into account for coordinates between sections, by measuring the distance between two coronal sections intersecting the centers of the two commissures. The most valuable contribution of this atlas is though the presentation of a

This atlas consists of printed sections and digital media, making it especially easy to use. The first section is a topographic and topometric atlas, in which *in vivo* and *in vitro* MRI scans are compared to whole head sections in horizontal, coronal and sagittal plans. The myeloarchitectonic atlas is based on serial coronal sections through a single hemisphere from a 24-year-old male. Although the templates give detailed information about cytoarchitectonic fields, it is hard to imagine how they could have been segmented from the sections presented on the book, since the staining allows very good contrast between gray and white matter; however, it is hard to define cytoarchitectonic borders with this method. It is a comprehensive study, and its most interesting features are the *in vivo* and *in vitro* MRI comparison to the cadaveric slices, and the user-friendly interface in the

**2.12 Stereotactic atlas of the human thalamus and basal ganglia ( Morel, 2007)**

In 2007, Morel presented this atlas with the main objective to combine high anatomical resolution (taking advantage of new staining methods) and stereotactic precision. This paper presents a collection of diagrams of the human thalamus, basal ganglia, and adjoining structures, consisting of a series of maps in the three stereotactic planes, and comparisons between brains with similar and differing intercommissural distances. This work, like the previously presented one ten years before, examines the distribution of different neurochemical markers in the thalamus and basal ganglia for comparison with non-human

Fiber tracts leading to the thalamus are also described, and there is a correlation between histological maps with MRI images. They compare also the individual representations,

This atlas is especially useful in functional neurosurgery for research purposes to those who want to understand the connections and plan new targets for neurosurgical interventions in

To give a better overview of various details, ranging from the number of cases studied to the reference coordinate systems, a table will summarize the salient features concerning

**2.13 Summary of materials and methods used in the preceding publications**

correlate specific markers to functional units.

chemoarchitectonic organization of the human thalamus.

**2.11 Atlas of the human brain (Mai et al.,1998, 2003)**

digital version.

primate data.

giving an idea of variability.

materials and methods applied.

diseases involving the thalamus and basal ganglia.

atlases) to fit other similar images (e.g. MRI). When two brains are normalized using this proportional system a " Talairach transformation" is performed.

Even with some known disadvantages, such as inter-slice distances variable between 2 and 5 mm, inconsistencies of orthogonal planes and assuming left/right symmetry, this atlas is widely used by neurosurgeons while planning interventions.

#### **2.10 Multiarchitectonic and stereotactic atlas of the human thalamus (Morel et al., 1997)**

Morel's et al. (1997) atlas is based on standard as well as histochemical and immuniohistochemical methods. Advanced neurochemical markers were used to further characterize thalamic nuclei and delimit subterritories of functional significance for stereotactic explorations. The objective was to improve the anatomical definition and precision of thalamic targets.

*The principal Talairach planes (ICp, VACp, VPCp) can be further parcelated by nine coronal planes (A-I), by four sagittal planes (a-d), and by twelve horizontal planes (1-12). Independent of individual hemispheric sizes, each hemisphere will consist of a total of 9x12x4 = 432 voxels. The coronal plane E, based on the AC-PC distance, represents a kind of core structure. Its dimensions are directly measured from the MRIs. The relative voxel sizes of A to D, F to I, a-d and 1-12 are a matter of convenience. They are derived from the maximal lobar extensions rostral, caudal, and lateral from VAC and VPC, and ventral from the AC-PC plane. They are expressed as fractions of the respective plane (1/4 in the coronal plane rostral and caudal to E, ¼ in the sagittal plane corresponding to a – d, 1/8 in horizontal planes dorsal to the AC-PC plane, and ¼ in horizontal planes ventral to the AC-PC plane).*

Fig. 4. Hemisphere inserted in Talairach's space

atlases) to fit other similar images (e.g. MRI). When two brains are normalized using this

Even with some known disadvantages, such as inter-slice distances variable between 2 and 5 mm, inconsistencies of orthogonal planes and assuming left/right symmetry, this atlas is

Morel's et al. (1997) atlas is based on standard as well as histochemical and immuniohistochemical methods. Advanced neurochemical markers were used to further characterize thalamic nuclei and delimit subterritories of functional significance for stereotactic explorations. The objective was to improve the anatomical definition and

*The principal Talairach planes (ICp, VACp, VPCp) can be further parcelated by nine coronal planes (A-I), by four sagittal planes (a-d), and by twelve horizontal planes (1-12). Independent of individual hemispheric sizes, each hemisphere will consist of a total of 9x12x4 = 432 voxels. The coronal plane E, based on the AC-PC distance, represents a kind of core structure. Its dimensions are directly measured from the MRIs. The relative voxel sizes of A to D, F to I, a-d and 1-12 are a matter of convenience. They are derived from the maximal lobar extensions rostral, caudal, and lateral from VAC and VPC, and ventral from the AC-PC plane. They are expressed as fractions of the respective plane (1/4 in the coronal plane rostral and caudal to E, ¼ in the sagittal plane corresponding to a – d, 1/8 in horizontal planes dorsal to the AC-PC plane, and ¼ in horizontal planes ventral to*

**2.10 Multiarchitectonic and stereotactic atlas of the human thalamus (Morel et al.,**

proportional system a " Talairach transformation" is performed.

widely used by neurosurgeons while planning interventions.

**1997)**

precision of thalamic targets.

*the AC-PC plane).*

Fig. 4. Hemisphere inserted in Talairach's space

They could observe the distribution of immunostaining among the thalamic groups and correlate specific markers to functional units.

One interesting technical aspect of this atlas is the correction for shrinkage factors. It was done by means of preoperative MRI from two patients who underwent medial thalamotomy for cancer-related neurogenic pain and who died later from their disease. Intercommissural distances were measured post-mortem at the end of histological processing, and an additional distortion factor was taken into account for coordinates between sections, by measuring the distance between two coronal sections intersecting the centers of the two commissures.  The first section is a topographic and topometric atlas, in which *in vivo* and *in vitro* MRI scans are compared to whole head sections in horizontal, coronal and sagittal plans. The myeloarchitectonic atlas is based on serial coronal sections through a single hemisphere from a 24-year-old male. Although the templates give detailed information about cytoarchitectonic fields, it is hard to imagine how they could have been segmented from the sections presented on the book, since the staining allows very good contrast between gray and white matter; however, it is hard to define cytoarchitectonic borders with this method. It is a comprehensive study, and its most interesting features are the *in vivo* and *in vitro* MRI comparison to the cadaveric slices, and the user-friendly interface in the digital version.

#### **2.12 Stereotactic atlas of the human thalamus and basal ganglia ( Morel, 2007)**

In 2007, Morel presented this atlas with the main objective to combine high anatomical resolution (taking advantage of new staining methods) and stereotactic precision. This paper presents a collection of diagrams of the human thalamus, basal ganglia, and adjoining structures, consisting of a series of maps in the three stereotactic planes, and comparisons between brains with similar and differing intercommissural distances. This work, like the previously presented one ten years before, examines the distribution of different neurochemical markers in the thalamus and basal ganglia for comparison with non-human primate data.

Fiber tracts leading to the thalamus are also described, and there is a correlation between histological maps with MRI images. They compare also the individual representations, giving an idea of variability.

This atlas is especially useful in functional neurosurgery for research purposes to those who want to understand the connections and plan new targets for neurosurgical interventions in diseases involving the thalamus and basal ganglia.

#### **2.13 Summary of materials and methods used in the preceding publications**

To give a better overview of various details, ranging from the number of cases studied to the reference coordinate systems, a table will summarize the salient features concerning materials and methods applied.

Review of Printed and Electronic Stereotactic Atlases of the Human Brain 155

Orientation of sections and interval

(1)19 cor.,5 sag., 6 horiz.from 1 brain, and 4 from another; (2) 20 cor., 17 sag.and20 horiz.

(1)54 plates, 1mm thick ;(2) 12 cerebellar plates

36 sagittal (1- 4mm int.); 38 frontal (5mm int); 27 horizontal (5mm int)

3 blocks sagittal, 4 horizontal and 2 coronal

17 horizontal;15 coronal and 8 sagittal; (2) 69 coronal sections

Range of Slices Coordinate

(1)57 mm ant. - 44 mm post. to AC; 0 - 22mm from midline; 18 above-20mm below ICL; 5-28mm below ICL; (2)16.5 mm ant.- 16.5mm post midcom. line 1.5-27.5 mm lat. Midline 16.0mm above to 9.5mm below ICL

(1)23mm rostral to 30mm caudal to FFL; 1mm rostral to 10mm caudal to FFL

Right: 18 sec 0- 62mm ;Left:18 sec 0-61mm ;0-65mm ant. To VAC; 0- 100mm post VAC;0-6.5 mm above ICL;0- 4.1mmbelow ICL

Region of interest: thalamus

(2) 60mm ant.AC-100 mm post. AC (100µm)

System MRI

No

No

Preopera tive MRI of 2 patients

Yes for the macroser ies

ICP MSP McomP

VFP FFL Fastigial point

> ICP MSP

ICP MSP VAC

ICP MSP VAC

VAC Yes

Reference

Schaltenbrand &Wahren, 1977

Afshar et

Talairach& Tournoux,1988

> Morel et al.,1997

Mai et al.,1998,2003

Nr. of brains and sections

34 sec. for macro and 57 sec.for microseries (frozen sec. 30µm) Paraffin used to brainstem and cerebellum

al.,1978 30 brains Myelin-stained

1 brain cut sagittaly, frontal and horizontal sections were interpolated

9 thalamic frozen blocks cut in 40µm thick slices from 5 brains

6 brains – macroseries (frozen, 1 cm thick) and 1 hemisphere for Microseries (paraffin)

Staining and other investigative tissue methods

Unstained(1), myelin-stained and Nissl (2)

Anatomic sections, that are drawn as a result of tracing the sections

Nissl, myelin and Immunohystoc hemestry parvalbumin (PV), calbindinD-28K (CB), and calretinin (CR)

(1)Sudan red or Sudan Black B (2)Hematoxyli n- or sudan black B for myelinated fibers

Orientation of sections and interval

(1)cor.,sag., horiz.and oblique through brainstem (5mm int.) (2)cor. (2-4 mm) and oblique through brainstem (5mm)

(1)16 coronal (1- 4 mm) and 18 sagittal (0.5 to 2.5 mm);(2)20 horizontal

21 coronal (1 mm) and 17 sagittal (1mm)

10 sagittal (0.5 to 4mm); 8 horizontal (3.5mm) ; 10 coronal sections

5 sag. and 5 cor. plates; 10 sag. and cor. whole brain sections

Range of Slices Coordinate

16.5 mm ant. to AC and 16.5 mm post.to PC;2.0 - 27.5mm lat. to midline;16mm above to 9.5mm below midcom. pt.

1-21 mm behind FM;3-20 mm from midline; includes thalamus, basal ganglia, and medial temporal lobes.

2-25mm lat.to midline parallel to ICP; 17mm above to 8.1 mm below IC; 23.4 mm ant. PC - 47mm post. PC

Cut at 9, 11,13.5,16 and 18 mm lat. to midline; 8.5, 10 , 11, 12.5 mm post. to the midcom. point

System MRI

MSP No

No

No

PC-PO line

ICP, MSP, VACp , VPCp

> ICP MSP McomP

FM-PC line

ICP MSP

ICP MSP VAC

TthL No

VAC No

No

Reference

Spiegel & Wycsis,1952

Talairach et al.,

Schaltenbrand & Bailey,1959

Andrew et al.,

Van Buren &Borke, 1972

Emmers& Tasker, 1975

<sup>1969</sup>19 brains

Nr. of brains and sections

1 brain (30 sections)

1957 1 brain Unstained and

111 brains (7 brains for variability study)

6 brains individual profiles and 25 hemisph. for gross anatomy variability

2 brains: 1 for macroseries 1 for microseries

Staining and other investigative tissue methods

Unstained (1) and myelinstained (2)

Myelin-stained

Unstained (1) and myelin – stained (2)

Nissl and myelin stained from 1 brain

Cresyl-violet, myelin and Golgi preparations

Electrical stimulation at 2mm intervals and mapping of the somesthetic responses elicited in awake patients; results projected into the templates

Review of Printed and Electronic Stereotactic Atlases of the Human Brain 157

**3.1 Creation of a three-dimensional atlas by interpolation from Schaltenbrand-Bailey's**

In 1987 Yoshida proposed the creation of a 3D atlas of the human brain based on the interpolation of the 2D templates of the Schaltenbrand-Bailey's atlas. Although the rendering resulted in great three-dimensional incoherency due to the lack of correction of the linear and non-linear tissue distortions present at the original sections, this atlas is important as the first of its kind and of the concept of referring to a volume instead of

**3.2 Multiple brain atlas database and atlas-based neuroimaging system (Nowinski et**

Nowinski and his collaborators started in 1997 a project in which four previously published atlases were digitized, enhanced, segmented (color coded or contoured), labeled, aligned, and organized into volumes for the purpose of developing an atlas-based neuroimaging system for analysis, quantification, and real-time manipulation of cerebral structures in two and three dimensions. A software was developed that is available in a CD-ROM called "Electronic Clinical Brain Atlas-ECBA." It is impressive in terms of three-dimensional visualization of the structures; however, due to some inaccuracies inherent in the original print atlases, three dimensional structures reconstructed from Schaltenbrand & Wahren atlas are often convoluted and displayed in unrealistic shapes. As noticed and discussed by the authors in this paper, a given point in the stereotactic space may have up to three different labels on the Talairach & Tournox atlas, due to inconsistent orthogonal plates. Nevertheless, important questions about the 3D accuracy of the most-used atlases were

**3.3 Automated atlas integration and interactive three-dimensional visualization tools**

The authors created a deformable volumetric atlas of the basal ganglia and thalamus from the Schaltenbrand and Wahren atlas (SW atlas) to help neurosurgeons navigate through MRI-invisible structures. They developed also a visualization platform that permits manipulation of the merged atlas and MRI data set in two- and three-dimensional views. A really interesting and new method of correction of errors was developed by this group. After digitizing the sections and the transparent overlays from the SW atlas, they aligned and segmented the nuclear contours based on the overlays and performed an interpolation to create a 3D volume. The alignment was based on the original grid, and they noted that the grid structure present in the atlas was placed on the cryotome images subsequent to photography, and so even precise alignment with respect to the grid is no guarantee that the underlying slices were not themselves distorted during the slicing process. However, they developed a methodology for matching automatically the slices with the 3D model. The atlas is registered point-to-point (250 homologous landmarks identified by a neuroanatomist) to a model MRI or standard reference volume. Even though any MRI could be chosen, they opted by the Colin27 that is the result of an average of 27 MRI scans of the same subject. As they have a labeled MRI based on the SW atlas, an algorithm called ANIMAL (Automated Nonlinear Image Matching and Anatomical Labeling) computes a nonlinear transformation to register the patient's MRI with the pre-labeled

**for planning and guidance in functional neurosurgery (St-Jean et al., 1998)**

confining exclusively on two-dimensional representations of the neural tissue.

**atlas (Yoshida, 1987)**

**al., 1997)**

evidenced.

*Definitions: PC-PO line: connects the center of posterior commissure (PC) to the ponto-medullary sulcus (posterior border of the pons) and the midsagittal plane (MSP); ICL- intercommissural line: line that passes through the superior edge of the anterior commissure (AC) and inferior edge of PC; ICP- intercommissural plane: plane obtained from the ICL, defines the horizontal plane; MSP-Midsagittal plane: obtained from midline; VACp:Verticofrontal plane is formed by VAC line, that is a vertical line traversing the posterior margin of the AC: VPCp is the verticofrontal plane perpendicular ICL crossing it at PC; MComP: Midcommissural plane is erected from the midcommissural point, that is the midpoint of the ICL;The FM-PC line is the distance between the posterior inferior margin of the foramen of Monro (FM) to the midpoint of the ventricular surface of PC;The total thalamic length (TthL) is the distance between the FM and the top of the pulvinar;The ventricular-floor plane (VFP) is the plane defined by the floor of the fourth ventricle. Perpendicular to this plane and reaching the fastigium (apex of the roof of the fourth ventricle) is the fastigium-floor line(FFL).*

Table 1. Printed stereotactic atlases based on number of cases studied, histological protocols, planes of section, coordinate systems, and reference to neuroimaging.


*2.4. Dataset*

The dataset used in this study is publicly available in [45]. The authors used a FLIR Vue Pro camera to capture thermographic images for the dataset. During image capture, participants looked at a fixed point while the camera was moved to nine equidistant positions, forming a semicircle around the volunteer. Thus, the dataset contained nine thermographic images of the face of each participant. Figure 5 displays examples of photos that comprise the dataset.

**Figure 5.** Examples of dataset images [45].

The complete dataset contained 998 images from 111 participants. However, to work on a balanced dataset concerning volunteers' gender, only 781 images were used. Of these, 658 were used for training and 123 for validation of transfer learning by the YOLO network. Aiming to evaluate the performance of YOLO for object detection, it is necessary to label each image with the annotations of their respective bounding boxes.

The face ROIs are the ear, eye, forehead, and whole face. These areas have known temperature thresholds for febrility and can be see directly; thus, they are suitable for screening febrile people using thermography [46]. However, not all regions are constantly visible on the person owing to the use of glasses, face masks, hair over the forehead or ear, and others.

LabelImg software, a free graphical tool for image and video annotation [47], was used to label all images used in this study. Figure 6 shows the graphical interface of the LabelImg software.

**Figure 6.** LabelImg software graphical interface.

#### **3. Results**

The training of the object detector was performed on Google Collaboratory, a computational environment that runs in the cloud and requires no configuration. It allows the writing and executing of code directly in the browser.

For the R-CNN training assessment, it was necessary to quantify the prediction accuracy by comparing the prediction made by this model with the real object location in the image. Thus, the mean average precision (mAP), which is one of the most common metrics for determining the accuracy of object detectors, was employed [48].

Other methods to evaluate the performance of the trained network were precision (P), recall (R), and F1-score (F1). For these metrics, a higher value indicates a better result. Additionally, the values of true positives (TP), false positives (FP), and false negatives (FN) were employed as performance metrics.

The resultant metric values of the trained R-CNN, with a confidence limit of 25% (conf\_threshold = 0.25), were as follows: TP = 452, FP = 46, FN = 9, P = 0.91, R = 0.98, and F1 = 0.94. The mAP with an intersection over union (IoU) greater than 50%, also known as<EMAIL_ADDRESS>was 0.97. From Table 1, it is possible to evaluate the performance of the model for each class.

Tests were carried out with photos of six volunteers, different from those present in the training dataset, to evaluate the prediction accuracy of new images. A Testo-885 thermographic camera captured the new images. After transfer learning, the object detector algorithm analyzed these images and detected all ROIs, even for volunteers wearing masks, caps, and with long hair. Figure 7 displays some of these images.

**Table 1.** Performance for each ROI during validation.

**Figure 7.** Images produced using the Testo-885 camera and analyzed by YOLO.

When identifying an object, the YOLO detector provides the coordinates, width, and height of the bounding boxes. This allows delimiting ROIs where the temperature is analyzed. From each ROI, the values of the pixels with the highest temperatures were extracted. Thus, the algorithm discards regions covered by hair, sweat, and fabric, which are generally at lower temperatures. The higher temperatures are shown as the lightest colors in Figure 8.

Figure 9 displays a boxplot of the pixel values in each ROI, as depicted in Figure 8. The distribution of the pixels in the forehead region displays lower values than those in the eye regions, indicating that the eyes are at a higher temperature than the forehead.

According to [25], the maximum or mean temperatures of ROIs can be adopted to assess human body surfaces. However, the segmentation of ROIs performed in this paper may include background images, parts of the surfaces of glasses, masks, and hair, decreasing the mean temperature of the ROI. Therefore, to avoid this issue, the maximum temperature for each ROI was adopted.

The temperature scale on the right side of Figure 8 indicates that: darker colors are close to a temperature of 24 ◦C and lighter colors approach 35 ◦C. In the thermal imager standard operating mode, these values are automatically generated by the camera's operating software, where the highest value indicates the maximum temperature of the objects in the thermal imager's field of view and the lowest value indicates the minimum temperature of the objects.

**Figure 8.** Extraction of ROIs to be analyzed.

Thermogram radiometric output is not always available, depending on the imager manufacturer. Thus, a method for reading temperatures in the region of interest directly in the thermal image was developed, so that the method can be widely used.

Along the temperature scale, there are 267 pixels, where the first one, pixel zero, has a value of 254. The last pixel of the scale has a value of 4. Figure 10 presents the relationship between the pixel values and their respective positions on the scale as a dashed line (in green). Equation (1) shows a first-order linear proportionality relation, the first-order polynomial, obtained through linear regression, with a coefficient of determination (R2) of 0.9941. The solid line (in red) on the graph shows the behavior of the equation of the straight line that describes this relationship.

$$
\dot{q} = -1.064\upsilon + 270.256,\tag{1}
$$

where *v* is the pixel value and *i* is the position on the temperature scale.

**Figure 10.** Pixel value as a function of position on the temperature scale. The dashed green line depicts the pixel values extracted from the image. The solid red line displays the behavior of the equation obtained by linear regression.

As Equation (1) indicates a first-order linear proportionality between pixel position on the scale and temperature, higher temperatures will produce higher pixel values. Thus, the pixels positioned at the beginning of the scale represent the highest temperatures, and pixels at the end indicate the lowest. Figure 11 depicts the relationship between the pixel positions and the respective temperatures of the scale, as shown in Figure 8. Equation (2) presents a straight line that describes this relationship.

$$T = \left[ (y\_2 - y\_1) / 266 \right] i + y\_1,\tag{2}$$

where *i* is the pixel position, obtained using Equation (1), *y*1 is the highest value recorded on the temperature scale, *y*2 is the lowest value recorded on the temperature scale, and *T* is the temperature (in ◦C) of the analyzed pixel.

**Figure 11.** Temperature as a function of pixel position.

Through algebraic manipulation of Equations (1) in (2), it is possible to obtain the value of the temperature *T* of the analyzed pixel, through the value, *v*, of the pixel, the highest value recorded on the temperature scale, *y*1, and the lowest value, *y*2. Equation (3) displays the algebraic manipulation:

$$T = 1/266 \left[ y\_1 (1.064v - 4.256) - y\_2 (1.064v - 270.256) \right]. \tag{3}$$

After obtaining the highest temperatures of each image ROI, the highest value among these temperatures represents the final temperature of the volunteer. Figure 12 shows images of 24 volunteers, and Table 2 lists the highest temperature recorded in each ROI for each person.

**Figure 12.** Images produced using the Testo-885 camera and analyzed by YOLO, with temperature estimated using Equation (3).

**Table 2.** Estimated Volunteer Temperatures.

Table 2 initially shows that there are small variations in temperature (less than 1 ◦C) among most volunteers. However, considering that the human being is homeothermic, this shows that the surface temperature undergoes variations not experienced by the body temperature. Furthermore, it is confirmed that face surface temperature is predominantly lower than body temperature, as shown by the mean temperature values of non-febrile volunteers.

One can note that for volunteers 6 and 22, only one ROI was visible in the image, permitting to obtain only one temperature for the analysis. The identification of only one ROI in volunteers 6 and 22 is an example of both the limitation of the intelligent system and that its objective was achieved. Volunteers were not in adequate direct sight of the thermal imager, in a way that would allow the identification of more ROIs. However, the system managed to identify at least one ROI, enabling the person's temperature analysis.

Unfortunately, only four febrile volunteers were obtained in the image production campaign with volunteers, who were previously diagnosed as feverish by a health team by checking body temperature. Their measured temperatures are identified as 21, 22, 23, and 24 on Table 2. Despite the low sampling of febrile people, it is noted that: the maximum temperature detected in the region of the face of the volunteers 21 and 24 (37.1 ◦C and 37.3 ◦C, respectively) did not exceed the usual fever threshold for central temperature (37.5 ◦C or 38.0 ◦C); the temperatures of different facial regions of volunteer 23 showed a 0.5 ◦C discrepancy, which is significant for the diagnosis of fever. This supports the hypothesis that the febrile diagnostic criteria of core body temperature (37.5 ◦C or 38.0 ◦C) should not be applied to human facial temperature.

#### **4. Conclusions**

This study employed a transfer deep learning method to detect and recognize ROIs, including the face, forehead, eyes, and ears, in thermographic images using the YOLO object detector. After training a CNN from a dataset made available by other researchers, images of new volunteers obtained in the laboratory served as input to the CNN to evaluate the detection performance of the ROIs.

Tests verified that ROI detection was feasible even with the use of masks, caps, helmets, or with features hidden by hair.

As displayed in Figure 9, there were variations in temperature among ROIs. The criterion of adopting the highest temperature within each ROI proved to be efficient, as areas without a direct target, such as those covered by hair, are disregarded.

This study presents a simple system for obtaining temperature values directly from thermographic images without significant computational processing. These improvements in detecting the maximum and minimum temperatures of ROIs can provide better results for identifying febrile people.

As infrared thermography measures surface (skin) temperature and not the core temperature of the human body, future work will apply adequate criteria to analyze the febrility of the individuals from the temperatures of the ROIs. This avoids the use of a single temperature threshold to indicate a feverish state for all regions of the human face. Screening people with fevers through infrared thermography should apply a different and adequate threshold temperature for each face region, typically higher than the body threshold temperature (37.5 ◦C). Additionally, expanding the dataset will improve the detection of ROIs and allow more reliable screening of febrile people.

Finally, other deep learning algorithms will be applied, evaluated, and compared to the results presented in this work.

**Author Contributions:** Conceptualization, R.B.N., J.S. and P.R.M.; data curation, J.R.d.S.; formal analysis, J.R.d.S., R.B.N. and J.S.; funding acquisition, M.A.d.S.L.C. and P.R.M.; investigation, J.R.d.S., G.M.d.A. and M.A.d.S.L.C.; methodology, J.R.d.S., G.M.d.A., M.A.d.S.L.C., H.L.M.C., R.B.N., J.S. and P.R.M.; project administration, P.R.M.; resources, M.A.d.S.L.C. and P.R.M.; software, J.R.d.S., G.M.d.A., M.A.d.S.L.C., R.B.N. and J.S.; supervision, G.M.d.A., M.A.d.S.L.C. and P.R.M.; validation, J.R.d.S., R.B.N. and J.S.; visualization, R.B.N.; writing–original draft, J.R.d.S.; writing–review & editing, R.B.N., J.S. and P.R.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by FAPES (Espírito Santo Research and Innovation Support Foundation), grant numbers 14/2019 (Master's scholarship), 03/2020 (Induced Demand Assessment–COVID-19 Project), and 04/2021 (Research Support). APC and text review were partially funded by the Federal Institute of Espírito Santo through the Institutional Scientific Diffusion Program (PRODIF).

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Ethics Committee of Federal Institute of Espírito Santo - Brazil (protocol code CAAE 33502120.2.0000.5072, approved on 29 July 2020).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Acknowledgments:** The authors thank Masterplace Mall, Construtora Paulo Octávio, Sagrada Família Church in Jardim Camburi, and the Energy Laboratory of Ifes Campus Vitória for collaborating in the field research with volunteers. This work was also supported by the Federal Institute of Espírito Santo and the National Council for Scientific and Technological Development (CNPq).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**