*4.3. E*ff*ect of Penetarting Element Size*

In LIBs, the electrodes are separated by a thin separator. The separator prevents the direct contact of electrodes. The electrodes and electrolytes are tightly packed, as in the case of the LIR2450 coin cell, by positive and negative caps. The generally-used polymer separator has safety issues when combined with high-density and high-capacity LIBs, because, during internal short circuit by penetration or crash, the separator shrinks in volume due to heat, making the direct contact of electrodes possible, which is a dangerous scenario [29]. The effect of different shapes of penetrating element on thermal behavior have been studied previously with ellipsoid, flat, cone, and sphere shapes [6]. In the current study, a cylindrical penetrating element in the battery active material (electrodes and electrolytes) with different sizes were considered. The penetrating element with diameters of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 mm were considered. During the operation of battery with penetrating element, the electrodes come into contact due to deformation and, subsequently, joule heat is produced at a very high rate. The heat is transferred from the point of penetration to the whole battery by conduction, and then to the outside by convection and radiation. In some cases, the high temperature activates the chemical reactions leading to exothermic behavior, and the cell components, including electrolyte and electrode, explode either from penetrating location or safety valve with flames [5]. The nail penetration tests are characterized by a localized hotspot and the propagation of heat to the whole battery through conduction [30]. Figure 5a shows the effect of the penetrating element size on the maximum coin cell temperature. It is evident from Figure 5a that a large penetrating element produces higher temperatures, and temperature continues increasing as the penetrating element size increases. The maximum temperature of the coin cell increases by 103 ◦C as the penetrating element diameter was increases from 0.5 to 3.5 mm. This thermal behavior, of a large increase in temperature for higher penetrating element size, is associated with a large short-circuit area occurring during internal short circuit, leading to a large cross-sectional area available for the flow of secondary current that developed due to the short circuit (i.e., the short-circuit current is proportional to the square of the penetrating element radius) [6]. In addition, a large penetrating element leads to a considerable reaction force and a high-buckling displacement [6]. The thermal behavior due to accumulated heat with penetrating element is closely related to battery internal resistance and contact resistance, which is a direct function of penetrating element size (i.e., diameter) [31].

The heat generation rate increases rapidly at the start due to the internal short circuit for all the cases of different diameters of penetrating element. The total heat generation rate is comprised of heat due to ohmic source, heat due to electrochemical reaction source, and heat due to short-circuit source. As the diameter of the penetrating element decreases, the heat due to the short circuit decreases, as shown in Figure 5b. As the diameter of the penetrating element increases, the contribution of heat due to the short circuit increases. The maximum total volumetric heat generation rate of 636,744.6 W/m3 is recorded for the penetrating element with diameter of 3.5 mm, whereas the maximum total heat generation rate of 57,784.6 W/m3 is recorded for diameter of 0.5 mm. Figure 5c shows the voltage response of a coin cell with different penetrating element sizes. It is seen from Figure 5c that as the penetrating diameter increases, from 0.5 to 3.5 mm, the time to attain the discharge cutoff voltage of 2.75 V reduces. The coin cell with the penetrating element of 0.5 mm diameter attained a discharge cut-off voltage of 2.75 V in 3270 s, whereas, for the penetrating element diameter of 3.5 mm only 780 s was needed. The results of maximum temperature, heat generation rate, and voltage profiles of the coin cell for different penetrating element diameters show that penetrating element size has a substantial effect on the thermal behavior of a coin cell.

(**a**)

**Figure 5.** *Cont.*

**Figure 5.** (**a**) Temperature profiles of the coin cell for different penetrating element sizes. (**b**) Heat generation rate profiles of the coin cell for different penetrating element sizes. (**c**) Voltage profiles of the coin cell for different penetrating element sizes. **Figure 5.** (**a**) Temperature profiles of the coin cell for different penetrating element sizes. (**b**) Heat generation rate profiles of the coin cell for different penetrating element sizes. (**c**) Voltage profiles of the coin cell for different penetrating element sizes.

(**c**)

#### *4.4. Effect of Initial State of Charge 4.4. E*ff*ect of Initial State of Charge*

The state of charge is an indicator of usable energy available compared to maximum rated usable energy. Figure 6a shows the effect of initial state of charge (SOC) on the maximum temperature of the coin cell. The maximum temperature of the coin cell increased with the increase in the initial SOC of the coin cell. Although temperature increased continuously for all SOC levels, the cases with high initial SOC levels showed high temperatures owing to stable high currents for sufficiently long periods [6]. Moreover, the rate of increase of temperature is slightly higher for low SOC level, because the available energy depleted quickly in the case of low SOC levels. The maximum and minimum temperatures of 113.5 and 67.7 °C are observed for 100% and 40% SOC levels, respectively. Similarly, the continuous heat generation rate is maintained for long periods with higher SOCs leading to higher temperatures, as shown in Figure 6b. The characteristic rapid rise of the heat generation rate at the start is observed for all cases of SOCs. The maximum and minimum total heat generation rates of 472707.2 and 340656.4 W/m3 are observed for 100% and 40% SOC levels, respectively. The results of various SOCs show that the thermal runaway temperature attainment is strongly dependent on the SOC level. The results support the findings of Cai et al., which showed that chances of partiallycharged LIBs (at 50% SOC or lower) to attain thermal runaway temperatures are rare during mechanical abuse [32]. The maximum temperature decreases as the SOC decreases. On the other hand, more energy is produced for higher SOC levels, as visible for the area below the heat generation curve shown in Figure 6a [31]. The thermal stability of LIB components is strongly dependent on SOC levels [33]. LIBs operate based on the intercalation–deintercalation phenomenon, in which lithiumion occupies porous structures of cathodes or anodes during charging–discharging. As the level of lithiation in negative electrodes increases, more lithium-ions are available for reactions that are exothermic in nature [34]. The exothermic reactions involving cathodes and electrolytes increase heat generation linearly with the increase in the SOC level, which indicates that SOC levels can play an The state of charge is an indicator of usable energy available compared to maximum rated usable energy. Figure 6a shows the effect of initial state of charge (SOC) on the maximum temperature of the coin cell. The maximum temperature of the coin cell increased with the increase in the initial SOC of the coin cell. Although temperature increased continuously for all SOC levels, the cases with high initial SOC levels showed high temperatures owing to stable high currents for sufficiently long periods [6]. Moreover, the rate of increase of temperature is slightly higher for low SOC level, because the available energy depleted quickly in the case of low SOC levels. The maximum and minimum temperatures of 113.5 and 67.7 ◦C are observed for 100% and 40% SOC levels, respectively. Similarly, the continuous heat generation rate is maintained for long periods with higher SOCs leading to higher temperatures, as shown in Figure 6b. The characteristic rapid rise of the heat generation rate at the start is observed for all cases of SOCs. The maximum and minimum total heat generation rates of 472,707.2 and 340,656.4 W/m3 are observed for 100% and 40% SOC levels, respectively. The results of various SOCs show that the thermal runaway temperature attainment is strongly dependent on the SOC level. The results support the findings of Cai et al., which showed that chances of partially-charged LIBs (at 50% SOC or lower) to attain thermal runaway temperatures are rare during mechanical abuse [32]. The maximum temperature decreases as the SOC decreases. On the other hand, more energy is produced for higher SOC levels, as visible for the area below the heat generation curve shown in Figure 6a [31]. The thermal stability of LIB components is strongly dependent on SOC levels [33]. LIBs operate based on the intercalation–deintercalation phenomenon, in which lithium-ion occupies porous structures of cathodes or anodes during charging–discharging. As the level of lithiation in negative electrodes increases, more lithium-ions are available for reactions that are exothermic in nature [34]. The exothermic reactions involving cathodes and electrolytes increase heat generation linearly with the increase in the SOC level, which indicates that SOC levels can play an important role in the event

of thermal runaway. This is experimentally shown by Mao et al., where batteries with 100% SOC were burnt owing to thermal runaway, and low SOC level batteries with 0% and 50% were not burnt [22]. batteries with 100% SOC were burnt owing to thermal runaway, and low SOC level batteries with 0% and 50% were not burnt [22]. Besides this, other parameters, including battery type, battery design, components of active

*Symmetry* **2020**, *12*, x FOR PEER REVIEW 13 of 23

important role in the event of thermal runaway. This is experimentally shown by Mao et al., where

Besides this, other parameters, including battery type, battery design, components of active battery material, battery shape, and size of battery, are key important factors that can affect the thermal behavior of LIBs. Therefore, the SOC level effect with penetrating element on the thermal behavior of LIBs is very specific, and may have different results (with above suggested parameters) for different batteries. In addition, the temperature non-uniformity is dependent on the size of the battery under consideration. In the present study, due to the compact size of the coin cell, the temperature non-uniformity study is not considerably relevant. The initial rise of the heat generation rate is maximum for 100% SOC and decreases as the SOC level decreases. Zao et al. discussed practical ways to prevent thermal runaway, and ruled out the decrease of SOC as a practical option to prevent thermal runaway in the case of nail penetration [31]. However, this strategy can be used for aircraft "cargo-only" transportation of batteries, where SOC should be kept at less than 30% to prevent thermal runaway [35]. Figure 6c shows the voltage response of the coin cell for different SOC levels. For different SOC levels, the initial voltage is different [9]. For low SOC, the capacity depletes soon after the stable current is established, with the attainment of a discharge cut-off voltage of 2.75 V. This results in a relatively less heat generation rate, leading to a comparatively low temperature. The voltage curve trends are the same for all the cases of SOC, as all other parameters are kept constant except SOC. The present model did not consider the discussions on thermal runaway behavior for temperatures exceeding 128 ◦C, as thermal runaway behavior including material decomposition is not included in the present study [6,36]. The results of maximum temperature, heat generation rate, and voltage profile of the coin cell for different SOC levels show that the initial SOC level has a substantial effect on the thermal behavior of the coin cell. battery material, battery shape, and size of battery, are key important factors that can affect the thermal behavior of LIBs. Therefore, the SOC level effect with penetrating element on the thermal behavior of LIBs is very specific, and may have different results (with above suggested parameters) for different batteries. In addition, the temperature non-uniformity is dependent on the size of the battery under consideration. In the present study, due to the compact size of the coin cell, the temperature non-uniformity study is not considerably relevant. The initial rise of the heat generation rate is maximum for 100% SOC and decreases as the SOC level decreases. Zao et al. discussed practical ways to prevent thermal runaway, and ruled out the decrease of SOC as a practical option to prevent thermal runaway in the case of nail penetration [31]. However, this strategy can be used for aircraft "cargo-only" transportation of batteries, where SOC should be kept at less than 30% to prevent thermal runaway [35]. Figure 6c shows the voltage response of the coin cell for different SOC levels. For different SOC levels, the initial voltage is different [9]. For low SOC, the capacity depletes soon after the stable current is established, with the attainment of a discharge cut-off voltage of 2.75 V. This results in a relatively less heat generation rate, leading to a comparatively low temperature. The voltage curve trends are the same for all the cases of SOC, as all other parameters are kept constant except SOC. The present model did not consider the discussions on thermal runaway behavior for temperatures exceeding 128 °C, as thermal runaway behavior including material decomposition is not included in the present study [6,36]. The results of maximum temperature, heat generation rate, and voltage profile of the coin cell for different SOC levels show that the initial SOC level has a substantial effect on the thermal behavior of the coin cell.

**Figure 6.** *Cont.*

**Figure 6.** (**a**) Temperature profiles of the coin cell for different levels of initial state of charge. (**b**) Heat generation rate profiles of the coin cell for different levels of initial state of charge. (**c**) Voltage profiles of the coin cell for different levels of initial state of charge. **Figure 6.** (**a**) Temperature profiles of the coin cell for different levels of initial state of charge. (**b**) Heat generation rate profiles of the coin cell for different levels of initial state of charge. (**c**) Voltage profiles of the coin cell for different levels of initial state of charge.

*4.5. Effect of the Location of the Penetrating Element*

The location of the penetrating element can have different effects on different types of batteries. In case of accidents, it is unpredictable as to how and where the battery will be impacted in terms of


**3.3.6 Age related factors**

Many studies have shown that hyperhomocysteinemia increases with age (Pennypacker et al., 1992). This is connected with inadequate nutrition, the changes in gastrointestinal function, B vitamins deficiency, enzyme defects, a higher occurance of the C677T mutation and numerous age-related physiological factors.

Pathophysiology and Clinical Aspects of 24 Venous Thromboembolism in Neonates, Renal Disease and Cancer Patients

Coffee consumption has been associated with several risk factors for coronary heart disease, including plasma total homocysteine and reduced B vitamin concentrations (Ulvike et al., 2008). Coffee drinking is associated with smoking and low intake of fruit and vegetables. Tea consumption, in contrast, was associated with lower homocysteine levels (Nygard et al.,

B vitamins have been suggested to play a critical role to maintain low homocysteine levels (Silaste et al., 2001). A diet poor in fresh fruit and vegetables or strict vegeterians may develop nutritional vitamin B12 deficiency (Miller et al., 1991). This is particulary serious in pregnancy, as the mother may not be able to supply the fetus with sufficient vitamin B12. Also modern food processing may destroy essential vitamins and it has recently been shown that microwave heating may destroy as much as 30 % of the vitamin B12 content in food (Watanabe et al., 1998). Prolonged heating may also destroy folate and vitamin B6. Besides, healthly subjects eating fish more than 3 times a week had lower homocysteine levels than

Physical activity plays an important role in our life, since it is the cheapest way of strengthening our health and reduces the risk of developing cardiovascular diseases. It has been confirmed that physical activity decreases the concentrations of total plasma homocysteine and thus the probability of developing a cardiovascular disease in healthy and already sick people. In the study called Bed rest "The influence of simulated weightlessness upon the human organism" performed in 2006, 2007 and 2008 at the Valdoltra Orthopaedic Hospital, Slovenia, in which young male participants (age 24-30 years) rested in horizontal position for 35 days, a statistically increased homocysteine concentration was documented (Plazar et al., 2008). The diet composition and the energy intake were daily supervised and monitored by a dietician. Volunteers were non-smokers, non-alcoholics, without history of cardiovascular and neuromuscular disorders*.* Several studies indicate physical activity as an independent lifestyle factor connected with lower homocysteine concentration. Besides, exercise is associated with a reduction in plasma fibrinogen concentrations, and with increasing activity levels of exercise a reduction in homocysteine was observed. So, this prolonged bed rest study confirms that increased levels of homocysteine in blood, negatively influences the cardiovascular system. Although, the precise mechanism is not well understood, similar consequences can result from prolonged

Many studies have shown that hyperhomocysteinemia increases with age (Pennypacker et al., 1992). This is connected with inadequate nutrition, the changes in gastrointestinal function, B vitamins deficiency, enzyme defects, a higher occurance of the C677T mutation

**3.3.3 Coffee consumption**

**3.3.4 Inadequate nutrition**

**3.3.5 Physical activity**

those eating fish less than one a month.

physical inactivity in everyday life.

and numerous age-related physiological factors.

**3.3.6 Age related factors**

1997b).

 Common risk factors inducing mild hyperhomocysteinemia

#### **4. Venous thromoembolism and hyperhomocysteinemia**

It has been recognized, since the first description of hyperhomocysteinemia, that arterial and venous thrombosis are common in these patients. Patients with homocysteinuria suffered of thrombotic events, cerebrovascular occlusions, deep vein thromboses, myocardial infarctions and peripheral vascular thromboses (Mudd et al., 1985) of which a quarter of all thrombotic events occured before the age of 16 and half before the age of 28, much earlier in life than would normally be expected for these types of events. Further, patients with CBS deficiency-induced homocysteinuria have high levels of plasma homocysteine (Carey et al., 1968). Interest in the hyperhomocysteinemia condition increased when a large number of studies (mainly retrospective) showed that also mildly elevated homocysteine levels are associated with venous thromboembolism (VTE), thrombotic stroke, and peripheral vascular disease (Wilcken & Wilcken, 1976, Mudd et al., 1985, Moll, 2004).

To conclude, mild hyperhomocysteinemia can be induced by a variety of risk factors of which the most common genetic factors are heterozygous CBS gene defects and polymorphism in the MTHFR gene at position 677, while as well numerous aquired conditions might be involved (Table 3).

Hyperhomocysteinemia: Relation to Cardiovascular Disease and Venous Thromboembolism 27

breast, pancreas, and gastrointestinal cancers are associated with a higher incidence of thrombosis. With more advanced stages of cancer there is a lower overall survival rate, but, also a greater risk of venous thromboembolism, additionally influencing the survival of

The associated pathophysiology of VTE and malignancies has not been precisely defined. However, it has been reported that cancer patients show increased levels of several procoagulant factors (Falanga et al., 1993). It is well established that women with advanced breast cancer show hyperhomocysteinemia, which explains the hight rate of venous thrombosis in women with metastatic breast malignancy (Smith et al., 2008). Other established contributors to the VTE increased risk are: chemotherapy, hormonal adjuvant therapy, surgery, central venous catheters, immobility and inherited thrombophilia, with

Under physiological conditions, non-protein bound homocysteine is subjected to glomerular filtration, and almost completely reabsorbed in the tubuli and oxidatively catabolized to carbon dioxide and sulphate in the kidney cells. The clearance is markedly reduced in renal failure with a strong, positive correlation between homocysteine levels, serum creatinine and the glomerular filtration rate (Hultberg et al., 1993). Hyperhomocysteinemia in patients with chronic renal failure induces an oxidative stress to the vascular endothelium, causing a failure in vasodilatation and an impairment of antithrombotic properties. In patients with end-stage renal disease (ESRD) the prevalence of hyperhomocysteinemia is 85-100% and of the fifty-nine ESRD patients undergoing hemodialysis treatment with supplemented B vitamines it was concluded that the MTHFR C677T mutation is an important genetic

Several studies have examined the relationship between hyperhomocysteinemia and retinal vein occlusion, a condition affecting approximately five out of 1000 of the general population over 64 years of age (David et al., 1988). The association between retinal vein occlusion, hyperhomocysteinemia and thermolabile MTHFR was confirmed (Janssen et al.,

Venous thrombosis in children occurs at a much lower frequency than in adults and the events are usually provoked by acquired risk factors like sepsis, cancer and central venous catheters. The association of VTE and hyperhomocysteinemia in children has been

Hyperhomocysteinemia during pregnancy, which is a consequence of perturbations in methionine and/or the folate metabolism, has been implicated in adverse outcomes such as

confirmed in two case control studies (Koch et al., 1999; Kosch et al., 2004).

the notion that oncological therapies do influence the immunological response.

determinant of elevated plasma homocysteine concentration level.

**4.2.4 Homocysteine and thrombosis in children**

**4.2.5 Hyperhomocysteinemia and pregnancy**

patients.

**4.2.2 Renal failure and VTE**

**4.2.3 Retinal Vein occlusion**

2005).

Independent from other factors hyperhomocysteinemia is associated with a 4.8 fold increased risk for VTE (Köktürk et al., 2010).


**1.2. General principles**

such as compressional, shear, surface, creep, lamb and torsional waves etc. These different modalities have widely different characteristics and they can coexist with possible mode conversions depending on the particular geometry and test scenario. In NDT, the co-existence or co-generation of this multimode propagation can be both advantages in some situations,

In processing and assessing an ultrasound image, one of the most striking differences between industrial NDT and medical imaging is that in the case of the latter, there is good preanatomical knowledge of the part of the anatomy being examined; often with the possibility of supplementary data obtained from other forms of imaging such as MRI, CT scans etc. However, for NDT inspection, the target features (shapes, sizes and orientation etc.) are almost always largely unknown, thus the interpretation is largely based on the reliability of the images being produced. This is a major challenge in industrial imaging applications. On the positive side, the targets being imaged in NDT applications e.g. a crack in a structure generally tends to appear as a good acoustic discontinuity yielding good Signal-to-noise ratio (SNR) compared to that from a tissue boundary, since the latter is dependent on small impedance contrast.

For all imaging applications, the signal dynamic range is an extremely important considera‐ tion. Usually this could be very high; far above that may be accommodated by display equipment. For medical Imaging, the dynamic range of signals could be of the order of order of 100dB covering both backscattered signals from tissues and specular reflections. (However, the range of interest is of the order of 40 to 50 dB, which is the range covered by backscattered signals). Although impedance contrast is generally high in NDT and sonar, dynamic range of signals can still be very large depending on the application, not necessarily because of high attenuation as in the case of biological tissues but because of the size of the targets such as micro-defects in NDT or beam divergence in the case of sonar. In the case of sonar, the range is extreme and the signals of interest could be of the order of several hundred millivolts to submicro volts. The dynamic range can be evaluated for homogeneous media, such as water, by considering signal loss between targets of same strength placed at different axial distances

(R1 & R2) form equation 1 below, which helps formulating compensating strategies.

*R*2

Where ∝ is the attenuation coefficient of the medium at the frequency of interest. Accommo‐ dating large dynamic range and depth gain compensation are therefore features common to all imaging systems; albeit for different reasons. Once the ranges and the loss characteristics are known, accommodation can be handled electronically, e.g. by using logarithmic compres‐ sion and Time-varying-gain (TVG) functions to accommodate the signals within the much

In order to understand the potential of imaging and areas of improvement, it is necessary to examine the fundamental characteristics of common imaging modalities, which is the subject

*R*1 + 2∝(*R*2 - *R*1) (1)

*Loss*(*dB*)= 20log

and equally become a nuisance in other cases.

270 Advancements and Breakthroughs in Ultrasound Imaging

*1.1.1. Dynamic range considerations*

limited display dynamic range.

The simplest form of ultrasonic visualisation is what is known as the A-scan. Here an ultrasonic transducer generates a short pulse of high frequency sound which is coupled to the test medium. Echoes generated from any acoustic discontinuities within the path of the ultrasound beam are received, usually by the same transducer, and are displayed on a screen or a monitor as intensity modulated signals. The technique essentially provides (a) Target-depth informa‐ tion (b) An indication of the extent of the discontinuity and to a much lesser extent information on its orientation or shape. Since the time required for displaying the echoes is virtually equal to the time of flight of the acoustic pulse within the object medium, it gives the maximum temporal resolution for moving targets. However, the interpretation and information obtained from an A-scan (1- spatial dimension) or that derived from it is very limited, highly operatordependent and therefore of limited use in diagnostic applications.

However, a collection of A-scans in a given plane in the test object could provide a twodimensional map (or an image) of the acoustic discontinuities. This requires acquisition, storage and display of successive A-scan lines requiring digital data storage and processing. These line-serial scans may be generated in a number of ways: mechanically moving the transducer along a given direction; using an array of transducers which are switched on in succession to mimic a mechanical movement (linear or curvilinear array scanning). Images thus generated represent a two-dimensional (2-D) view of the target medium and are com‐ monly known as B-scans.

Alternatively, the ultrasound beam may be electronically steered using a transducer array (phased array scanning) or by a mechanically rotating transducer. The 2-D images thus obtained are normally referred to as sector B-scans.

#### *1.2.1. Real-time versus determinism*

One of the confusing terminologies in imaging (or any other form of data presentation) is *"Realtime"*. The naturally implied meaning of the term real-time is: *"as it happens".* This meaning is incompatible with laws of Physics since nothing can be observed at the same instant as something happens. So, image presentation can only approach this ideal of real-time, provided that what an observer sees in an image is close enough to what was happening to the object as a whole, both in time and spatial resemblance. Then the expression "real-time" can be considered appropriate. For example, an observer seeing a trajectory of a meteoroid views this in real-time because the velocity of light is at least 6 to7 orders of magnitude higher than the moving object. But this is not the case, for example, seeing a moving heart valve in an ultra‐ sound image taking a few milliseconds to form just one frame. Obviously, the image is temporally and spatially distorted. Therefore a target that appears to be moving in a succession of image frames is not seen "as it happens" and therefore not true real-time. The term *"Pseudo* *Real-time"* is a more meaningful definition in such cases where the image is seen to be live, while the velocity ratio (i.e. the speed of the target to that of sound) is significant, or forming an image frame takes significant time.

It is also important to note that there has been renewed interest in some of the early develop‐ ments that uses direct imaging technology such as ultrasonic cameras and other imaging

Breaking Through the Speed Barrier — Advancements in High-Speed Imaging

http://dx.doi.org/10.5772/56378

273

A fundamental limitation with all of the above existing technology, which arises from the need for line-serial scanning, is the loss of temporal resolution. Attempts to improve this aspect by increasing computing power alone cannot give the full-potential of real-time imaging and may

The second limitation, for example when using linear-array technology, is the use of smaller effective aperture compared to the size of the total array aperture, hence affecting the lateral resolution achievable. These two limitations are analysed below in order to reveal the extent to which they affect potential performance in ultrasonic imaging. Although the discussion is based on using linear arrays for clarity, the same considerations are valid for other configu‐

Figure 1 below shows a linear array transducer coupled to a test medium. The test object is assumed to be homogeneous and have a depth (d). The array consists of (N) elements with a total aperture size (A). The elements are usually switched on as a group (as shown) rather than single elements to improve lateral resolution, since this depends on the effective aperture, as depicted in equation 3. The next group in the firing sequence advance by only one element

Test medium

Group fired

Between individual scanning lines, a rest period (trest) is applied before firing the second consecutive group of elements as a practical requirement to allow multiple echoes to die down

A elements

Transducer Array (N)

thus keeping the line resolution equal to the spacing of the array elements.

**2. Limitations of the conventional technology**

only give improvements with largely diminishing returns.

modalities [10, 11].

**2.1. Basic limitations**

rations.

d

**Figure 1.** Line-serial scanning

Direction of Scan

and time for acquisition, storage and display of data.

On the other hand, *"Determinism"* can be an important concept in defining a key aspect of image integrity. This can be a predominant requirement – for example in quantitative image analysis where accurate spatial information is important. The use of the term "determinism" in this context does not really mean "as it happens" (although the time interval between a moving target and its presentation as an image is usually small). Determinism essentially means that the spatial integrity of a moving target is persevered in the image to a high degree. In the case of ultrasound the theoretical limit of spatial determinism can only be reached if a single ultrasound pulse emitted by an imaging device produces a complete image field of the object within its time of flight. This is clearly not achievable with the existing line-serial imaging technology, but it is one of the main features of the new hybrid imaging system described later in this chapter.

#### **1.3.  Producing a single frame of such an image may have taken about 20 minutes then, but with the advancement of computing power and processing techniques, live 3-D and 4-D volume imaging reaching pseudo-real time volumes of surface features have now come into existence [4, 5].

One of the fairly recent additions to B-mode imaging is the so called Zone-sonography [6]. A main feature in this system is the acquisition of data from a relatively low number of zone sectors, as opposed to line-by-line acquisition, thereby significantly improving the temporal artefacts inherent with the conventional methods. It has been claimed that in some cases, this approach could produce speed improvements of up to 10 times compared to conventional lineserial imaging systems [6], although the zones in themselves are produced using serial scanning.

In addition, in the case of medical imaging, various echo enhancement techniques such as Coded Excitation (CE) and Digitally Encoded Ultrasound (DEU) which improve sensitivity, penetration and contrast are now been widely used [7, 8]. CE is a pulse compression technique which is designed to differentiate and boost weak return signals from deep within the body. This is done by transmitting an encoded pulse sequence, isolating the coded return signal and amplifying only this signal, while regaining longitudinal resolution which is distributed within the insonifying pulse train [7]. Some systems using CE can also show blood flow together with the body tissue as a B-mode image, without the need for overlay as would be the case with Doppler imaging. Tissue Harmonic Imaging (THI) is another widely used technique which reduces haze, clutter and image artefacts. In this approach, higher harmonic components of the echoes generated by the tissues due to non-linear propagation are used instead of the fundamental [9].

It is also important to note that there has been renewed interest in some of the early develop‐ ments that uses direct imaging technology such as ultrasonic cameras and other imaging modalities [10, 11].