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Computed tomography

 Computed tomography (CT), was originally known as "EMI scan" as it was developed at a research branch of EMI, a company best known today for its music and recording business. It was later known as computed axial tomography (CAT or CT scan) and body section roentgenography. It is a medical imaging method employing tomography where digital geometry processing is used to generate a three-dimensional image of the internals of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation. The word "tomography" is derived from the Greek tomos (slice) and graphein (to write). CT produces a volume of data which can be manipulated, through a process known as windowing, in order to demonstrate various structures based on their ability to block the X-ray beam. Although historically (see below) the images generated were in the axial or transverse plane (orthogonal to the long axis of the body), modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures.

Although most common in healthcare, CT is also used in other fields, for example nondestructive materials testing. Another example is the DigiMorph project at the University of Texas at Austin which uses a CT scanner to study biological and paleontological specimens.

Additional recommended knowledge



In the 1930s the Italian radiologist Alessandro Vallebona proposed a method to represent a single slice of the body on the radiographic film. This exam was known as tomography. The idea is based on simple principles of projective geometry: moving synchronously and in opposite directions the radiogen tube and the film, the image created by the points on the focal plane appears sharper, while the images of the other points annihilate as noise.

Tomography has been one of the pillars of radiologic diagnostics until the early 80's, when the advent of computer and of the circular methodic, this last due to Godfrey Newbold Hounsfield and Allan McLeod Cormack, gradually supplanted it in favour of CT.

The first commercially viable CT scanner was invented by Sir Godfrey Newbold Hounsfield in Hayes, United Kingdom at EMI Central Research Laboratories using X-rays. Hounsfield conceived his idea in 1967, and it was publicly announced in 1972. Allan McLeod Cormack of Tufts University, Massachusetts, USA independently invented a similar process, and both Hounsfield and Cormack shared the 1979 Nobel Prize in Medicine.



The CT scanner was "the greatest legacy" of the Beatles, with the massive profits resulting from their record sales enabling EMI to fund scientific research[1], including into computerised tomography. The first production X-ray CT machine (in fact called the "EMI-Scanner") was limited to making tomographic sections of the brain, but acquired the image data in about 4 minutes (scanning two adjacent slices) and the computation time (using a Data General Nova minicomputer) was about 7 minutes per picture. This scanner required the use of a water-filled Perspex tank with a pre-shaped rubber "head-cap" at the front, which enclosed the patient's head. The water-tank was used to reduce the dynamic range of the radiation reaching the detectors (between scanning outside the head compared with scanning through the bone of the skull). The images were relatively low resolution, being composed of a matrix of only 80 x 80 pixels. The first EMI-Scanner was installed in Atkinson Morley's Hospital in Wimbledon, England, and the first patient brain-scan was made with it in 1972.   In the U.S., the first installation was at the Mayo Clinic. As a tribute to the impact of this system on medical imaging the Mayo Clinic has an EMI scanner on display in the Radiology Department.


Previous studies


CT's primary benefit is the ability to separate anatomical structures at different depths within the body. A form of tomography can be performed by moving the X-ray source and detector during an exposure. Anatomy at the target level remains sharp, while structures at different levels are blurred. By varying the extent and path of motion, a variety of effects can be obtained, with variable depth of field and different degrees of blurring of 'out of plane' structures.






Types of Modern CT acquisition








Helical is a very fast way to examine the target anatomy. The volume is scanned very quickly because the table is in constant motion as the gantry rotates continuously. There is no interscan delay between slices as in a Axial acquisition.


A Digitally Reconstructed Radiograph (DRR) is used to help plan the treatment. The DRR is corrected for the fan beam effect. This is the prime difference from the Scout/Pilot image.


Since the introduction of the first clinical system by Hounsfield, several generations of scanners have been produced, with distinguishing tube-detector configuration and scanning motion (Canadian Association of Medical Radiation Technologists CT Imaging 1-Theory textbook).

Generation configuration detectors beam min scan time
First translate-rotate 1~2 pencil thin 2.5 min
Second translate-rotate 3~52 narrow fan 10 sec
Third Rotate-rotate 256~1000 wide fan 0.5 sec
Fourth Rotate-fixed 600~4800 wide fan 1 sec
Fifth electron beam 1284 detectors wide fan electron beam 33 ms

Although numbered sequentially, the 3rd and 4th generation designs developed at approximately the same time. The concept of electron beam CT, which some authors have called 5th generation, followed later. Some authors have described up to 7 generations of CT design. However, it is only generations one to four that are widely, and consistently, recognised.

In the first and second generation designs, the X-ray beam was not wide enough to cover the entire width of the 'slice' of interest. A mechanical arrangement was required to move the X-ray source and detector horizontally across the field of view. After a sweep, the source/detector assembly would be rotated a few degrees, and another sweep performed. This process would be repeated until 360 degrees (or 180 degrees) had been covered. The complex motion placed a limit on the minimum scan time at approximately 20 seconds per image.

In the 3rd and 4th generation designs, the X-ray beam is able to cover the entire field of view of the scanner. This avoids the need for any horizontal motion; an entire 'line' can be captured in an instant. This allowed simplification of the motion to rotation of the X-ray source. Third and fourth generation designs differ in the arrangement of the detectors. In 3rd generation, the detector array is as wide as the beam, and must therefore rotate as the source rotates. In 4th generation, an entire ring of stationary detectors are used.

The third generation design suffers because it is highly sensitive to detector performance. Because of the fixed relationship of a detector to a specific part of the beam, any miscalibration or malfunction of an individual detector will appear as a ring in the final reconstructed image. As the detectors moved and were exposed to physical stress, loss of calibration and subsequent 'ring artifacts' were commonplace. The fourth generation, with its fixed detectors benefited not just from improved reliability of the detectors, but because the detectors could be automatically calibrated as the X-ray beam approached, and because the different reconstruction geometry meant that a malfunction would lead only to subtle loss of image contrast (fogging) rather than a visible ring.

Solving the issue of detector stability has led 3rd generation designs to the dominant position in contemporary designs. 4th generation designs suffered very high cost (due to the large number of detectors) and had very high susceptibility to 'streak artifacts' (due to compton scattering). All current CT scanners are of the 3rd generation design.

Electron Beam CT

Electron beam tomography (EBCT) was introduced in the early 1980s, by medical physicist Andrew Castagnini, as a method of improving the temporal resolution of CT scanners. Because the X-ray source has to rotate by over 180 degrees in order to capture an image the technique is inherently unable to capture dynamic events or movements that are quicker than the rotation time.



Helical or Spiral CT

See main article Helical cone beam computed tomography

Helical, also called spiral, CT was introduced in the early 1990s, with much of the development led by Willi Kalender and Kazuhiro Katada. In older CT scanners, the X-ray source would move in a circular fashion to acquire a single 'slice', once the slice had been completed, the scanner table would move to position the patient for the next slice; meanwhile the X-ray source/detectors would reverse direction to avoid tangling their cables.




Multislice CT

Multislice CT scanners are similar in concept to the helical or spiral CT but there are more than one detector ring. It began with two rings in mid nineties, with a 2 solid state ring model designed and built by Elscint (Haifa) called CT TWIN, with one second rotation (1993): It was followed by other manufacturers. Later, it was presented 4, 8, 16, 32, 40 and 64 detector rings, with increasing rotation speeds. Current models (2007) have up to 3 rotations per second, and isotropic resolution of 0.35mm voxels with z-axis scan speed of up to 18 cm/s.[2]. This resolution exceeds that of High Resolution CT techniques with single-slice scanners, yet it is practical to scan adjacent, or overlapping, slices - however, image noise and radiation exposure significantly limit the use of such resolutions.

The major benefit of multi-slice CT is the increased speed of volume coverage. This allows large volumes to be scanned at the optimal time following intravenous contrast administration; this has particularly benefitted CT angiography techniques - which rely heavily on precise timing to ensure good demonstration of arteries.

Computer power permits increasing the postprocessing capabilities on workstations. Bone suppression, volume rendering in real time, with a natural visualization of internal organs and structures, and automated volume reconstruction really change the way diagnostic is performed on CT studies and this models become true volumetric scanners. The ability of multi-slice scanners to achieve isotropic resolution even on routine studies means that maximum image quality is not restricted to images in the axial plane - and studies can be freely viewed in any desired plane.

Dual Source CT

Siemens introduced a CT model with dual X-ray tube and dual array of 64 slice detectors, at the 2005 Radiological Society of North America (RSNA) medical meeting. Dual sources increase the temporal resolution by reducing the rotation angle required to acquire a complete image, thus permitting cardiac studies without the use of heart rate lowering medication, as well as permitting imaging of the heart in systole. The use of two x-ray units makes possible the use of dual energy imaging, which allows an estimate of the average atomic number in a voxel, as well as the total attenutaion. This permits automatic differentiation of calcium (e.g. in bone, or diseased arteries) from iodine (in contrast medium) or titanium (in stents) - which might otherwise be impossible to differentiate. It may also improve the characterization of tissues allowing better tumor differentiation.

256+ Slice CT

At RSNA 2007, Philips announced a 256 slice scanner, while Toshiba announced a "dynamic volume" scanner based on 320 slices. The majority of published data with regard to both technical and clinical aspects of the systems have been related to the prototype unit made by Toshiba Medical Systems. The recent 3 month Beta installation at Johns Hopkins Press Release using a Toshiba system tested the clinical capabilities of this technology JHU Gazette. The technology currently remains in a development phase but has demonstrated the potential to significantly reduce radiation exposure by eliminating the requirement for a helical examination in both cardiac CT angiography and whole brain perfusion studies for the evaluation of stroke.

Inverse geometry CT



Diagnostic use



  Diagnosis of cerebrovascular accidents and intracranial hemorrhage is the most frequent reason for a "head CT" or "CT brain". Scanning is done with or without intravenous contrast agents. CT generally does not exclude infarct in the acute stage of a stroke. However, CT is useful to exclude an intra-cranial hemorrhage as a cause, or complication of the stroke. For the detection of acute hemorrhage, especially subarachnoid hemorrhage, CT is the test of choice as it is more sensitive than MRI.


CT has an important role in evaluation of the functioning of a ventriculoperitoneal shunt by demonstrating the volume of the ventricular system. Although CT cannot assess intracranial pressure, it can demonstrate several important causes of raised intracranial pressure. Despite the limitation that CT may not detect raised intracranial pressure, it may help in the clinical decision to perform lumbar puncture and is often performed in this context.




  CT is excellent for detecting both acute and chronic changes in the lung parenchyma. A variety of different techniques are used depending on the suspected abnormality. For evaluation of chronic interstitial processes (emphysema, fibrosis, and so forth), thin sections with high spatial frequency reconstructions are used - often scans are performed both in inspiration and expiration. This special technique is called High resolution CT (HRCT)....... Note: HRCT is normally done with thin section with skipped areas between the thin sections. Therefore it produces a sampling of the lung and not continuous images. Continuous images are provided in a standard CT of the chest.



Pulmonary Angiogram


Diagnostic use

It is a preferred choice of imaging in the diagnosis of PE due to its minimally invasive nature for the patient, whose only requirement for the scan is a cannula (usually a 20G). Before this test is requested, it is usual for the referring clinician to have carried out a D-dimer blood test and requested a chest X-Ray to rule out any other possible differential diagnosis.


  • Images are usually taken on a 0.625mm slice thickness, although 2mm is sufficient.
  • 50 - 100 mls of contrast is given to the patient at a rate of 4 ml/s.



  A normal CTPA scan will show the contrast filling the pulmonary vessels, looking bright white. Ideally the aorta should be empty of contrast, to reduce any partial volume artefact which may result in a false positive. Any mass filling defects, such as an embolus, will appear dark in place of the contrast, filling / blocking the space where blood should be flowing into the lungs.




It is uncertain whether this modality will replace invasive coronary catheterization. Currently, it appears that the greatest utility of cardiac CT lies in ruling out coronary artery disease rather than ruling it in. This is because the test has a high sensitivity (greater than 90%) and thus a negative test result means that a patient is very unlikely to have coronary artery disease and can be worked up for other causes of their chest symptoms. This is termed a high negative predictive value. A positive result is less conclusive and often will be confirmed (and possibly treated) with subsequent invasive angiography. For the record, the positive predictive value of cardiac CTA is estimated at approximately 82% and the negative predictive value is around 93%.



The latest MSCT scanners acquire images only at 70-80% of the R-R interval (late diastole). This prospective gating can reduce effective dose from 10-15mSv to as little as 1.2mSv in follow-up patients acquiring at 75% of the R-R interval. Effective doses at a centre with well trained staff doing coronary imaging can average less than the doses for conventional coronary angiography.

Abdominal and pelvic







Advantages and hazards

Advantages over projection radiography


Radiation exposure



The radiation dose for a particular study depends on multiple factors: volume scanned, patient build, number and type of scan sequences, and desired resolution and image quality. Additionally, two helical CT scanning parameters that can be adjusted easily and that have a profound effect on radiation dose are tube current and pitch.[5] CT scans of children have been estimated to produce non-negligible increases in the probability of lifetime cancer mortality leading to calls for the use of reduced current settings for CT scans of children.[6] A 2007 report in the New England Journal of Medicine suggested that the radiation from current CT-scan use may cause as many as 1 in 50 future cases of cancer.[7]

According to the USAToday, and members of the American Heart Association, an average CT scan can expose a patient to between 1,000 to 10,000 millirems of radiation, depending on the exact machine and the examination being performed. However, Japanese people who were 1 mile from ground zero received only 3,ooo millirems of radiation, on average. [8]

Typical scan doses

Examination Typical effective dose (mSv) (milli rem)
Chest X-ray 0.1 10
Head CT 1.5[9] 150
Abdomen CT 5.3[9] 530
Chest CT 5.8[9] 580
Chest, Abdomen and Pelvis CT 9.9[9] 990
Cardiac CT angiogram 6.7-13[10] 670 - 1300
CT colonography (virtual colonoscopy) 3.6 - 8.8 360 - 880

Adverse reactions to contrast agents














  • Aliasing Artifact or Streaks

These appear as dark lines which radiate away from sharp corners. It occurs because it is impossible for the scanner to 'sample' or take enough projections of the object, which is usually metallic. It can also occur when an insufficient mA is selected, and insufficient penetration of the x-ray occurs. These artifacts are also closely tied to motion during a scan.

  • Partial Volume Effect

This appears as 'blurring' over sharp edges. It is due to the scanner being unable to differentiate between overlying structures and respective HU's. The processor tries to average out the two densities or structures. This can be partially overcome by scanning using thinner slices.

  • Ring Artifact

Probably the most common mechanical artifact, the image of one or many 'rings' appears within an image. This is due to a detector fault.

  • Noise Artifact

This appears as gaining on the image and is caused by a low signal to noise ratio. This occurs more commonly when a thin slice thickness is used. It can also occur when the kV or mA is too low.

  • Motion Artifact

This is seen as blurring which is caused by movement of the object being imaged.

  • Windmill


  • Beam Hardening


Three dimensional (3D) Image Reconstruction

The principle


Multiplanar reconstruction






3D rendering techniques

Surface rendering
Volume rendering

3D rendering software

Some examples of CT 3D surface rendering software include
VGStudio, Mimics, 3D doctor, Amira....etc

Image segmentation

Main article: Segmentation (image processing)










  1. ^ The Beatles greatest gift... is to science. Whittington Hospital NHS Trust. Retrieved on 2007-05-07.
  2. ^ Supplement to Applied Radiology, "Cardiac CT: Where are we today and where are we going?".
  3. ^ Hart, D; Wall B F (2002). "Radiation exposure of the UK population from Medical and Dental X-ray examinations". NRPB report W-4.
  4. ^ Hart, D.; Wall (2004). "UK population dose from medical X-ray examinations". European Journal of Radiology 50 (3): 285-291.
  5. ^ Donnelly, Lane F.; et al (February 2001). "Minimizing Radiation Dose for Pediatric Body Applications of Single-Detector Helical CT". American Journal of Roentgenology 176 (2): 303-6.
  6. ^ Brenner, David J.; et al. (2001). "Estimated Risks of Radiation-Induced Fatal Cancer from Pediatric CT". American Journal of Roentgenology (176): 289-296.
  7. ^ Template error: argument title is required. 
  8. ^ Template error: argument title is required. 
  9. ^ a b c d Shrimpton, P.C; Miller, H.C; Lewis, M.A; Dunn, M. Doses from Computed Tomography (CT) examinations in the UK - 2003 Review
  10. ^ Radiation Exposure during Cardiac CT: Effective Doses at Multi–Detector Row CT and Electron-Beam CT
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Computed_tomography". A list of authors is available in Wikipedia.