Basic principles o f computed tomography

CT is an anatomical sectional imaging technique that was developed in the early 1970’s [Hounsfield, 1973]. Initially, CT scanners were used to image the brain and with technological progress applications were found for CT imaging of the thorax, abdomen, pelvis and the extremities. These are the only parallels that CT has with PET. The basic principle of CT is that the tube produces an x-ray beam, which is directed through the patient. The x-rays are either absorbed or transmitted through individual body structures depending on the density o f that structure. A detector placed at 180® picks up the resultant beam. In a CT scanner, the patient lies on a couch that is incrementally moved through a gantry that houses an x-ray source (tube) and a detector situated at 180® on the other side of the circular assembly. The tube and detector rotate around the patient as they are moved tiirough the scanner and this

produces a series of thin “slices”, each a cross-sectional image at a particular level through the patient’s body (figure 1.2).

The physical linkages between the power cables and the x-ray tube mean that the tube is unable the rotate in continuous motion. This limitation has major consequences as after each image “slice” the system must stop, rotate back before proceeding to the next slice of tissue. Typically the time taken is 1 second per slice, which makes imaging o f a region relatively slow with the risk of motion artefact being significant [Conall and Hanlon, 2002]. Depending on the resolution required, each slice can be between 1 and 10mm in thickness. The relationship between the amount of the x-ray beam that passes through a particular tissue is inversely proportional to the density of that tissue. The detected incident x-ray beam is converted into an electron stream, which is digitised and assigned a number, known as Hounsfield Units (HU). This represents the amount of energy absorbed by the body tissue it traverses and data is acquired for each angle o f rotation. This data set can be reconstructed into various shades o f black, grey and white using appropriate computer software. Water is assigned an HU o f zero with air appearing black and bone white. This data can be stored on magnetic or digital tapes or be printed out like a plain radiograph. Images can also be viewed on a monitor. Unlike a plain radiograph the resultant image is not a “shadow” image o f the structure under investigation in one plane, but a map of x-ray absorbtion in two dimensions.

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Figure 1.2 Diagram showing the different modes o f data acquisition for conventional 2D CT (top), spiral CT (middle) and multislice CT (bottom)

Figure 1.3 Sequential transaxial spiral CT ^^slices” (a) at the level o f the liver. This patient had previously been treated for CRC There is a suggestion that a CLM is present on these images (circled). It is possible to reconstruct images in the sagittal and coronal planes. This is demonstrated in image (b), a sagittal reconstruction through the pelvis in a different patient who had a symptomatic rectal carcinoma (circled).

CT is extremely well suited to displaying and delineating anatomical information primarily because of the method of producing the image. Contrast media can be injected intravenously or administered orally if certain gastrointestinal tract structures are being studied (for example the liver or intestinal lumen from mouth to anus). The principal function of contrast media (usually iodine based) is to enhance images of vessels, soft tissue structures and tumours.

A more recent advance to conventional CT has been the development of spiral or helical CT. Whereas conventional CT imaged each section or slice and then moved to the next; spiral CT obtains data in a different fashion. The patient moves continuously through the gantry as the x-ray source and detector circle the patient (also

continuously) so that a volume of tissue is imaged. The advance was only possible due to the development of “slip ring” technology which allowed the tube and detector to rotate continuously without physical constraint. Data acquisition is substantially faster than conventional CT and contrast studies and resolution are much improved. This hardware has been augmented with the development of software that is en ab le of processing helical data and reconstructing images in coronal and sagittal as well as the transaxial plane (figure 1.3).

Spiral CT has revolutionised the throughput of patients in departments of radiology. With significantly reduced scan times, more patients can be imaged and the actual scan for individual patients is improved in terms o f obtaining whole abdominal images in one breath hold. The ability to study differential arterial and venous phases of liver perfusion after the administration of intravenous contrast is just one more benefit of the technique. The capability of spiral CT in CRC imaging is, therefore, significant (e.g. CTC) with sensitivity for lesions and pathology being higher than with traditional CT methodology. However, this improved sensitivity is not matched by an equal improvement in specificity and it is in this area that PET and other functional modalities may complement CT.

The natural progression has been to join several detectors (4 to 8) together and these can detect x-ray data across a larger volume of tissue making scaning even faster with an improved resolution [Rubin et al., 1999]. This is known as multishce CT. The thorax, abdomen and pelvis can be imaged in a single breath hold, ^proximately 20

seconds). In terms o f cost, a spiral CT scanner is about one third to one half die cost of a multislice CT machine.

CTC was possible with spiral CT scanners, but the advent of multislice CT has heralded the possibility o f accurate “virtual colonoscopy”, more correctly termed three-dimensional CTC [Hara et al., 2001]. This is a further extension of multislice CT technology that uses computer software to reconstruct data sets obtained from spiral CT of the abdomen into a “virtual” image of the mucosal lining of the colon and rectum [Vining, 1997; Amin et al., 1996]. Images can be reconstructed in two and three dimensions. In the three-dimensional mode and using a computer mouse to navigate it is possible to “fly th ro u ^ ” the virtual colon and inspect the mucosa for lesions. This technique is in its’ infancy, but provides the tantalising prospect of avoiding the hazards of an invasive colonoscopy in order to diagnose pathology in the colon and rectum [Harvey et al., 1998; Fenlon et al., 1999; Morra et al., 1999]. The technique is expensive mainly because o f the costs of software and the need to renew CT hardware, but this situation is likely to change if it proves to be a sensitive and specific clinical investigation, which can be used to replace diagnostic colonoscopy (figure 1.4). The second obvious disadvantage of the technique is that tissue

specimens cannot be retrieved as in conventional colonoscopy, but this has to be weighed up against the potential o f avoiding conventional colonoscopy for diagnosis.

Figure L4 Photograph (a) shows a pedunculated adenomatous colonie polyp which has been marked with an arrow at its ’ base. Image (b) is a two

dimensional multislice CT reconstruction o f the same lesion, again arrowed at its’ base. The black area is the colonic lumen. Image (c) is a three

dimensional reconstruction o f the multislice CT data. This is the equivalent o f a single frame in a video clip o f a virtual colonoscopy.

In addition, it is important to note that CT accounts for 40% of medical radiation, but represents only 4% o f radiological examinations [Shrimpton and Edyvean, 1998]. Any expansion in the use of CT in CRC imaging has to be balanced against the risks of radiation. The National Radiological Protection Board has made specific reference to the likelihood of inappropriate examinations by CT [National Radiological

Protection Board, 1990] and it is noteworthy that multislice CT tends to expose patients to a shghtly higher dose o f radiation compared to spiral CT This debate must also be bourne in mind when considering PET as an imaging modality in CRC, both when used as an alternative or complimentary investigation.