Computed tomography and its uses in veterinary medicine (Proceedings)


Computed tomography and its uses in veterinary medicine (Proceedings)

Aug 01, 2009

Computerized axial transverse scanning was first announced in April 1972 by G.N. Hounsfield. The basic concept was quite simple: A thin cross-section of the head, a tomographic slice, was examined from multiple angles with a pencil-like x-ray beam. Tomography is imaging that depicts a cut, slice or section of the body free of superimposition by overlying structures. Hounsfield and a physicist from Tufts University, Alan Cormack, shared the Nobel Prize in physics in 1982 for their work on CT. Let's review regular radiographs and compare them to CT. When imaging the abdomen with conventional radiographs, the image created directly on the film is relatively low in contrast. The image is not as clear as one might expect because of superimposition of all the anatomic structures within the abdomen. Scatter radiation further degrades the visibility of image detail. One of the main advantages of CT over conventional radiography is the ability to eliminate superimposition. A CT scan results in transverse or axial images. Transverse images are those perpendicular to the long axis of the body. Collimation is required during CT scanning for precisely the same reasons that it is required in conventional radiography. Proper collimation reduces patient dose and restricts the volume of tissue irradiated. More importantly, it enhances image contrast by limiting scatter radiation. CT uses x-rays and computer processing to create cross sectional (transverse) slices of internal structures. CT images are not only clear but can isolate a specific internal region. Each CT slice is formatted from multiple x-ray exposures captured as the scan completes a 360 degree rotation. Transmitted x-ray energy is recorded by detectors positioned opposite the patient. The x-ray energy is converted to an electric signal and sent to the CT computer for processing. The CT computer translates the electronic single to numeric (digital) information, which in turn is used to display images on the computer monitor.

Spiral CT permits rapid acquisition of a volume of data, thus producing high quality two and 3-dimensional images with very short scanning times. Within the spiral CT scanner, the patient is advanced through the gantry as it is continuously rotated so that the movement of the x-ray tube around the patient simulates the threads of the screw. The advantages of spiral CT include 1) no motion artifact, resulting in improved lesion detection, 2) reduced partial volume artifact due to reconstructing smaller intervals, 3) optimized intravenous contrast obtained during peak enhancement, 4) reduced scanning time, and 5) multiplanar images which result in higher quality reconstruction, because there are no gaps in data. These advantages allow CT to compete with the resolution and versatility of MRI. The rapid acquisition of spiral CT images is especially useful for angiographic procedures, because dynamic scanning can be performed during maximum contrast medium opacification without the use of selective catheterization. Time to peak enhancement can be measured for an individual during a test run consisting of a dynamic scan centered over the area of interest after bolus injection of contrast medium. One clinical application of spiral CT angiography is the diagnosis of pulmonary thromboembolism. Spiral CT can non-invasively identify PTE by detecting filling defects within the contrast filled arteries.

Multislice CT (MSCT) allows for simultaneous acquisition of 4, 8, and 16 slices respectively. With this, combined with a reduction in scanner rotation time, (to less than 0.5 seconds) imaging time can be accelerated by a factor of 8 to 32. Vessels with very small diameters are clearly visualized. There is improved spatial resolution along the length of the body allowing for high quality secondary reconstructions or 3-D visualization techniques.

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