Computed tomography and its uses in veterinary medicine (Proceedings)

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.

CT is particularly valuable for imaging intracranial lesions as well as those involving the nose and sinuses, middle ear, and the periorbital region. In these areas, CT is superior to either standard radiography or ultrasound. Although MRI is considered the optimum technique for central nervous system diagnosis, good quality CT can provide adequate diagnostic information for many brain lesions. Intracranial masses and fluid-filled cavities usually are easily demonstrated with CT. Vascular lesions are usually not as well-defined, but intravascular contrast media aid in this regard. Image interpretation involves standard principles of radiography including an evaluation of size, shape, location, and density of visible structures. The ventricular system, tentorium cerebelli, and midline falx cerebri are readily visible on CT scans. The remaining brain tissue is relatively uniform and homogeneous. Alteration in ventricular size, shape, or position, deviation of midline, and calcification of masses are easily seen on survey images and are important signs of mass lesions. Peripheral edema may be seen as decreased density surrounding the mass. Areas of acute hemorrhage may appear as radiodense due to presence of hemoglobin. As clot resorption occurs, the radiographic density decreases until it may appear radiolucent and appear similar to edema. Intravascular contrast agents usually enhance masses, demonstrating vascular alterations and areas of disruption of the blood brain barrier. Certain canine brain tumors may have distinguishing features on CT images based on location and pattern of contrast enhancement. Meningiomas typically are peripherally located and homogeneously enhance. Astrocytomas and gliomas typically show peripheral enhancement with central lucency. Choroid plexus masses are often relatively dense and enhance uniformly. Pituitary tumors may be recognized by their location and typically show uniform contrast enhancement.

Other regions of the head besides the brain are also well imaged with CT. CT is the preferred diagnostic imaging method for evaluating the nose, sinuses, and periorbital areas, including cases with possible invasion of the cranial vault. If there is destruction of major skull bones, 3-D reconstruction can be dramatic. Reformatted images in the sagittal and dorsal planes often are very useful in evaluating CT scans of the head. CT can also aid in the diagnosis of nasopharyngeal stenosis. The bony tympanic bullae, internal acoustic meatus, semicircular canals, and ossicles are all well visualized by CT enabling accurate diagnosis of otitis media and interna. Chronic otitis externa can have mineralization which is exquisitely visualized with CT. Contrast enhancement allows differentiation between ceruminous debris and masses/ thickening in the horizontal ear canal.

In intervertebral disc protrusion or herniation, CT may provide valuable additional information following standard radiographic examination. It is essential that the lesion be localized by neurologic examination and standard radiography prior to performing CT. CT should be used to obtain additional information regarding a specific spinal lesion at a specific site, not as a fishing expedition over a long segment of the spine. With the exception of lesions in the caudal lumbar and sacral region, a myelogram with standard radiographs should be performed first, followed immediately by CT, if indicated. Thus, the CT is a myelographic study. Good quality CT images demonstrate very thin contrast columns not visible on standard radiographs. CT can be used to demonstrate lateralization of a lesion. Spinal cord compression is more accurately assessed and usually appears more dramatic on CT as compared with standard radiographs. For the lumbosacral area, evaluation of bone remodeling, evidence of cauda equina compression by either soft tissue or bone remodeling within the spinal canal, and comparison of size and density of intervertebral foramina, both between right and left sides at the same space and between different intervertebral disc spaces is possible.

Cortical bone detail cannot be seen with MRI no matter what pulse sequence is used. Therefore CT is the method of choice for imaging ulnar coronoid processes. Its' superior resolution over standard radiography for this purpose cannot be overstated. Radiographic evaluation of joint diseases with CT uses the same principals as standard radiographs. Radiographic changes include subchondral bone lucency and sclerosis, fracture lines, osteophytosis, soft tissue swelling, and soft tissue mineralization. The advantage of CT is that superb detail allows accurate evaluation of lesions seen minimally, if at all, on standard radiographs. CT allows visualization of both the secondary arthritic changes typically seen radiographically, but also clearly shows the fragmentation of the coronoid process that can be difficult to visualize with certainty with radiographic examination. CT arthrography has been studied and can be helpful in diagnosing partial cranial cruciate tears. Complex fractures and their association to the previous anatomic structure can be depicted with CT. The ability to reconstruct images in three dimension can help visualize the pathologic abnormality and help plan for the best therapeutic remedies. CT performed on patient's with pelvic trauma identified soft tissue lesions not seen by radiographic examination, including hemorrhage and muscular trauma. In most cases, CT also identified bone lesions not seen radiographically.

Brachial plexus tumors are a cause of forelimb lameness and can be identified with CT. Special patient positioning and large fields of view allow optimum visualization of these tumors which are often outside of the spinal canal.

CT is underused for thoracic examinations in veterinary medicine. Evaluation of pulmonary and mediastinal masses, including lymphadenopathy and evaluation of spine and rib involvement with thoracic masses are examples of candidates for CT scanning. CT is considered the most sensitive method for detection of human pulmonary metastases. CT evaluation of vaccine associated sarcomas has proven to determine the full extent of disease, often altering recommendations based on the CT images, including preoperative radiation therapy when surgery alone was deemed inadequate to affect local tumor control. Alterations in the surgical approach were also made due to CT findings.

Contrast agents are of considerable value when evaluating abdominal organs. Gastrointestinal contrast is helpful to identify and differentiate the gastrointestinal tract from other structures, particularly in the cranial abdomen. The usefulness of CT in evaluating canine adrenal masses is well established. CT has proven to be an excellent technique for obtaining samples of abnormal tissues. Fluoroscopy, ultrasonography, and computed tomography can all be used to allow direct placement of a needle into a specific site both for diagnostic as well as therapeutic purposes. However with fluoroscopy, demonstration of lesions in two planes is required for accurate needle placement and resolution is, at times, suboptimal. Gas, bone, or surface irregularities may obscure deep-seated lesions when ultrasonography is employed. CT-guided biopsy has been recommended for use in humans when lesion visibility or accessibility by other imaging modalities is unsatisfactory. Via virtue of its high contrast resolution, excellent display of anatomic topography, and disregard for overlying gas or bone, CT may offer superior visualization of lesions, the needle pathway, and tip of the needle as well as surrounding structures. In humans, CT is often selected over fluoroscopy for biopsy guidance of thoracic lesions that are small, subpleural, ill-defined, or juxta vascular. Biopsies of the pelvis, retroperitoneum, adrenals, pancreas, and small orbital masses are often preferentially performed with CT rather than ultrasound guidance.

Direct injection of contrast media into a popliteal lymph node is an easy technique for identification of thoracic ducts, and this technique can be applied to the diagnosis of diseases associated with chest lymphatic drainage. Single and dual phase computed tomographic angiography has been used to evaluate the portal vein, its tributaries, and intrahepatic branches. CT angiography is a fast, minimally invasive procedure that will image all portal tributaries and branches as they fill with contrast medium during a single peripheral venous injection. CT angiography is able to diagnose single and multiple portosystemic shunts, located both within and outside of the liver. Trans-splenic CT portography can also be performed. In one study, the information gained from CT portography resulted in a decrease in surgical time necessary compared with similar surgeries performed without angiographic information.

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