In computed tomography (CT), an x-ray tube moves around the body and continuously projects a thin fan of x-rays through the body. Electronic detectors opposite the tube continuously monitor the number of x-rays passing through the body and the angle at which the beam is being projected. The number of x-rays reaching the detector changes as the beam passes through different tissues because of the tube movement. A computer mathematically evaluates the data and determines the most probable density of any point within the volume of tissue scanned. This density is then displayed on a monitor. Together, all of the densities make an image of the cross-section of the portion of the body through which the beam passed. This tomographic image is usually referred to as a slice, and each of the individual attenuation points in the image is referred to as a voxel (volume element). The animal is then moved a few millimeters and the process repeated. By sequentially scanning a body area, the entire volume of interest can be imaged without any superimposition of structures. CT also has much better contrast discrimination than standard radiographs, so structures such as individual parts of the brain or individual muscle bellies are seen as separate and distinct on the CT scan. X-ray contrast media is frequently used to further enhance the contrast between structures and help characterize lesions.
Modern multi-slice CT scanners can acquire up to 128 cross-sectional images per rotation; each rotation may be as short as ½ sec. These systems are capable of continuous rotation (helical or spiral scanning) and can perform a complete scan of the abdomen or thorax in a human on a single breath hold (∼10 sec). The image reconstruction time is correspondingly short, and the entire study can be completed in less time than was required to acquire a single image 15 years ago. Veterinary practices now have 16-slice scanners that can complete a scan in less time than it takes to position the patient on the scanning table. Even with such extra-ordinarily fast systems, veterinary patients must still be anesthetized and immobilized to perform the studies, but the period of anesthesia is short and the value of the information derived is great. Modern reconstruction algorithms also allow 3-dimensional reconstruction of structures with a given density. Bones can be depicted without the overlying soft tissues and vascular structures that have been contrast enhanced can be depicted without any overlying tissues. The newest scanners can produce images of vessels that rival those obtained by conventional contrast angiography.
In addition to some of the imaging procedures that are unique to CT, this modality is replacing conventional radiography for evaluation of some structures and disease traditionally assessed by radiography. CT scans of the skull in any species are far more informative and diagnostic than any conventional radiograph. The complex anatomy of the skull results in a pattern of overlying structures on a radiograph; however, this is vastly simplified on a CT scan, making the diagnosis much more specific and accurate. CT scans of the feet and legs of horses can detect structural changes in bones and hoof structures that are not readily evident on even detailed radiographic images of the same body part.
CT is also rapidly replacing myelography for evaluation of spinal cord disease because of its greater safety and speed and because it is able to directly image the disks and vertebrae. In addition, CT is being used as a screening procedure to evaluate the lungs and other organs for evidence of meta-stasis in cancer patients. Metastatic lesions in the lungs are far more evident on CT scans than on radiographs. Unfortunately, there are still no scanners capable of scanning the abdomen or thorax of adult horses. Were such systems available, it is likely that CT would rapidly become the imaging modality of choice for these areas in horses. In years to come, CT scanners may also be used to evaluate the heart with a speed and accuracy that rivals echocardiography, as has already occurred in human medicine.
CT is useful to guide the acquisition of biopsies and aspiration of samples from areas of the body (eg, lungs and brain), that cannot be approached using ultrasound or other imaging modalities. The same approach may be used to perform image-guided therapy (eg, radiofrequency ablation of hepatic tumor nodules).
CT scans can detect structural changes deep within the body, including tumors, abscesses, vascular abnormalities, occult fractures, and hematomas. The radiologist must have a firm knowledge of anatomy and be able to ascertain the identity of structures in any plane through the body. Knowledge of physiology and artifacts are also important when evaluating CT scans.
Last full review/revision March 2012 by Jimmy C. Lattimer, DVM, MS, DACVR, DACVRO