| 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 x-ray 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 body through which the beam passed, referred to as a slice. 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. |
| The first scanners were slow and cumbersome with limited resolution. However, modern multi-slice CT scanners with supercomputer capabilities can acquire up to 16 cross-sectional images per rotation; each rotation may be <1 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 (≤17 sec). The image reconstruction time is correspondingly short, and the entire
study can be completed in less time than was originally required to acquire a single image. Even with such extraordinarily 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. |
| CT scans can be used to 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 paramount in evaluating CT scans. Extensive experience and training are required to become adept at the interpretation of these images. Because
of the expense and size of the equipment, the need for specialized experience, and the number of animals for which it is indicated, CT scanners are typically limited to specialty and academic practices. |
