Many people are familiar with x-rays
through their own personal experience or through their pet having them done. Computed tomography, commonly referred to as a 'CT' or 'CAT' scan, is becoming more common also. This technology is very useful when looking at parts of the body such as the brain or spine.
Computed tomography was first used in 1971 in human medicine in England. The image produced is reconstructed from many radiographic images taken of the animal. Very fine x-ray beams are sent through the body to detectors, which then send the signals to a computer which processes the image.
The tissue images that show up on the monitor consist of white, black, and shades of gray, called the gray scale. The amount of gray shows the ability of the tissue to absorb the x-ray beam. This helps to identify tissues that are normal or abnormal, such as brain tumor versus normal brain tissue. The computer typically images the height and width of the tissue, while newer machines can give a 3-D look. Better quality of the CT image is obtained by taking thinner slices with the x-ray beams.
CT scans provide better differentiation of soft tissue than conventional x-rays, since the gray scale can be controlled by the technician. A contrast media (dye) can also be injected into the patient's bloodstream to further increase the differentiation of tissue. The CT scan also does not superimpose the bone over the soft tissue, as it is only looking at a slice of the body at a time.
Animals who are candidates for a CT scan include those with neurological disorders, such as seizures, behavior changes, or problems walking. The animal is anesthetized with a general anesthesia during the procedure, since she must remain completely still. The table on which the animal is laying is slowly advanced into the part of the machine that performs the scan (called the gantry). An x-ray tube rotates 360º around the patient to record the x-rays from many angles. The number of images taken depends on the area and size of the suspected problem. When the computer finishes processing the information, an image appears on a monitor. The image is saved for further review and to compare to later scans to determine the effectiveness of treatment.
If a tumor is found, the CT scan can pinpoint its size and location. This is necessary for the radiologist and surgeon to know, so a treatment plan can be formulated. A biopsy of a tumor can be taken using the CT scan to guide the needle. This helps in the diagnosis and treatment also.
CT scans are not always perfect. If the patient moves during the procedure, streaking of the image will occur, which severely affects the image quality. Artifacts may occur, if there are problems with the x-ray beam or detector. The doctors reading the scans need to be experienced with the images in order to correctly diagnose a condition. Some soft tissue masses cannot be seen on the CT scan. These patients may also need an MRI.