X-rays are a form of electromagnetic radiation outside of the visible light spectrum. A radiograph is produced when X-rays pass through a body part and onto an X-ray detector. Since X-rays are absorbed in varying degrees depending on the radiological density of different tissues, a two-dimensional picture of the body part is created. For example, because bone and metal are radiologically dense, they absorb most of the X-rays and appear bright white on radiographs. Soft tissue structures, such as organs and blood vessels, absorb only some of the X-rays and appear in varying shades of gray.
Radiographs originally were projected onto film, but digital technology revolutionized the process. Digital technology improves image detail, allows for image manipulation, and markedly increases the speed and efficiency of producing an image. In canine oncology, radiographs are particularly useful for evaluating boney abnormalities, and scanning for and monitoring internal masses.
Computed tomography, or CT scan, uses a motorized X-ray source that rotates around the patient. Narrow beams of X-rays pass through the patient and are digitally projected onto a screen. For each pass around the patient, sophisticated computer calculations create a two-dimensional image, or “slice.” These images can be viewed individually, or stacked together to create a three-dimensional image of the patient. CT provides more detailed information than conventional X-rays making it useful in areas of complex anatomy such as the brain, and it is the preferred modality for bone imaging. In veterinary oncology, CT is useful for imaging nasal, brain, and lung masses. A CT scan is required to determine the exact size and location of a tumor before radiation therapy.
PET stands for positron emission tomography and a PET scan or PET/CT is a form of nuclear medicine which uses radioactive tracers to diagnose and treat disease. An injection of radioactive material is given and then a CT scan is performed to show where the tracer has localized. PET scans allow us to visualize cancer as a functional change rather than waiting for the distortion of anatomy. Since rapidly growing cancer cells use more glucose than normal cells, the most common tracer used in canine oncology is a form of glucose called FDG (fluorodeoxyglucose) which is rapidly concentrated in the tumor tissue.
Instead of using radiation, magnetic resonance imaging uses strong magnets and a radiofrequency current to disrupt protons in tissue. Sensors detect changes in these protons to create a three-dimensional picture of the tissues. In veterinary medicine, general anesthesia is required since the patient must remain still inside the machine. However, work is ongoing to demonstrate that dogs can be trained to remain still and awake in the tube.1 This allows for the evaluation of functional MRI images used to evaluate which areas of the brain activate with various cognitive tasks. MRI is best suited to evaluate the soft tissue structures of the body. It provides highly detailed information on the brain and spinal cord.
Diagnostic ultrasound uses sound waves emitted from and detected by a transducer, a device that converts one form of energy to another. When an electric field is applied to ceramic crystals inside the transducer, sound waves are created. These sound waves enter the body and bounce off boundaries between tissues in the body (e.g., the boundary between fluid and tissue). The reflected sound waves then are detected by the transducer and converted back into an electric signal. Using the speed of sound and the time of each echo’s return, a two-dimensional image is created on a computer screen. Diagnostic ultrasound frequently is used to evaluate the canine abdominal cavity. The size, shape and internal architecture of organs and tumors are readily visualized. Ultrasound also can guide the aspiration or biopsy of internal tissues. Since sound waves do not travel well through air or bone, the lungs and bones are not easily visualized with ultrasonography. However, proper positioning and/or the presence of abnormalities can allow for evaluation of masses within the chest cavity.
(Information in this article was sourced from the National Institute of Biomedical Imaging and Bioengineering, National Institutes of Health, www.nibib.nih.gov.)
(1) Berns GS, Brooks AM, Spivak M (2012) Functional MRI in Awake Unrestrained Dogs. PLoS ONE 7(5): e38027.