Nuclear medicine imaging involves dosing the patient with a very small amount of a gamma ray-emitting radioisotope. The material is then detected within the body with a gamma camera. The isotope may be injected, ingested, or inhaled as appropriate for the study being performed. The radioisotope is usually part of a larger molecule that has a specific affinity for the tissue or organ of interest. For instance, some organic phosphonates have an affinity for bone, and isotopes bound to sulfur colloids will localize in the liver and spleen. Very few radioisotopes have direct affinity for a given tissue; iodine is the notable exception and localizes very strongly in the thyroid. Inhaled gases or aerosols localize in the airways and lungs and may be absorbed into the bloodstream. In veterinary medicine, the most commonly used isotope is metastable technetium 99 (99mTc), although radioactive iodine, indium, and thallium are also used in specific instances.
The data collected by the gamma camera can be displayed directly on a monitor or projected onto a film as a permanent record. Most modern systems also send the data to a computer system for analysis, which allows enhancement of count differences and determination of organ margins. The operator can select regions of interest to analyze for count content and for counts over time. When the study uses a radiopharmaceutical that is metabolized or has a limited residence time in an organ, organ function can be determined. These dynamic studies can be used to evaluate the function of organs such as the lungs, kidneys, and heart. Such studies may reveal abnormalities that static forms of anatomic imaging cannot detect. Functional imaging is the great strength of nuclear medicine studies and allows disease detection earlier and more readily compared with anatomic imaging systems. Advanced MRI studies can emulate this functional aspect of scintigraphic imaging, but those systems are much more limited in scope and availability.
Single photon emission computed tomography (SPECT) and positron emission tomography (PET) are advanced scintigraphic imaging techniques that are widely used in human medicine for detection and evaluation of disease. Both of these techniques yield a CT-like cross-sectional image based on the deposition of radionuclides within the body. Such images have greater sensitivity than planar images and improved specificity as well. PET imaging is routinely used in the staging and evaluation of many diseases, especially cancer. This technology, which is based on the use of positron-emitting isotopes of lighter elements such as oxygen, nitrogen, carbon, and fluorine, can evaluate the metabolism and localization of these elements with great sensitivity.
The major issue with using nuclear medicine imaging in veterinary medicine is the regulations surrounding the acquisition and use of radiopharmaceuticals. All use must be strictly documented and, unlike human medicine, the patient generally must remain in the hospital after the study is performed to allow the agents to be mostly cleared from the body. A second reason for limited use is the physiologic nature of the lesions which results in images of poor spatial resolution in some disorders.
Last full review/revision March 2012 by Jimmy C. Latimer, DVM, MS, DACVR, DACVRO