Nuclear Medicine Imaging in Animals
Nuclear Scintigraphy, Positron Emission Tomography
Although around for >60 years, nuclear scintigraphy is still relatively unused in veterinary medicine because it uses radionuclides, which are expensive and heavily regulated. In addition, the images derived from the studies are physiologic in nature and therefore quite unfamiliar to most veterinarians. This is unfortunate, because nuclear scintigraphy provides information on pathologic and physiologic processes that cannot otherwise be obtained.
Diagnostic nuclear medicine imaging involves dosing the patient with a very small amount of a gamma ray–emitting radioisotope. The location and distribution of the radioisotope within the body is then detected with a gamma camera, a device specifically designed to collimate and detect gamma rays. 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 or may not 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 and stored in a digital file as a permanent record. Most modern systems 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 isotope content and rate of accumulation 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 and allows disease detection earlier and more readily than anatomic imaging systems. Advanced MRI studies can emulate this functional aspect of scintigraphic imaging in some cases, but those systems are much more limited in scope and availability, as well as costing an order of magnitude more.
Single photon emission computed tomography (SPECT) and positron emission tomography (PET) are advanced scintigraphic imaging techniques widely used in human medicine for detection and evaluation of many diseases. In both of these techniques, a CT-like cross-sectional image based on the deposition of radionuclides within the body is generated. Such images have greater sensitivity than planar images and improved specificity as well.
PET imaging in particular has seen tremendous growth in the past decade and is routinely used in the staging and evaluation of many diseases, especially cancer and some neurodegenerative disorders. This technology, based on the use of positron-emitting isotopes of lighter elements such as oxygen, nitrogen, carbon, and fluorine that can be inserted into compounds normally metabolized by the body, can evaluate the metabolism and localization of these elements with great sensitivity. Because of the unique properties of positrons, the location at which these particles are deposited in the body can be determined within a couple millimeters, even in very large patients.
PET imaging is available at most academic centers, and its use exceeds that of traditional nuclear scintigraphy in some centers. These instruments are extremely sensitive and can often define the presence of or characterize the extent of some disease processes long before they can be evaluated by anatomic imaging systems such as MRI or CT. When these images are combined with CT or MRI images, tremendous sensitivity for the detection of numerous diseases results. PET/CT and PET/MRI scanners are hybrid imaging systems that combine the anatomic sensitivity of CT with the physiologic sensitivity of PET to provide localization of foci of disease that could easily be missed using either instrument alone. These hybrid images are now considered standard of care in the management of many cancerous and metabolic diseases in humans.
The major issue with using nuclear medicine imaging in veterinary medicine is not the availability of gamma cameras or the technical expertise required to operate them. Cameras are readily available on the used market, and training of technologists to operate them is not prohibitively complex. Rather, it is the regulations surrounding the acquisition and use of radiopharmaceuticals in animals. All use must be strictly documented and, unlike in human medicine, the veterinary patient generally must remain in the hospital after the study is performed to allow any elimination of radionuclides from the body to be essentially complete. This is done in order to limit exposure of owners to the radionuclides. A second reason for limited use is the physiologic nature of the lesions, which results in images of poor spatial resolution even though they are exquisitely sensitive to some disease processes. The special training required for interpretation of these images is provided as part of a veterinary radiology residency program available at only a few centers.