Nuclear Medicine Imaging
(Nuclear scintigraphy) |  |
| 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. 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 image data is collected by a device called a gamma camera, which detects the gamma rays emitted by the radioisotope. The data collected can be displayed directly on a monitor or projected onto a film as a permanent record. Most 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 be analyzed 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. Only advanced MRI studies can emulate this functional aspect of scintigraphic imaging, but those systems are much more limited in scope and availability. |
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Single photon emission computed tomography and positron emission tomography (PET) are advanced scintigraphic imaging techniques that are widely used in human medicine for detection and evaluation of many diseases. 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 in particular has seen tremendous growth in the last decade and is now 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. Unfortunately, the cost of the
instruments and radiopharmaceuticals and the lack of expertise in veterinary medicine to interpret the studies currently limits their use to research and rare clinical cases in teaching institutions. |
