Nuclear Medicine Imaging
Positron emission tomography (PET) is a nuclear medicine imaging technique that exploits the unique decay physics of positron-emitting radionuclides (Sidebar 2.9) and produces a three-dimensional image of radionuclide distribution. For example, the radiopharmaceutical fluorine-18-fluorodeoxyglucose (FDG) is a form of sugar labeled with a radionuclide [fluorine-18] that is imaged using PET. This imaging technique, which is commonly known as FDG-PET, detects differences between cancer and normal cells in the consumption of glucose. Cancer cells, particularly those from aggressive tumors, proliferate more rapidly than normal cells and consume considerably larger amounts of glucose. Not only can tumor sites be pinpointed through the detection of increased FDG consumption, but differences in FDG consumption in tissues can be detected. However, FDG may be taken up by other lesions, such as infectious foci, and not just tumors, so the diagnostic specificity of FDG-PET is limited.
In the future, the network of cyclotron/radiopharmacies that are now focused exclusively on making FDG are well positioned to provide distribution of other fluorine-18-labeled radiopharmaceuticals to regional hospitals as these are developed and approved for clinical use. In addition, development and regional deployment of lower cost radionuclide-producing machines may make other radiopharmaceuticals based on radionuclides with shorter half-lives such as carbon-11 more widely available.
Single photon emission computed tomography (SPECT) is another common nuclear medicine imaging device. SPECT uses gamma cameras to obtain three-dimensional images. To acquire SPECT images, the gamma camera is rotated around the patient and multiple images from multiple angles are obtained. A computer can then reconstruct the images. Radiopharmaceuticals used for SPECT are labeled with gamma-emitting radionuclides such as technetium-99m, iodine-123, and thallium-201. SPECT is used extensively to study cardiac health (e.g., blood flow to the heart through myocardial perfusion imaging) and to image blood flow to the brain.
PET and SPECT each have distinct advantages and disadvantages that make them useful for detecting certain conditions. Each technique uses different properties of radioactive elements in creating an image. For example, one of the advantages of SPECT compared with PET is that more than one radiotracer can be used at a time. In addition, the longer half-life of radionuclides used with SPECT makes this imaging procedure more readily available to the medical community at large. However, PET images have higher sensitivity than SPECT images by a factor of 2 to 3 and use radiopharmaceuticals that provide more physiological information.