A Closer Look At Nuclear Medical Imaging

Author(s): 
By Molly S. Judge, DPM, CNMT, and Nellie L. Kelty, MAS, CNMT

When a patient with Charcot neuroarthropathy presents with an acutely symptomatic limb, joint or a non-healing wound, the differential diagnosis always includes infection. Bone and joint infections are naturally associated with a high degree of potential morbidity. This potential morbidity is significantly increased when there is a delay in diagnosis or when the diagnosis is missed altogether. Therefore, validating a definitive diagnosis of infection and clarifying when it involves bone is essential to providing appropriate and timely treatment. Nuclear medicine leukocyte imaging (NMLI) allows prompt confirmation of the presence of infection and identifies the location of a focus of infection when it exists.1 In addition, these studies have a predictable appearance in the face of an active Charcot arthropathy without infection. Given the limb threatening consequences of an infected Charcot joint, using NMLI allows one to differentiate between Charcot and osteomyelitis.2 Clinicians can repeat NMLI after the completion of antibiotic therapy in order to rule out the complication of indolent infection. In the absence of infection, these imaging studies provide supplemental data to support the pursuit of an alternative diagnosis in the differential list. A Brief Overview Of The History Of Nuclear Imaging Nuclear medicine imaging techniques for identifying and isolating infection have been used since the 1950s. Gallium was one of the first isotopes used to localize infections and other pathologic processes. This agent binds with transferrin, an iron bound protein found within the cytoplasm of white blood cells (WBCs). An intravenous injection of gallium citrate provides an in vivo labeling of leukocytes and bacterial organisms, which allows one to identify inflammatory processes.1 Since the introduction of gallium imaging for infection, research has brought about the development of alternate, radiolabeled WBC studies to enhance the specificity and imaging quality of these exams. In 1976, indium-oxine leukocyte Imaging (111In-Oxine-WBC imaging) came to the forefront and has since enjoyed a large degree of clinical usage.3-18 Indium will delineate leukocyte accumulation, providing a “faithful mirror” of WBC activity within 24 hours. Over time, the 111In-Oxine-WBC imaging has earned its place in the study of both acute and chronic infectious processes. Indium imaging, like gallium, suffers from inherently poor imaging characteristics as it emits dual high-energy alpha rays to produce its images. This results in poor spatial resolution and a low target to background ratio. This technique often requires a minimum of 24 hours to localize within an area of WBC accumulation. Depending on the differential diagnosis, one may obtain serial imaging at four to six hours, 24 hours, 48 hours and 72 hours. In cases of positive uptake, the region of isotope localization will become more discrete as time progresses due to the combined effect of the physical and biologic half-life of the compound. Improved localization occurs over time as a result of lowered background radiation and an improvement in target to background ratio. At the time, this was the best imaging agent available for the noninvasive investigation of infection. However, indium leukocyte imaging has remained a primary imaging agent for localization of infectious processes despite the agent’s poor imaging characteristics and low spatial resolution. Interest in nuclear medicine imaging rejuvenated in the late 1980s as a new and improved radiolabeled white cell technique came to the forefront. In 1986, technetium-99m hexamethyl propylene amine oxime (Tc-HMPAO), under the trade name Ceretec, was developed for cerebral perfusion imaging using the isotope technetium. The excellent imaging characteristics of the isotope 99mTc significantly enhanced nuclear medicine tomography in cerebral imaging. It was later discovered that the agent, HMPAO, has a high affinity and avidity in labeling to white blood cells. Technetium is a very “photogenic” isotope due to its low energy of emission and short half-life. In other words, technetium has favorable imaging characteristics, providing improved spatial resolution in imaging that is not achievable using either indium or gallium.

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