Radionuclide imaging of nonosseous infection

Nuclear medicine is an important tool in the diagnostic evaluation of patie nts with a variety of nonosseous infections. In the immunocompetent populat ion labeled leukocyte imaging is the radionuclide procedure of choice, with Gallium imaging reserved for those situations in which the leukocyte study is nondiagnostic or cannot be performed. Fever of unknown origin is caused by infection in less than one-third of cases, and therefore die number of positive leukocyte studies will be relatively low. The negative leukocyte s tudy is also useful, however, as it has been demonstrated that a negative s tudy excludes, with a high degree of certainty, focal infection as the caus e of an FUO, In the cardiovascular system, labeled leukocyte scintigraphy i s very useful for diagnosing mycotic aneurysms and infected prosthetic vasc ular grafts, with a sensitivity of about 90%. The specificity of the study is somewhat more variable - false positive results have been described in p erigraft hematomas, graft thrombosis, bleeding, and pseudoaneurysms. In the central nervous system labeled leukocyte imaging can provide important inf ormation about the etiology of contrast enhancing brain lesions identified on computed tomography, i.e., distinguishing been een neoplasm and infectio n. In the immunocompromised population, typified by the AIDS patient, Galli um scintigraphy is the radionuclide procedure of choice for diagnosing oppo rtunistic diseases. In the thorax, a normal Gallium scan, in the setting of a negative chest X-ray, virtually excludes pulmonary disease. A negative G allium scan in a patient with an abnormal chest X-ray and Kaposi's sarcoma study suggests that the patient's respiratory problems are related to Kapos i's sarcoma Focal pulmonary parenchymal uptake is most often associated wit h bacterial pneumonia, although Pneumocystis carinii pneumonia can occasion ally present in this fashion. Diffuse pulmonary parenchymal uptake of Galli um call be due to numerous causes,but in general, the more intense the upta ke, the greater the likelihood that the patient has P, cariniii pneumonia. Lymph node uptake is most often due to lymphoma or mycobacterial disease. I n the abdomen, Gallium Is also useful for detecting nodal disease, but is n ot reliable for detecting large bowel disease. Labeled leukocyte imaging sh ould be performed when colitis is a concern. Both (18)FDG PET and Tl-201 SP ECT imaging of the brain are useful for distinguishing between central nerv ous system lymphoma and toxoplasmosis in the HIV (+) patient. On both studi es, lymphoma manifests as a focus of increased tracer uptake, whereas toxop lasmosis shows Little or no uptake of either tracer.