Infection, Inflammation and Musculoskeletal

While relatively simple techniques (such as three-phase bone scintigraphy for suspected osteomyelitis of the extremities) rely on indirect scintigraphic visualization of the pathophysiologic events associated with the process (such as local hyperperfusion, edema, osteoblastic reaction), other techniques employ radiopharmaceuticals that accumulate “specifically” at the site of infection. Perhaps the oldest tracer of this category is ⁶⁷Ga-citrate, an agent that targets transferrin and lactoferrin receptors (that are abundantly expressed on the cell surfaces of granulocytes and of other inflammatory cells). The most accurate radionuclide infection-imaging is based on direct labelling of leukocytes (mostly granulocytes), which is achieved either by systemic administration of certain radiopharmaceuticals (such as anti-granulocyte antibodies) or by isolation of autologous leukocytes from the patient’s own peripheral blood, having been labelled through in-vitro incubation with adequate radiotracers (such as, e.g., ¹¹¹In-oxine or ^99mTc-HMPAO), followed by re-infusion into the patient. More recent developments rely on PET with [¹⁸F]FDG, based on increased (though nonspecific) accumulation at sites of abnormally increased metabolism (such as tumours or infection) or areas of abnormally decreased metabolism (e.g. decreased marrow activity in the distribution of prior radiation therapy). Also, in the case of infection imaging, the highest clinical benefit is achieved by relying on tomographic imaging (SPECT or PET), preferably with hybrid image fusion analysis (SPECT/CT or PET/CT) for anatomic localization and attenuation correction.