Opportunistic infection and nuclear medicine

Immunosuppression as a result of T- or B-cell dysfunction can be associated with a variety of illnesses as a result of the underlying disease or treatment causing the dysfunction, infection or, indeed, therapy. Immunodeficiency may be primary or secondary. Secondary causes of immune deficiency are more common and more frequently encountered during imaging. Immune deficiency can arise in patients with conditions such as leukemia and lymphoma; from infectious agents such as the human immunodeficiency virus (HIV); from the administration of drugs, including chemotherapy agents and steroids; and as a result of metabolic diseases such as renal failure and diabetes. A condition that often presents challenges in the interpretation of abnormal uptake within radionuclide imaging is the patient with HIV infection. This has been compounded in some ways by the introduction of highly active antiretroviral therapy and the advent of the immune reconstitution inflammatory syndrome. Imaging abnormalities are found in association with the underlying disease, eg, lymphoma, HIV which, on occasion, may be difficult to separate from an opportunistic infection. The primary value of radionuclide imaging and in particular (18)F-fluorodeoxyglucose-positron emission tomography is to rapidly establish the probable site of disease to direct biopsy or aspiration so that the underlying pathology can be confirmed. The value of single-photon emission computed tomography and positron emission tomography has been enhanced by the introduction of hybrid imaging so that the computed tomography element of the scan localizes the site of disease more accurately than imaging without the computed tomography. Interest in monitoring response to treatment of infection is increasing but care has to be taken as inflammatory uptake attributable to immune reconstitution inflammatory syndrome can be similar to a worsening of infective uptake and this can lead to misinterpretation of the effect of treatment. It is important for the imager to be aware of the effects of underlying treatments on functional imaging and therefore to have a full history of the disease and the drug treatments that the patient is taking.