Liver Scintigraphy
Liver scintigraphy is the only imaging modality which shows liver function. Although it has a high sensitivity and is useful in the diagnosis of diseases such as cirrhosis, hepatitis and metabolic disorders as well as detecting liver lesions, it is non-specific. Technetium-99m (^99mTc) sulphur or tin colloid and ^99mTc albumin colloid are the most commonly used radiopharmaceuticals, but other imaging agents can also be used such as ^99mTc labeled red blood cells, and Gallium-67 (⁶⁷Ga) citrate to raise the specificity. Liver scintigraphy has largely fallen away because ultrasound and CT have greater specificity.

HIV/AIDS

Lung Complications
Several radiopharmaceuticals have been used to demonstrate pneumocystis carinii pneumonia (PCP) ( 2). These include ⁶⁷Ga citrate, Indium-111 (¹¹¹In) labeled non-specific polyclonal IgG ( 1), and ^99mTc labeled PCP specific monoclonal IgG ( 3 ). ⁶⁷Ga citrate scans have been abnormal in 85-95% of the cases ( 1). A normal chest xray and normal ⁶⁷Ga scan excludes the diagnosis of PCP (1). Where the chest xray is normal and the ⁶⁷Ga scan abnormal the specificity is increased to 100% ( 3). Classical findings on a ⁶⁷Ga scan show diffuse heterogeneous pulmonary uptake that is greater than liver uptake ( 1). However there are other conditions that can result in diffuse lung uptake of varying intensities. This reduces the specificity to 50% ( 1). ¹¹¹In-oxine labeled leukocytes lung scans for PCP have a sensitivity of 40%. They are more sensitive for bacterial pneumonias that are also positive on chest xray. Labeling HIV patients' leukocytes produce certain problems. Firstly these patients have low white cell counts which make labeling difficult and secondly autologous labeling techniques involve venepuncture which carries the risk of needle stick injury to the health care worker ( 1). ¹¹¹In oxine labeled polyclonal IgG has a low specificity for PCP because the label is non-specific ( 3). ^99mTc labeled monoclonal IgG is more specific and has a sensitivity of 86% and a specificity of 87% ( 3). ^99mTc DTPA aerosols are a sensitive marker in early PCP infection when the chest xray may be normal or equivocal but there is a high index of suspicion for PCP. Unfortunately the technique is not specific as other lung infections can show the same pattern on scan. Alveolar clearance of the aerosol can be measured. In normal individuals there is a slow monoexponential clearance. Clearance is rapid and biphasic in severe alveolar damage as seen in PCP and legionella pneumonia ( 3).

⁶⁷Ga has also been used to scan mycobacterium avium intracellulare (MAI). Uptake is lobar and asymmetric, but histoplasmosis will appear similar on scan ( 1). Cytomegalo virus (CMV) will have low grade uptake with perihilar prominence in the lungs and frequently associated multiple organ uptake or eye uptake due to retinitis ( 1) .

Central Nervous System (CNS) Complications
The most common CNS complications in HIV/AIDs patients are toxoplasmosis, primary CNS lymphoma, progressive multifocal leukoenchephalopathy (PML) and HIV encephalopathy ( 1,3). Nuclear medicine imaging of the CNS is limited. Infections will be seen as hypometabolic lesions in Fluorine-18 flurodeoxyglucose (¹⁸F-FDG) PET imaging and lymphomas will show as hypermetabolic lesions ( 3). There is no role for ⁶⁷Ga citrate to play in CNS imaging as it does not cross the intact blood brain barrier adequately ( 1). ¹¹¹In labeled white cells are better for diagnosing infectious lesions ( 1). HIV encephalopathy results from direct infection to the brain tissue primarily involving white matter and subcortical nucleii ( 1). ^99mTc hexamethylpropyleneamine (HMPAO) SPECT images will show typical multifocal perfusion defects affecting the frontal and temporal lobes but the pattern is similar to cocaine abuse and Altzheimers dementia ( 3). ¹⁸F-FDG PET imaging of HIV encephalopathy shows subcortical hypermetabolism in the early stages and cortical and subcortical hypometabolism in the late stage ( 1).

HIV Related Neoplasia
Kaposi' sarcoma is the most common cancer in AIDS patients ( 1). In non-AIDS patients it is an indolent tumor, localised in the skin, but in AIDS patients it is more aggressive and multicentric (1). ⁶⁷Ga scans are usually negative but Thallium-201 (²⁰¹Tl) is taken up avidly by Karposi sarcoma ( 1,3,4). ²⁰¹Tl sequential imaging can help differentiate between infection and Karposi sarcoma. Early images taken between 20 and 60 minutes will be positive for both infection and Karposi sarcoma but the delayed images at 3 hours will show clearance of ²⁰¹Tl from the infectious lesions whilst Karposi sarcoma will still be positive ( 4). ⁶⁷Ga and ²⁰¹Tl imaging are useful in differentiating between Karposi sarcoma, undifferentiated lymphoma and infection (Table 1) ( 4).

Tabla 1. Differential diagnosis using ²⁰¹Tl and ⁶⁷Ga