If a radioactive colloidal preparation is injected intravenously, it is picked up by the reticuloendothelial system. The Kupffer cells of the liver take up most of the radioactive material in normal circumstances, with a small amount being deposited in the spleen and bone marrow. If a sensitive radioactive counting device is placed over the liver, a two-dimensional image or map can be obtained of the distribution of radioactivity. A similar procedure can be done with thyroid and kidney using radioactive material that these organs normally take up (e.g., iodine in the case of the thyroid). Certain diseases may be suggested on liver scan if the proper circumstances are present:

1. Space-occupying lesions, such as tumor or abscess, are often visualized as discrete filling defects if they are more than 2 cm in diameter.
2. Cirrhosis typically has a diffusely nonuniform appearance accompanied by splenomegaly, but the cirrhotic process usually must be well established before scan abnormality (other than hepatomegaly) is seen. The most typical picture is obtained in far-advanced cases, but the scan appearance may differ somewhat even in these patients.
3. Fatty liver has an isotope distribution like that of cirrhosis, but only if severe.
4. Liver scanning may be useful to differentiate abdominal masses from an enlarged liver.

Undoubtedly, more sensitive equipment will become available and, perhaps, better radioactive isotopes. At present, useful as the liver scan may be, it is often difficult to distinguish among cirrhosis, fatty liver, and disseminated metastatic carcinoma with nodules less than 2 cm in diameter. Liver scan is reported to detect metastatic carcinoma in 80%-85% of patients tested (literature range, 57%-97%) and to suggest a false positive diagnosis in 5%-10% of patients without cancer. The majority of these false positive studies are in patients with cirrhosis, hepatic cysts, hemangiomas, or a prominent porta hepatis.