What is the role for nuclear medicine studies in evaluating hepatic mass lesions?
The traditional liver and spleen scan using an intravenous injection of 99m Tc-sulfur colloid has largely been replaced by US and dynamic multiphase CT and MRI. In addition to superior resolution with CT and MRI, adjacent structures can also be evaluated. If results are inconclusive, nuclear medicine testing can provide additional information, which can lead to the proper diagnosis.
Sulfur colloid is composed of small particles (0.3 to 1 μm) that are phagocytosed by the reticuloendothelial systems, including Kupffer cells in the liver. Lesions that lack Kupffer cells in the liver will not accumulate sulfur colloid. Virtually all neoplasms, including metastasis, focal inflammatory and infectious diseases of the liver, and vascular malformations, manifest as decreased radionuclide activity (cold) on both liver-spleen and hepatobiliary imaging. However, focal nodular hyperplasia (FNH) can demonstrate a nonspecific appearance on CT, MRI, and US. If a lesion appears isointense (warm) or hyperintense (hot) compared with the rest of the liver, it can be presumed to be FNH because no other hepatic lesion contains a sufficient number of Kupffer cells to concentrate sulfur colloid. Occasionally, FNH can appear cold if there are not enough Kupffer cells to accumulate a sufficient amount of sulfur colloid, which unfortunately does not differentiate it from other hepatic masses. Additional imaging with cholescintigraphy will demonstrate early and prolonged uptake of the radiopharmaceutical because of the presence of hepatocytes in FNH with impaired clearance of the radiopharmaceutical from these lesions.
The evaluation of hepatic lesions is limited on planar imaging to approximately 1 to 2 cm. To evaluate smaller lesions, single-photon emission computed tomography (SPECT) imaging, which is produced using rotating gamma camera heads and reconstructing the data into three dimensions, can be used in the evaluation of lesions in the subcentimeter range.
Using multiphasic imaging with CT or MRI, evaluation for hepatic hemangiomas is excellent. However, if atypical features are noted, imaging using SPECT with 99m Tc-labeled red blood cells (RBCs) can provide additional information for hemangiomas larger than 2 cm and close to the hepatic surface, frequently at lower cost and without intravenous contrast injection. Additional SPECT imaging also improves the ability to evaluate smaller hemangiomas.
