Detection of lipid in abdominal tissues with opposed-phase gradient-echo images at 1.5 T: techniques and diagnostic importance.

T1-weighted gradient-echo magnetic resonance images can be acquired with an echo time such that water and lipid spins are in phase or opposed phase. Observation of relative loss of signal intensity on opposed-phase images compared with that on in-phase images allows qualitative assessment of relatively small amounts of lipid in tissues. Conversely, frequency-selective fat saturation techniques are useful primarily for identifying predominantly fatty masses such as angiomyolipomas. Both in-phase and opposed-phase images should be acquired with similar parameters because unequivocal identification of lipid requires comparison with in-phase images to control for T1 and T2* effects. Opposed-phase imaging has been used to differentiate adrenal adenomas, which contain lipid, from adrenal metastases, which do not. The technique can be expanded to examine a spectrum of intraabdominal tumors and conditions that are characterized by intracellular lipid. These include hepatic steatosis, hepatocellular neoplasms, myelolipoma, adrenocortical carcinoma, angiomyolipoma, and renal cell carcinoma. In liver masses, the presence of lipid is largely restricted to primary hepatocellular tumors. Renal and adrenal masses may contain focal fat (angiomyolipomas and myelolipomas, respectively) or diffuse lipid (clear cell renal carcinomas and adenomas, respectively).