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 wi th an echo time such that water and lipid spins are in phase or oppose d phase. Observation of relative loss of signal intensity on opposed-p hase images compared with that on in-phase images allows qualitative a ssessment of relatively small amounts of lipid in tissues, Conversely, frequency-selective fat saturation techniques are useful primarily fo r identifying predominantly fatty masses such as angiomyolipomas, Both in-phase and opposed-phase images should be acquired with similar par ameters because unequivocal identification of lipid requires compariso n with in-phase images to control for T1 and T2 effects. Opposed-phas e imaging has been used to differentiate adrenal adenomas, which conta in 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 carci noma, angiomyolipoma, and renal cell carcinoma. In liver masses, the p resence of lipid is largely restricted to primary hepatocellular tumor s. Renal and adrenal masses may contain focal fat (angiomyolipomas and myelolipomas, respectively) or diffuse lipid (clear cell renal carcin omas and adenomas, respectively).