One application of the unique capability of magnetic resonance (MR) imaging for characterizing soft tissues is in the specific detection of lipid. Adipose tissue may be abundant in the body, but its presence in a lesion can greatly limit differential diagnostic considerations. This article reviews MR imaging fat detection techniques and discusses lesions in the abdomen and pelvis that can be readily diagnosed by using these techniques. Traditional fat detection methods include inversion-recovery and chemically selective fat-suppression pulse sequences, with the former being less sensitive to field heterogeneity and less tissue specific than the latter. Chemical shift-based sequences, which exploit the inherent resonance frequency difference between lipid and water to depict intracytoplasmic fat, have great utility for evaluating hepatic steatosis and lesions such as adrenal and hepatic adenomas, hepatocellular carcinoma, focal lipomatosis of the pancreas, and adrenal cortical carcinoma. The signal from large amounts of fat can be suppressed by using a narrow radiofrequency pulse for selective excitation of fat protons (ie, fat saturation imaging), a technique that increases image contrast resolution and highlights lesions such as contrast-enhancing tissue, edema, and blood products. This technique is especially useful for evaluating renal angiomyolipomas, adrenal myelolipomas, ovarian teratomas, and liposarcomas. MR spectroscopy is a promising method for quantifying absolute liver fat concentration and changes in hepatic triglyceride content during treatment. New and evolving techniques include magnetization transfer and modified Dixon sequences. A solid understanding of these techniques will help improve the interpretation of abdominal and pelvic imaging studies.
(©)RSNA, 2013.