).Abbreviations FNH = focal nodular hyperplasia, FSE = fast spin echo, HCC = hepatocellular carcinoma, SI = signal intensity.
ABSTRACTThe vast majority of focal liver lesions are hyperintense on T2-weighted magnetic resonance (MR) images. Rarely, however, hepatic nodules may appear totally or partially hypointense on those images. Causes for this uncommon appearance include deposition of iron, calcium, or copper and are related to the presence of blood degradation products, macromolecules, coagulative necrosis, and other conditions. Although rare, low signal intensity relative to surrounding liver on T2-weighted images may be seen in a wide spectrum of lesions. Examples include cases of focal nodular hyperplasia, hepatocellular adenoma, hepatocellular carcinoma, metastases, leyomioma, siderotic or dysplastic nodules, nodules in Wilson disease, granuloma, and hydatid cyst. On fat-suppressed T2-weighted images, nodules with a lipomatous component, such as lipoma, angiomyolipoma, hepatocellular adenoma, and hepatocellular carcinoma may also appear partially or totally hypointense. The conjunction of other MR imaging findings and their integration in the clinical setting may allow a correct diagnosis in a considerable proportion of cases. The cause for T2-weighted hypointensity may not be, however, always recognized, and only pathologic correlation may provide the answer. The aims of this work are to discuss the causes and mechanisms of hypointensity of liver lesions on T2-weighted images and proposing an algorithm for classification that may be useful as a quick reminder for the interested reader.
INTRODUCTIONMagnetic resonance (MR) imaging of liver lesions is of pivotal importance in clinical practice. In abdominal MR imaging, T1-and T2-weighted sequences remain a basic requirement for lesion characterization. MR signal intensity (SI) of focal liver lesions is affected by numerous pathologic factors. Histologic features strongly affect T1 and T2 relaxation times and MR imaging is unique in its ability to explore intracellular content and recognize the presence of substances that may govern signal behavior. Owing to their high cellular water content, the majority of focal liver lesions appear hyperintense on T2-weighted images. In contrast to this most frequently seen MR imaging pattern, liver nodules may show total or partial hypointensity on T2-weighted images. The hypointense appearance can be classified as absolute or relative, depending on the native SI of adjacent liver parenchyma and sequence parameters (Fig 1). Although relative hypointensity depends on the native SI of liver, which can be modified by several circumstances (eg, inflammation, diffuse iron deposition, etc), it may be more commonly encountered when
