26 Giant hemangiomas may have regions of fibrosis and/or thrombosis, resulting in a central scar with strands of T2 hypointensity.26 Caution should be exercised in differentiating hemangiomas from hypervascular metastases, such as neuroendocrine find more tumors, which can be markedly T2-hyperintense and arterially enhanced.32-35 Small flash-filling hemangiomas may require MR follow-up, as differentiation from metastases can be difficult.

Metastases may demonstrate a continuous targetoid rim of enhancement compared to the discontinuous rim displayed by hemangiomas. With metastases, the arterial enhancing rim may washout, or become hypointense relative to the liver during the portal venous phase. With HSA, hemangiomas demonstrate expected enhancement during the dynamic phase images and are hypointense during the hepatocyte phase, mirroring the signal intensity of the portal veins. This imaging appearance has been referred to as “pseudo-washout.”30, 36, 37 This hypointensity during the hepatobiliary phase is expected given the lack of hepatocytes within the lesion. Although the imaging appearance on T2-weighted and dynamic postcontrast sequences should allow for accurate diagnosis, HSA may not be the best option for suspected hemangiomas. FNH, common in asymptomatic patients, pathologically consists of nonneoplastic hepatocytes in a disorganized array surrounding a central

scar with anomalous vessels. As FNH are composed of hepatocytes, they are STK38 relatively stealthy (barely discernable from normal parenchyma) on noncontrast images and show a characteristic enhancement Bcl-2 inhibitor pattern.38-43 A typical enhancement pattern with ECA is early nodular arterial enhancement, which equilibrates, or becomes isointense, with the background liver on portal venous phase images (Fig. 2). Some lesions contain a T2 hyperintense central scar. The scar may be hypointense during the arterial phase and show delayed enhancement with ECA. HSA-enhanced MRI is the study of choice for FNH. On hepatobiliary phase images, FNH are iso- or hyperintense to the background

liver, reflecting uptake of contrast by lesional hepatocytes. A multicenter study of 550 consecutive patients with FLL characterized on Multihance MRI demonstrated that 95% (289/302) of FNHs were iso- or hyperintense on hepatobiliary phase images.43 In the same study, the overall diagnostic performance of hepatobiliary MRI in differentiating benign from malignant lesions demonstrated sensitivity of 96.6%, specificity 87.6%, and positive predictive value of 85%.43 Zech et al.39 demonstrated hepatobiliary MRI with Eovist yielded confident diagnosis of FNH in 88% of patients. Graziolo et al.44 in a study of Multihance MRI in differentiating HCA from FNH found 97% sensitivity and 100% specificity in diagnosing FNH. Although HSA yields reliable results in diagnosing FNH, some caution may be warranted.