We retrospectively reviewed the MRI images of patients with anatomopathological diagnosis of hepatocellular adenomas . According to recent studies, HCAs can be classificated in different subtypes depending on the different genetic and histophatological findings, and are currently categorized into three distinct subtypes: inflammatory hepatocellular adenomas, hepatocyte nuclear factor 1 alpha (HNF-1a)-mutated hepatocellular adenomas, and β-catenin-mutated hepatocellular adenoma . These subtypes show variable imaging findings in MRI. The imaging findings of these subtypes have been described in figure 1.
Inflammatory hepatocellular adenomas
Inflammatory hepatocellular adenoma is the most common subtype (around 30-50% of all HCAs). These lesions are mainly seen in young women with history of contraceptive use.
Inflammatory HCAs are associated with a increased risk of bleeding (around 30% of all inflammatory HCAs) and a lower risk of malignant transformation (5%-10%).
At MR imaging, inflammatory HCAs are strongly and diffusely hyperintense on T2-weighted sequences with a higher signal intensity in the periphery of the mass, relating dilated sinusoids (atoll sign). On T1-weighted sequences inflammatory hepatocellular adenomas are iso- or mildly hyperintense with minimal or no signal drop-off on chemical shift sequences. Most of them show diffusion restriction on DWI. After administration of gadolinium-based constrast, inflammatory hepatocellular adenomas may appear as a hypervascular lesion with persistent enhancement during dynamic evaluation and may show a variable uptake in the hepatobiliary phase ( 25%-46% of cases show hyper-isointensity on hepatobiliary phase). Prominent T2 hyperintensity associated with persistent enhancement in delayed phases has a sensitivity of around 85% and a specificity of 87% for the diagnosis of inflammatory hepatic adenomas ( Figure 2,3, 4).
HNF-1a-mutated hepatocellular adenomas
HNF-1a-mutated HCA is the second most common subtype and they suposse about 30-35% of all hepatocellular adenomas. Inactivating hepatocyte nuclear factor 1a is the responsible for the development of HNF-1a-mutated hepatocellular adenomas, and sometimes they can be associated with diabetes and familial hepatic adenomatosis. These HCA´s are nearly exclusively seen in female patients, most frequently with a history of oral contraceptive use.
HNF-1a-mutated HCA is the least aggresive subtype of all hepatocellular adenomas. Small tumors (< 5 cm) show minimal risk of bleeding and no risk for malignant transformation.
At MR imaging, HNF-1a-mutated HCA often shows a significant and diffuse signal dropout on chemical shift T1-weighted images corresponding to fatty deposition. Iso- or hyperintensity on T1-weighted sequences may correspond to glycogen component or less commonly with haemorrhage. On T2-weighted images the mass often appear iso-or slightly hyperintense without significant restriction on DWI. But some complicated adenomas or adenomas containing different tissues can show restriction. After administration of gadolinium-based constrast, HNF-1a-mutated hepatocellular adenomas usually show moderate enhancement in arterial phase with rapid wash-out on portal and delayed phases and may show a hypointense signal intensity on hepatobiliary phase. A homogeneous signal drop-off on chemical shift sequences has a sensitivity of around 86% and a specificity of 100% for the diagnosis of HNF-1a-mutated HCA .
β-catenin-mutated hepatocellular adenoma
β-catenin-mutated HCA constitute around 10%-15% of all hepatocellular adenomas. β-catenin gene mutation´s are the responsible for the development of these HCA´s subtype. They appear more commonly in men and can be associated with glycogen storage disease, male hormone administration and familial adenomatosis polyposis.
β-catenin-mutated HCA is associated with a increased risk of malignant transformation in hepatocellular carcinoma. β-Catenin-mutated HCA can be interpreted by MRI as boderline lesions between hepatocellular adenoma and hepatocelullar carcinoma (Figure 9, 10) .
At MR imaging β-catenin-mutated HCA show a variable and no specific MR patterns and can mimic hepatocelullar carcinoma. Most often they show heterogeneous intensity signal in T1- and T2- weighted sequences. After administration of gadolinium-based constrast, these HCA´s subtype may appears as a hypervascular mass on arterial phase with persistent or non-persistent enhancement in portal and delayed phases and may show a hypointense signal intensity on hepatobiliary phase (Figure 5, 6 ).
Unclassified hepatocellular adenoma
About 10% of all hepatocellular adenomas are without specific pathologic and/or genetic abnormalities. Sommetimes, the presence of haemorrhage can be one of the reasons that justify the unclassified categorization of the mass. Because of imaging experience is very limited to identify unclassified HCA´s, no specific MR imaging findings have been described yet ( Figure 7, 8) .