Benign Hepatocellular Nodules: Hepatobiliary Phase of Gadoxetic Acid-enhanced MR Imaging Based on Molecular Background

Gadoxetic acid is a contrast agent for magnetic resonance (MR) imaging with hepatocyte-specific properties and is becoming increasingly important in detection and characterization of hepatocellular carcinoma and benign hepatocellular nodules, including focal nodular hyperplasia (FNH), nodular regenerative hyperplasia (NRH), hepatocellular adenoma (HCA), and dysplastic nodule. In these hepatocellular nodules, a positive correlation between the grade of membranous uptake transporter organic anion-transporting polypeptide (OATP) 1B3 expression and signal intensity in the hepatobiliary (HB) phase has been verified. In addition, it has been clarified that OATP1B3 expression is regulated by activation of beta-catenin and/or hepatocyte nuclear factor 4 alpha. On the other hand, recent studies have also revealed some of the background molecular mechanisms of benign hepatocellular nodules. FNH commonly shows iso- or hyperintensity in the HB phase with equal or stronger OATP1B3 expression, with map-like distribution of glutamine synthetase (a target of Wnt/beta-catenin signaling) and OATP1B3 expression. NRH shows doughnut-like enhancement with hypointensity in the central portion in the HB phase with OATP1B3 expression. The majority of HCAs show hypointensity in the HB phase, but beta-catenin-activated HCA exclusively demonstrates iso- or hyperintensity with increased expression of nuclear beta-catenin, glutamine synthetase, and OATP1B3. Dysplastic nodule commonly shows iso- or hyperintensity in the HB phase with similar to increased OATP1B3 expression, but one-third of high-grade dysplastic nodules can be demonstrated as a hypointense nodule with decreased OATP1B3 expression. Knowledge of these background molecular mechanisms of gadoxetic acid-enhanced MR imaging is important not only for precise imaging diagnosis but also understanding of the pathogenesis of benign hepatocellular nodules. (C) RSNA, 2016