Figure 1: Axial VIBE image demonstrates a low signal intensity mass in the right lobe of the liver.

Figure 2: Axial T2-weighted fat saturated image demonstrates increased signal intensity in the mass in the right lobe of the liver.

Figure 3: Axial post-contrast 2D VIBE image obtained during the arterial phase demonstrates intense homogeneous enhancement of the entire lesion.

Figure 4: Axial post-contrast 2D VIBE image obtained during the delayed phase demonstrates that the lesion becomes less intense relative to the surrounding liver parenchyma, and the central scar has enhanced.

Focal Nodular Hyperplasia (FNH)

FNH is defined as a nodule composed of benign-appearing hepatocytes occurring in a liver that is otherwise histologically normal or nearly normal. FNH is the second most common benign liver tumor after hemangioma and has a reported prevalence of 0.9%. The male-to-female ratio is 1:8, and the tumors occur in relatively young patients. Approximately 20% of the patients have multiple FNH lesions. The combination of multiple FNH lesions and hemangiomas is considered to be multiple FNH syndrome. The pathogenesis of this lesion is not well understood. Vascular malformation and vascular injury have been suggested as possible underlying mechanisms. Recent studies have demonstrated that there is no association between steroids and development of FNH.

Currently, FNH is divided into two types: classic and nonclassic. The nonclassic type contains three subtypes: (a) telangiectatic FNH, (b) FNH with cytologic atypia, and (c) mixed hyperplastic and adenomatous FNH.

Typical FNH is often not well visualized on ultrasound ( US ). There may be only a subtle change in echogenicity compared with the surrounding normal liver parenchyma. The conspicuity of the lesions at US may improve with a relatively large or prominent central scar. The lesions may be slightly hypoechoic, isoechoic, or slightly hyperechoic. Some lesions may show a hypoechoic halo surrounding the lesion. This halo most likely represents compressed hepatic parenchyma or vessels surrounding the lesion. The halo may be more prominent around FNH with fatty infiltration that is located within a liver with steatosis as well. In such cases, the compressed liver parenchyma surrounding the lesion is devoid of fat and has relatively low echogenicity compared with the liver and the lesion. The outer contours of the lesions may be well defined, although the internal structure of FNH, including the central scar, is often not well visualized . Use of color and power Doppler US may add information concerning the vascularity of the suspected FNH. Despite these possibilities, US is currently not considered the modality of choice for characterization of focal liver lesions.

Typical FNH may have lobulated contours at CT. On unenhanced CT, the lesions are either hypoattenuating or isoattenuating to the surrounding liver. In the arterial phase, the lesions become hyperattenuating due to the homogeneous intense enhancement of the entire lesion, except the central scar. In the portal and later phases, the lesions become more isoattenuating with the surrounding liver and the central scar may show some enhancement .

MR imaging has higher sensitivity (70%) and specificity (98%) for FNH than US and CT. The central scar is more often detected with MR imaging than with CT (78% and 60%, respectively). The higher sensitivity and specificity of MR imaging may be due to the fact that state-of-the-art MR imaging provides information concerning the soft-tissue characteristics as well as the vascularity of the lesions. Typically, FNH is iso- or hypointense on T1-weighted images (94%–100%), is slightly hyper- or isointense on T2-weighted images (94%– 100%), and has a hyperintense central scar on T2-weighted images (84%). FNH shows intense homogeneous enhancement in the arterial phase and enhancement of the central scar in the later phases of gadolinium-enhanced imaging . Hepatocellular adenomas show less intense enhancement and lack a central scar.

The amount of scar tissue within FNH and the size of the central scar may vary. The central scar is typically high in signal intensity on T2-weighted images and low in signal intensity on T1-weighted images. It shows visible enhancement on delayed contrast-enhanced images.

Multiple FNH lesions occur in approximately 20%–25% of patients with FNH . Multiple FNH lesions are more often of the non-classic type and may contain atypical features at imaging. In addition, multiple FNH lesions can be associated with other benign lesions, such as cysts, hemangiomas, and adenomas. Distinction from multiple adenomas and multifocal HCC, particularly in noncirrhotic livers, may be challenging.

References:

1. Basaran C, Karcaaltincaba M, Akata D, et al. Fat-Containing Lesions of the Liver: Cross-sectional Imaging Findings with Emphasis on MRI. American Journal of Roentgenology . 2005 Apr; 184(4): 1103-10.

2. Hussain SM, Terkivatan T, Zondervan PE, et al. Focal Nodular Hyperplasia: Findings at State-of-the-Art MR Imaging , US , CT, and Pathologic analysis. Radiographics. 2004 Jan-Feb; 24(1): 3-17.