From the Guidelines
Focal nodular hyperplasia (FNH) is typically characterized by small, hyperenhancing, and rounded lesions on CT and MRI, which turn isodense/isointense to the surrounding parenchyma in the portal venous phase, with mild T2 iso-to hyperintensity and isointensity in the T1 signal on MRI. The imaging features of FNH can be summarized as follows:
- On CT, FNH is often identified in the arterial contrast phase as small, hyperenhancing, and rounded lesions that turn isodense to the surrounding parenchyma in the portal venous phase 1.
- On MRI, FNH-like nodules are commonly homogeneous in attenuation on all series, with mild T2 iso-to hyperintensity, isointensity to slightly diffusion, and isointensity in the T1 signal, and mild or no restricted diffusion 1.
- After injection of a hepatocyte-specific contrast agent, FNH-like nodules are typically iso-to hyperintense compared to the surrounding parenchyma in the late hepatocyte phase 1.
- The finding of washout in the delayed phase can also be seen in benign FNH-like nodules, which generally remain stable in size during follow-up and rarely disappear 1. The characteristic imaging features of FNH reflect its pathophysiology, which consists of normal hepatocytes arranged in abnormal architecture with malformed biliary structures and prominent arterial vessels. The hypervascular nature of FNH explains its enhancement pattern, while the fibrous central scar accounts for the delayed enhancement due to slow contrast accumulation in the fibrous tissue.
From the Research
MRI Findings of Focal Nodular Hyperplasia
- FNH is typically iso- or hypointense on T1-weighted images 2
- FNH is slightly hyper- or isointense on T2-weighted images 2
- A hyperintense central scar on T2-weighted images is a characteristic feature of FNH 2, 3
- FNH demonstrates intense homogeneous enhancement during the arterial phase of gadolinium-enhanced imaging 2
- Enhancement of the central scar during later phases of gadolinium-enhanced imaging is also a feature of FNH 2
CT Findings of Focal Nodular Hyperplasia
- FNH frequently shows hypodensity on precontrast CT scans 3
- Transient immediate enhancement after bolus injection is a common feature of FNH on CT scans 3
- Homogeneity is a characteristic feature of FNH on CT scans, seen in 85% of cases 3
- A scar is noted in 31% of FNH cases on CT scans 3
Differential Diagnosis
- FNH can be distinguished from other hypervascular liver lesions such as hepatocellular adenoma, hepatocellular carcinoma, and hypervascular metastases using MRI and CT findings 2, 3
- A multimodality approach is essential for the correct diagnosis of FNH to prevent unnecessary surgery 3
Diagnostic Criteria
- The typical MR triad of isointensity on T1- and/or T2-weighted images, homogeneity, and a scar which shows hyperintensity on T2-WI is seen in only 12% of FNH cases 3
- Histologic assessment of liver biopsy can record major diagnostic features such as fibrous bands, thick-walled vessels, reactive ductules, and nodularity, which are helpful in diagnosing FNH 4, 5