Appearance of FNH on US CT and MRI
General: FNH is the second most common benign hepatic tumor and it is more common in women. FNH contains all of the normal liver elements, but in an abnormal arrangement. It is typically smaller than 5 cm in diameter and solitary. The characteristic feature of FNH is the central scar, containing radiating fibrous tissue with vascular and biliary elements. The central scar may be seen with other lesions such as fibrolamellar HCC. Therefore, although a characteristic feature of FNH, it is not specific for FNH.
US: Often subtle; therefore, minimal contour abnormalities or vascular displacement should raise the possibility of FNH. A well-demarcated hypo- to isoechoic mass, possibly demonstrating a central scar, may be identified. Doppler images, especially if a stellate arterial pattern is present, are suggestive of FNH.
MDCT: FNH is hypo- to isodense on NCCT and without calcifications. FNH is hyperdense on HAP images because it is supplied by the hepatic artery. On PVP images, it is isodense to normal liver with a hyperdense pseudocapsule. The central scar is present in 35% of lesions smaller than 3 cm and 65% of lesions larger than 3 cm. The scar has lower attenuation than the normal liver on HAP and PVP images, but becomes hyperdense on 5- to10-minute delayed images. Enlarged feeding arteries and draining veins may be seen, especially with the use of MPRs.
MRI: FNH is T1-w hypo- to isointense and T2-w iso- to hyperintense to liver. The central scar is T1-w hypointense and T2-w hyperintense, unlike HCC in which the central scar is T2-w hypointense. The lesion demonstrates diffuse enhancement in the HAP except for the central scar, which demonstrates delayed enhancement similar to CT. Unlike HCC and HCAs, capsular enhancement is not identified in FNH. FNH has delayed enhancement with gadoxetic acid.