Focal Nodular Hyperplasia (FNH)
The cellular structure of FNH is similar to that of the normal hepatic parenchyma apart from the presence of an abnormal biliary system. Since FNH contains the same elements as normal liver but with a disordered architecture, it mimics the appearance of normal liver and may be difficult to detect using any of the imaging modalities without exogenous contrast agents. On unenhanced MR imaging, the lesion is iso- to hypointense compared to normal liver on T1-weighted images, frequently with a hypointense central fibrous scar. On T2-weighted images it is isointense to mildly hyperin-tense compared to normal liver, with a hyperin-tense central scar. On dynamic Gd-enhanced T1-weighted images, the lesions show intense enhancement in the arterial phase that washes out rapidly on portal-venous and subsequent equilibrium phase images. A key characteristic of this enhancement is a homogeneous appearance (other than the central scar) which aids in differentiating these lesions from fibrolamellar hepatocellular carcinoma.
Typical of fibrous tissue, the central scar is slower to allow entry of extracellular-type contrast materials and slower to allow wash-out. This leads to a characteristic hypointense appearance on early arterial phase images but an iso- to hyperintense appearance on portal-venous and equilibrium phase images due to a prolonged retention of contrast agent compared to the wash-out from normal liver parenchyma. This can be very helpful in differentiating the central scar of FNH from the tumor necrosis often seen in malignant tumors.
Although Gd-enhanced MRI is considered the most sensitive method for the characterization of FNH, atypical features can frequently confound interpretation: in a recent study 86% of small (< 3 cm) FNH did not have a visible scar on unen-hanced or enhanced dynamic phase scans . Similar findings have been reported for CT . Although the absence of a scar in small FNH cannot be considered "atypical", it may make it more difficult to distinguish these lesions from other hy-pervascular neoplasms on dynamic imaging alone. Hence, the availability of MR contrast agents with liver-specific properties may be helpful for the accurate characterization of FNH.
Gd-BOPTA and Gd-EOB-DTPA offer the possibility to perform both dynamic and delayed phase imaging, unlike conventional Gd agents and some SPIO agents. Gd-BOPTA-induced enhancement of FNH on T1-weighted dynamic phase imaging is indistinguishable from that of conventional nonspecific Gd agents (Fig. 1). Thereafter, Gd-BOPTA offers the advantage over conventional agents of permitting additional hepatobiliary phase T1-weighted imaging (Fig. 2). On these delayed T1-weighted images, substantial enhancement is usually noted within the parenchyma of the vast majority of FNH lesions indicating the presence of functioning hepatocytes able to take up Gd-BOP-TA. Conversely, the central scar, which is the principal site of biliary metaplasia, appears consistently hypointense.
FNH lesions typically appear iso- or hyperin-tense compared to the surrounding liver parenchyma on hepatobiliary phase images after Gd-BOP-TA, with three different patterns of enhancement frequently observed: homogeneous, peripheral or heterogeneous . Similar findings have been observed with mangafodipir trisodium (Fig. 3) , although the inability to perform dynamic imaging with this agent is an obvious limitation (see Chap. 4, Figs. 23,25).
Since FNH lesions usually contain Kupffer cells and are thus able to take up SPIO and ultrasmall superparamagnetic iron oxide (USPIO) particles, the lesions typically demonstrate reduced signal intensity (SI) on delayed post-iron oxide T2-weighted images (Fig. 4). However, this finding is not specific, as well-differentiated HCC may also take up iron oxide particles, although usually to a lesser extent. Moreover, the amount and distribution of Kupffer cells within the nodules can vary and yield different patterns of signal decrease: some small FNH (< 3 cm) show a homogeneous signal drop similar to that observed in the surrounding parenchyma, while large FNH may show a heterogeneous signal drop. The central scar does
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