HBP-enhancing hepatocellular adenomas and how to discriminate them from FNH in Gd-EOB MRI

Background Recent studies provide evidence that hepatocellular adenomas (HCAs) frequently take up gadoxetic acid (Gd-EOB) during the hepatobiliary phase (HBP). The purpose of our study was to investigate how to differentiate between Gd-EOB-enhancing HCAs and focal nodular hyperplasias (FNHs). We therefore retrospectively included 40 HCAs classified as HBP Gd-EOB-enhancing lesions from a sample of 100 histopathologically proven HCAs in 65 patients. These enhancing HCAs were matched retrospectively with 28 FNH lesions (standard of reference: surgical resection). Two readers (experienced abdominal radiologists blinded to clinical data) reviewed the images evaluating morphologic features and subjectively scoring Gd-EOB uptake (25–50%, 50–75% and 75–100%) for each lesion. Quantitative lesion-to-liver enhancement was measured in arterial, portal venous (PV), transitional and HBP. Additionally, multivariate regression analyses were performed. Results Subjective scoring of intralesional Gd-EOB uptake showed the highest discriminatory accuracies (AUC: 0.848 (R#1); 0.920 (R#2)—p < 0.001) with significantly higher uptake scores assigned to FNHs (Cut-off: 75%-100%). Typical lobulation and presence of a central scar in FNH achieved an accuracy of 0.750 or higher in at least one reader (lobulation—AUC: 0.809 (R#1); 0.736 (R#2); central scar—AUC: 0.595 (R#1); 0.784 (R#2)). The multivariate regression emphasized the discriminatory power of the Gd-EOB scoring (p = 0.001/OR:22.15 (R#1) and p < 0.001/OR:99.12 (R#2). The lesion-to-liver ratio differed significantly between FNH and HCA in the PV phase and HBP (PV: 132.9 (FNH) and 110.2 (HCA), p = 0.048 and HBP: 110.3 (FNH) and 39.2 (HCA), p < 0.001)), while the difference was not significant in arterial and transitional contrast phases (p > 0.05). Conclusion Even in HBP-enhancing HCA, characterization of Gd-EOB uptake was found to provide the strongest discriminatory power in differentiating HCA from FNH. Furthermore, a lobulated appearance and a central scar are more frequently seen in FNH than in HCA.


Introduction
Focal nodular hyperplasia (FNH) and hepatocellular adenoma (HCA) are among the most benign solid neoplasms of the liver and have the highest incidence in young adolescent to middle aged women [1]. Especially for HCA, a correlation with intake of oral contraceptive pills (OCPs) has been reported [2][3][4]. Reliable noninvasive differentiation by liver imaging is crucial as treatment differs for FNH and HCA. FNHs are commonly treated conservatively, while HCA should be resected because subtypes tend to bleed spontaneously or may undergo malignant transformation [5][6][7][8][9][10][11][12][13].
Published studies report significant hepatobiliary phase (HBP) Gd-EOB uptake for the majority of FNHs, resulting in iso-to hyperintensity to the surrounding liver, while most HCAs appear hypointense [8,14,15,20,21]. Occasional, FNHs may show atypical HBP appearances such as partial hypointensity or rim enhancement [5,15,22]. In contrast, a significant portion of HCAs, have recently been shown to appear (partially) hyperintense during the HBP, even mimicking the appearance of FNHs on Gd-EOB MRI [23][24][25]. These findings point to important pitfalls in differentiating FNH and HCA which could lead to unnecessary procedures like biopsies or surgery.
The purpose of this study was to determine how significantly Gd-EOB-enhancing HCA and FNH can be differentiated [26].

Patients
Our institutional review board (Charité Ethics Committee, Charité -University Medicine Berlin) approved this retrospective study (internal registration number EA2/016/14) and waived informed consent due to its retrospective nature. The study protocol conforms to the ethical guidelines of the 2002 Declaration of Helsinki. Imaging datasets were evaluated by two independent radiologists specialized in abdominal imaging (> 10 years of liver MRI experience). Primary endpoint was the discrimination between benign focal liver lesion, either FNH or HCA.
In a second step, two readers rated intralesional Gd-EOB uptake in steps of 25%. Both readers in consensus identified 40 HCA lesions with an intralesional Gd-EOB uptake greater than 25%, which were defined as significantly enhancing lesions [25]. In a third step, all histopathologically confirmed FNHs from January 2008 to March 2019 were retrospectively identified (n = 28) from the institutional databases who were examined with a Gd-EOB MRI. This inclusion process resulted in a final sample of 68 lesions-28 FNHs and 40 HCAs.
Some of the HCA patients participated in previous studies [15,19,25].

Imaging*
MRI was performed at either 1.5 T or 3.0 T MR scanner. Phased-array body coils were used in every patient. Standard imaging protocols included the following sequences: pre-contrast T2-weighted (T2-w) sequences with and without fat saturation (FS); T1-weighted (T1-w) sequences with and without FS (including in-/opposedphase technique). After intravenous injection of Gd-EOB (0.025 mmol/kg body weight; flow rate of approximately 1-2 mL/s) and a 40 mL saline flush, multiphase T1-w 3D sequences with FS were acquired during breath-holds (arterial phase with a fixed delay of 18 s, portal venous phase with 55 s delay, and transitional phase with 90 s delay). The hepatobiliary phase was acquired with a 3D T1-w FS sequences 20 min after contrast administration [25].

Fig. 1
Intralesional Gd-EOB uptake in the HBP was rated subjectively as iso-to hyperintensity percentage on a 5-point scale (score 0: 0%; score 1: 10-25%; score 2: 25-50%; score 3: 50-75%; score 4: > 75%). Lesions with scores of 0-1 were defined as hypointense and excluded during compilation of the sample. Lesions with scores of 2-3 were defined as "significantly enhancing lesions" while a lesion with a score of 4 was defined as "entirely enhancing lesions" Fig. 2 A/a shows a homogeneously and strongly HBP Gd-EOB-enhancing FNH in the left liver lobe. The lesion was classified as "entirely enhancing lesion" based on intralesional Gd-EOB uptake of more than 75% (score of 4). B/b shows HCA (inflammatory subtype (IHCA)) with heterogeneous / patchy HBP Gd-EOB uptake of 50-75% (score of 3), hence defined as "significantly enhancing lesion" • Atoll sign (defined as a hyperintense rim in T2/T2fs sequences probably representing dilated sinusoid spaces with the periphery of adenomas): yes vs. no.
Some of these features are displayed in Fig. 3. After analyzing the qualitative features of the lesions. Each of the two readers had to conclude a diagnosis of either FNH or HCA.

Quantitative analysis* Dynamic CE behavior
For each liver lesion, a 2D region of interest (ROI) including the entire lesion at its maximum cross-sectional diameter was placed manually. First relative signal intensity (SI) for each lesion and the liver was measured: Therefore, ROIs were placed in the arterial phase sequence and cloned to the subsequent pre-contrast and post-contrast phase sequences (PV, transitional, and 3 A/a shows an entirely enhancing FNH with a heterogeneous uptake pattern during the HBP. The FNH has a central scar and also appears lobulated. B/s also shows another entirely enhancing FNH during the HBP with a pseudocapsule. In C/c, the same lesion as in B/b clearly shows a T2-hyperintense central scar in the T2w FS sequence. D/d shows the classic atoll sign in an inflammatory subtype of HCA in the right liver lobe. An atoll sign is characterized by a hyperintense rim and a hypointense center in T2w images HBP). An additional circular ROI with a fixed 10-mm diameter was placed in healthy liver parenchyma sparing vessels and bile ducts. Furthermore, relative (Rel.) lesionto-liver enhancement was calculated for the different contrast phases as follows:

Statistics*
Statistical analysis was performed with XLSTAT (Version 2011.0.01; Addinsoft SARL, New York, New York). Proportional distributions were calculated using contingency tables. Descriptive parameters are given as mean and standard deviation. Based on histograms and quantile plots, normal distribution was not assumed for metric parameters, and therefore nonparametric tests were performed. Differences in contrast enhancement between FNHs and HCAs were analyzed using the Mann-Whitney U-test. Cross-tables and the Pearson chi-square test were used to investigate the association of categorical variables. To test inter-reader correlation, a pairwise twosided Spearman rank correlation test was performed. Diagnostic accuracy in terms of sensitivity and specificity was tested throughout by performing ROC curve analysis. After identifying the parameters with significant univariate accuracy, a multivariate regression was performed for the three most accurate parameters. All tests were two-sided with a level of significance of 0.05 [25]. *As mentioned above some of the patients participated in previous studies with similar methods (see Material and Methods section: Imaging, Quantitative analyses and Statistics) [25].

Gd-EOB MRI characteristics
In the total study population, 60% (41/68) of all lesions were rated as significantly enhancing (scores of 2-3) and 40% were rated as entirely enhancing (40/79) (score > 4) in HBP. Eighty-two percent of all FNH lesions (23/28) were classified as entirely enhancing lesions and 12.5% (5/40) as "only" significantly enhancing. In the HCA group, 87.5% of all lesions (35/40) were classified as significantly enhancing. The difference between the two groups of lesions was statistically significant. Interreader agreement of Gd-EOB scoring in HBP was excellent for both readers (p < 0.001-0.872).

Multivariate regression model
The Multivariate Regression Model was calculated for both readers out of the three univariate parameters with the highest accuracy levels (HBP-behavior, lobulation and scar). For both readers only the HBP appearance was significant with p = 0.001 / OR: 22.15 (R#1) and p < 0.001 / OR: 99.12 (R#2). For both readers none of the other parameters turned out to be significant (p = 0.067-0-715 / ORs: 0.40-5.62) (Fig. 5).

Dynamic CE behavior
ROI-based relative lesion-to-liver enhancement in the HBP was found to differ significantly between the two types of liver lesions (p < 0.001) with a higher mean ratio for FNH than for HCA. Portal-venous wash-out was found to be stronger for HCA than for FNH (p = 0.048).
Mean ratios in the arterial and venous phases did not differ significantly (p = 0.279 and p = 0.107). All results are displayed in Table 1 and Fig. 6.

Discussion
Our results show that Gd-EOB MR imaging provides additional information for differentiating FNH and HCA based on enhancement during the hepatobiliary phase even in Gd-EOB enhancing HCAs. FNHs can be differentiated reliably based on their typical strong and homogeneous HBP uptake behavior, corresponding to more than 75% enhancement of the lesion area, hence defined as entirely enhancing lesion using the system presented here. Adding morphologic features such as a lobulated appearance and the presence of a central scar, can support the discrimination between FNH and Gd-EOB enhancing HCAs. Our scoring system of Gd-EOB uptake appears to be a promising tool for differentiate these two entities even in significantly Gd-EOB enhancing HCA providing a potential solution for not false diagnosing these lesions as FNH.
Although both FNHs and HCAs are rare benign lesions of the liver, their non-invasive differentiation is important for determining further treatment algorithms. FNH does not have a risk for malignant transformation or bleeding and can be described as a so called "do not touch" lesions. In HCA, depending on their size and histological subtype, surgical resection is recommended due to the risk for malignant transformation or bleeding. It is therefore important to discriminate routinely between FNH and HCA with the help of MRI correctly [8,28]. Earlier studies provide evidence that Gd-EOB-enhanced MRI contributes to the differentiation of FNH from HCA with the former appearing hyperintense and the latter hypointense to the surrounding liver [5,15]. However, the most recent data show that signal hyperintensity is not uncommon in HCA, and atypical appearances have been reported for both entities [15,[23][24][25]. Thomeer et al. (2014) reported areas of iso-to hyperintensity in 12 of 17 patients (71%) with histopathologically proven HCA of the inflammatory subtype (IHCA) after gadobendate dimenglumine injection [23]. They concluded that, for IHCAs, other imaging features than for noninflammatory HCAs should be used to distinguish between HCAs and FNHs [23]. Agarwar et al. (2014) even conclude that IHCA can mimic the appearance of FNH on Gd-EOB MRI including HBP hyperintensity [24]. These results are in line with a recently published study from our group, in which we presented results suggesting an added benefit of Gd-EOB-enhanced MRI for HCA subtype differentiation [25]. In line with the data of Thomeer et al. and Agawar et. al., we found that the majority of significantly enhancing lesions were of the inflammatory subtype [25]. Unlike Thomeer et al., however, we assume that HBP imaging still provides the highest discriminatory power to distinguish HCA and FNH in the noncirrhotic liver. In the present study, we only analyzed lesions with more than 25% intralesional Gd-EOB uptake. As reported before, we classified intralesional Gd-EOB uptake in steps of 25% [25]. Additionally, we introduced the term "entirely enhancing" for lesions with uptake of more than 75% (Fig. 1). The two readers in our study achieved the highest accuracy rates with sensitivities of 82-96% and specificities of 87.5% when then study lesions were subdivided into "entirely enhancing" and "significantly enhancing" lesions. In comparison to Gd-EOB uptake behavior, only two other features-lobulated appearance of FNHs (sensitivity: 82.0-89.5% and specificity: 65.0-72.5%) and presence of a central scar in FNHs (sensitivity: 46.5-89.0% and specificity: 67.5-72.5%)-achieved acceptable accuracy levels. The Gd-EOB uptake pattern, divided into homogeneous versus heterogeneous enhancement, achieved high sensitivity rates (86.0-93.0%), while specificity (30-42.5%) was poor, as 30% of all HCAs were classified as homogeneously enhancing as well.
As expected, arterial hyperintensity provided high levels of sensitivity without any strong discriminatory power as both FNH and HCA are characterized by hyperintensity in the arterial phase [5,19,30]. Even though Grazioli et al. (2012) reported higher arterial signal intensity in FNH than in HCA, it must be borne in mind that IHCA shows greater arterial enhancement than the other subtypes, which is important for this study as the HCA subgroup mainly consisted out of IHCA [4,5,31,32]. In our study, IHCAs accounted for 92.5% of all HCAs, thus making arterial enhancement characteristics an even more unsuitable feature to differentiate HCAs from FNHs. The high percentage of IHCAs also distorts results for portal-venous phase, which is mainly described for HHCA. while, in IHCA, which have wider interstitial spaces, the contrast agent tends to persist during transitional/equilibrium phases. This behavior potentially mimicks FNH, Fig. 6 Relative lesion-to-liver (%) enhancement in the arterial (AP), portal venous (PVP), transitional (TR), and hepatobiliary (HBP) phases for the FNH subgroup (A) and the HCA subgroup (B). C shows a significant difference between the means of FNH and HCA which appears iso-to hyperintense during portal venous and transitional / equilibrium phases [4,31,32].
In our cohort, five HCAs were rated as entirely enhancing lesions. We believe that this small group of lesions is highly interesting as these are HCA lesions (4 × IHCA and 1 × bHCA) with the potential of mimicking FNH in Gd-EOB MRI. However, entirely enhancing HCAs are so rare that a statement on how to distinguish them from FNH is only possible in the context of further studies.
The results of our quantitative analysis in terms of CE lesion-to-liver ratios corroborate our qualitative results regarding the discriminatory power of the intralesional Gd-EOB uptake scoring system as the higher mean value in FNH (110.3 ± 39.2), corresponds to a score of 4, and was significantly higher than in HCA (59.2 ± 38.0-p < 0.001). Referring to Thomeer et al. we assume that, although HBP hyperintensities occur frequently in HCA, it is still possible to differentiate FNH and HBP and overcome MRI pitfalls by characterizing their uptake behavior more specifically, as done in our analysis. As also supported by our quantitative lesion-toliver ratios, the Gd-EOB scoring system presented here may become a tool easy to use in daily clinical routine to discriminate between FNH and significantly enhancing HCA.
Our study has limitations. First, we performed a retrospective study and patients were not enrolled consecutively and although basic MRI sequences were consistent, acquisition parameters differed, as due to their rarity, the collective was enrolled during a time-period of eleven years. Also, the fixed delay after injection of contrast enhanced sequences could lead to missing the optimal timing for some lesions. Second, lesion-to-liver ratios were measured in ROIs and not volumetrically. Third, we only analyzed FNHs and HCAs. Hepatocellular carcinomas can also present as HBP enhancing lesions. To minimize this bias, only patients without cirrhosis were included. Fourth, although readers were blinded, they were aware of the study design, which may have introduced detection bias, especially as only FNH were included who were surgically resected.
Even in Gd-EOB enhancing HCA during the HBP, characterization of Gd-EOB uptake was found to provide the strongest discriminatory power to differentiate HCA from FNH. The scoring system we introduce may facilitate interpretation of MRI and provide an easy-to-use diagnostic tool to overcome the imaging pitfall encountered in the differentiation of significantly enhancing HCA and FNH. Furthermore, a lobulated appearance and the presence of a central scar, which are more common in FNH than in HCA, can be used as additional features.