DISCUSSION

Our study results show the tumor size measurement varies with imaging modalities and each phase. It is still debate which phase should be used to measure the exact size of tumor. The Organ Procurement and Transplantation Network (OPTN) protocol recommends size measurements in the late hepatic arterial or early portal venous phase.(13) On the other hand, LI-RADS allows to measure the size in the most visible phase.(3) The American Association for the Study of Liver Diseases (AASLD) practice guidelines does not specify size measurement methods.(11) Our results showed that the measurement errors of tumor diameter measured by MR were smaller than those measured by CT. It indicates that MR is favorable for size measurement, especially in beginners. Interestingly, there were no statistical significances of the measurement errors between novices and experts on both CT and MR.

Davenport MS et al. reported an inter-observer agreement of the major imaging features in MR, substantial agreement for arterial hyperenhancement (κ = 0.67), moderate for washout appearance (κ = 0.48), and moderate for capsule appearance (κ = 0.52).(8) In contrast, the degree of inter-observer agreement demonstrated in this study was moderate (к = 0.475) for AP hyperenhancement, substantial (к = 0.630) for washout appearance, and fair (к = 0.303) for capsule appearance. These results are lower than those of previous reports, which can be explained in relation to the characteristics of gadoxetic acid. In MR imaging using gadoxetic acid, which shows not only hepatocyte uptake and also biliary excretion, the hepatic AP may appear less intense than with conventional agents.(14) These characteristics of gadoxetic acid can affect low inter-observer agreement in

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assessing arterial hyperenhancement. In HBP, normal liver parenchyma appears uniformly bright on T1- weighted images because gadoxetic acid accumulates in the extracellular space.(14) Enhanced normal parenchyma can obscure capsule and it could be a cause of low inter-observer agreement.

Comparing the major imaging features, the inter-observer agreement was higher in CT (к = 0.566) than in MR (к = 0.475) for arterial enhancement. When reviewed by expert, the arterial enhancement was better detected in MR than in CT (96.8% vs. 92.1%). Novice’s readings showed a frequency of arterial enhancement similar to that of expert (92.1%) in CT, but revealed a significantly lower detection rate in MR, compared to that of expert’s (90.5% vs. 96.8%). It might be due to characteristics of gadoxetic acid. Since gadoxetic acid may show less intense enhancement in the AP than other contrast agents, it is important to familiarize with the images using gadoxetic acid.

In terms of washout appearance, the inter-observer agreement was higher in MR (к = 0.630) than in CT (к = 0.364) and it might be caused by higher tissue contrast in MR than in CT.

In capsule appearance, the inter-observer agreement was lower in MR (к = 0.303) than in CT (к = 0.491). Specifically, the frequency of capsule appearance in CT and MR was similar (50.8%, 54.0%) in expert, however, there was a significant difference in the frequency of capsule appearance between CT and MR (54.0%, 68.3%) in novice. It suggests that sufficient experience in MR imaging using gadoxetic acid is needed for precise image reading in GAeMR. The frequency of capsular appearance was higher in MR than in CT, not only in expert but also in novice. Strong association between the imaging features of subjective washout and

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capsule in hypervascular liver nodules may be partially derived from an optical illusion and/or reader predisposition.(15) Therefore, the inter-observer agreement could be lower in capsule appearance.

Ancillary imaging features that favor HCC in LI-RADS includes mild to moderate T2 hyperintensity, restricted diffusion, corona enhancement, mosaic architecture, nodule-in-nodule architecture, intra-lesional fat, lesional iron sparing, lesional fat sparing, blood products, diameter increase less than threshold growth, distinctive rim, HBP hypointense rim, HBP hypointensity.(3) But in our study, limited numbers of cases used ancillary imaging features, because we already knew pathological results. Because there are no standard guidelines, the use of ancillary features may cause bias by radiologist’s discretion.

There was no significant difference between expert and novice in the frequency of final categorization of LI-RADS. When evaluating the value of ICC, it was found that the inter-reader agreement of MR was lower than that of CT (ICC:

0.574 in CT, 0.318 in MR). Although not significant, there was a trend toward increased proportion of LI-RADS 5 lesions at MRI compared to CT.

There are several limitations in our study. First, shrinkage factor after formalin fixation should be considered when reporting tumor size. The tumor size may slightly larger in vivo condition, and that may differ according to the various imaging modalities. So we used the gross pathological size of HCC as reference standard. Second, in our institution, the pathological specimens are usually sectioned in the sagittal or coronal plane, coronal and sagittal images are not always available in retrospective study with CT and GAeMR. Accordingly, we used mainly axial images. Third, we measured the size of each phases of one HCC at the same session, bias could exist. The differences between each phases was very small. It is difficult to measure the size of the HCC with irregular peritumoral

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enhancement on AP, but it is easier than that to measure the size of HCC with clear outline on HBP image.

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