The prevalence of hepatic steatosis has been increasing rapidly, and it is currently detected in over 25% of people undergoing routine health check-ups in Japan . There is a long history of using ultrasound to diagnose hepatic steatosis, starting when Joseph et al. proposed the concept of a “bright liver” in 1979 , and it is still often routinely used for this purpose in clinical practice. New findings such as hepato-renal echo contrast , deep attenuation, and vascular blurring were subsequently added to form a set of 4 characteristic features to aid in the diagnosis of this condition. However, improved beam penetration resulting from advances in ultrasound systems has altered how the characteristic features of hepatic steatosis appear on ultrasound. Specifically, deep attenuation has become difficult to capture with modern ultrasound systems because attenuation correction methods cause deep attenuation to be displayed as if there were no attenuation at all. Thus, it is actually becoming more difficult to accurately assess hepatic steatosis using conventional methods alone. Also, there is now a critical need to assess whether hepatic steatosis occurs in > 5% of the liver because this is the definition of steatosis used in the diagnosis of NASH, which can progress to cirrhosis . Studies have also shown that the level of hepatic steatosis correlates with cardiovascular events , and that steatosis ≥ 25% clearly worsens the prognosis for liver transplantation . Accurate quantification of hepatic steatosis is therefore necessary, but liver biopsy has been the only quantitative method available to date. However, liver biopsy is a poorly suited diagnostic test for such a prevalent condition because of the costs, possible risks, and invasiveness of the procedure .
One method currently available for quantifying steatosis is the attenuation imaging (ATI) modality offered by Canon Medical Systems. Tada et al.  compared ATI results against liver biopsy results in 38 patients with NAFLD using ROC curve analysis and found good diagnostic performance for steatosis scores of 1, 2, and ≥ 3 (AUROC = 0.77, 0.88, and 0.86, respectively). Bae et al.  compared ATI results with liver biopsy results in 108 patients with diffuse liver disease using ROC curve analysis and also found good diagnostic performance for steatosis scores of 1, 2, and ≥ 3 (AUROC = 0.843, 0.886, and 0.926, respectively).Other methods have also been evaluated. Hyodo et al.  evaluated the utility of computed tomography (CT) for quantifying steatosis. They compared dual-energy CT results with liver biopsy results in 33 NAFLD patients by ROC curve analysis and found good diagnostic performance for steatosis scores of ≥ 1 based on AUROC. Two studies have evaluated the utility of magnetic resonance imaging (MRI). Igarashi et al.  compared multi-slice and multi-point MRI findings with liver biopsy findings in 52 patients with NAFLD by ROC curve analysis and found good diagnostic performance for steatosis scores of 1, 2, and ≥ 3 (AUROC = 0.975, 0.929, and 0.969, respectively). Imajo et al.  compared MRI-based proton density fat fraction results with liver biopsy results in 142 NAFLD patients by ROC curve analysis and found good diagnostic performance for steatosis scores of 1, 2, and ≥ 3 (AUROC = 0.98, 0.90, and 0.79, respectively).
In this study, we tested the performance of the UGAP developed as an attenuation imaging method for quantifying relative attenuation in the liver caused by the properties of living tissues for the quantification of steatosis in patients with NASH. We found a significant positive correlation between LFC (%) obtained by liver biopsy and AC values (r = 0.83, p < 0.01), as well as good diagnostic performance of AC values for steatosis scores of 1, 2, and ≥ 3 in ROC curve analysis (AUROC = 0.95, 0.96, and 0.93, respectively). Our results indicate that ultrasound diagnosis of steatosis using UGAP has comparable performance to that previously reported for diagnosis of steatosis by ultrasound with ATI, CT, and MRI. The assessment results obtained using UGAP were favourable probably because this technology automatically detected 2 different positions within the ROI that were in the most suitable condition for measuring liver signals to determine attenuation.
In 36 patients with NASH, AC values showed a positive correlation with ALT (r = 0.50, p < 0.01), PLT (r = 0.44, p < 0.01), and PT% (r = 0.56, p < 0.01). Also, in patients with NASH, comparison of the AC value with fibrosis, which is one of the liver pathological factors, showed that the AC value decreased significantly with the progression of fibrosis. When NASH progresses to cirrhosis, the degree of steatosis decreases (so-called burnout NASH), and this may be one of the possible reasons for the positive correlations of AC with PLT and PT%, and for the decreases in AC with the progression of fibrosis.
The positive correlation between AC values and ALT suggests the degree of liver inflammation influences the AC value, but this needs to be investigated further in the future.
Our study has some limitations. This was a single-centre study with a small sample size, and the results will need to be validated in a multicentre study with a larger sample size. Research in different racial groups will also need to be conducted because we evaluated only Japanese patients in this study.