Hepatic steatosis assessment using ultrasound homodyned-K parametric imaging: the effects of estimators

Background: The homodyned-K (HK) distribution is an important statistical model for describing ultrasound backscatter envelope statistics. HK parametric imaging has shown potential for characterizing hepatic steatosis. However, the feasibility of HK parametric imaging in assessing human hepatic steatosis in vivo remains unclear. Methods: In this paper, ultrasound HK mu parametric imaging was proposed for assessing human hepatic steatosis in vivo. Two recent estimators for the HK model, RSK (the level-curve method that uses the signal-to-noise ratio (SNR), skewness, and kurtosis based on the fractional moments of the envelope) and XU (the estimation method based on the first moment of the intensity and two log-moments, namely X- and U-statistics), were investigated. Liver donors (n=72) and patients (n=204) were recruited to evaluate hepatic fat fractions (HFFs) using magnetic resonance spectroscopy and to evaluate the stages of fatty liver disease (normal, mild, moderate, and severe) using liver biopsy with histopathology. Livers were scanned using a 3-MHz ultrasound to construct mu(RSK) and mu(XU) images to correlate with HFF analyses and fatty liver stages. The mu(RSK) and mu(XU) parametric images were constructed using the sliding window technique with the window side length (WSL)=1-9 pulse lengths (PLs). The diagnostic values of the mu(RSK) and mu(XU) parametric imaging methods were evaluated using receiver operating characteristic (ROC) curves. Results: For the 72 participants in Group A, the mu(RSK) parametric imaging with WSL=2-9 PLs exhibited similar correlation with log(10)(HFF), and the mu(RSK) parametric imaging with WSL = 3 PLs had the highest correlation with log(10)(HFF) (r=0.592); the mu(XU) parametric imaging with WSL = 1-9 PLs exhibited similar correlation with log(10)(HFF), and the mu(XU) parametric imaging with WSL = 1 PL had the highest correlation with log(10)(HFF) (r=0.628). For the 204 patients in Group B, the areas under the ROC (AUROCs) obtained using mu(RSK) for fatty stages >= mild (AUROC(1)), >= moderate (AUROC(2)), and >= severe (AUROC(3)) were (AUROC(1), AUROC(2), AUROC(3)) = (0.56, 0.57, 0.53), (0.68, 0.72, 0.75), (0.73, 0.78, 0.80), (0.74, 0.77, 0.79), (0.74, 0.78, 0.79), (0.75, 0.80, 0.82), (0.74, 0.77, 0.83), (0.74, 0.78, 0.84) and (0.73, 0.76, 0.83) for WSL = 1, 2, 3, 4, 5, 6, 7, 8 and 9 PLs, respectively. The AUROCs obtained using mu(XU) for fatty stages >= mild, >= moderate, and >= severe were (AUROC(1), AUROC(2), AUROC(3)) = (0.75, 0.83, 0.81), (0.74, 0.80, 0.80), (0.76, 0.82, 0.82), (0.74, 0.80, 0.84), (0.76, 0.80, 0.83), (0.75, 0.80, 0.84), (0.75, 0.79, 0.85), (0.75, 0.80, 0.85) and (0.73, 0.77, 0.83) for WSL = 1, 2, 3, 4, 5, 6, 7, 8 and 9 PLs, respectively. Conclusions: Both the mu(RSK) and mu(XU) parametric images are feasible for evaluating human hepatic steatosis. The WSL exhibits little impact on the diagnosing performance of the mu(RSK) and mu(XU) parametric imaging. The mu(XU) parametric imaging provided improved performance compared to the mu(RSK) parametric imaging in characterizing human hepatic steatosis in vivo.