## 5 Discussion  
Envelope statistics is an important group of QUS parameters for ultrasound tissue characterization. Ultrasound envelope statistics parametric imaging has potential in assessing hepatic steatosis. As this imaging method is based on ultrasound backscattered signals, it is compatible with a conventional pulse-echo ultrasound imaging framework. To the best of our knowledge, this paper is the first to review the state-of-the-art ultrasound envelope statistics parametric imaging techniques for characterizing hepatic steatosis. Table 1 provides a summary of ultrasound envelope statistics parametric imaging techniques for hepatic steatosis assessment, in terms of study type, case number, reference standard, and performance. The reviewed ultrasound envelope statistics parametric imaging techniques include ASQ imaging, ultrasound Nakagami imaging, HK imaging, kurtosis imaging, and entropy imaging. ASQ imaging, Nakagami imaging, and HK imaging are based on statistical models, while kurtosis imaging and entropy imaging are not. ASQ imaging, Nakagami imaging, kurtosis imaging, and entropy imaging have been applied to clinical studies, but HK imaging have not. The performance of HK imaging in assessing human hepatic steatosis should be explored in future developments. Statistical model-based and non-model-based ultrasound envelope statistics parametric imaging techniques are compared as follows. A prerequisite for using statistical models describing the statistical properties of ultrasound backscattered signals is that the ultrasound backscattered data must conform to the used distribution [22; 58]. This may not always be satisfied because of the differing signal detection and processing hardware and software designs among different ultrasound imaging systems. In this sense, non-model-based ultrasound envelope statistics parametric imaging, which can be computed using any data regardless of the distribution model, may have more flexibility in tissue characterization [22]. Table 2 compares the advantages and limitations of different ultrasound envelope statistics parametric imaging techniques. ASQ imaging has been commercialized with higher availability, but inconsistent findings for characterizing human hepatic steatosis have been reported [26; 27]. Ultrasound Nakagami imaging has low computational complexity, but the \(m\) parameter plateaus around 1 for higher scatterer concentrations [43]. Ultrasound HK imaging is based on parameters of a physical meaning, but the analytical complexity for estimating HK parameters is high. Ultrasound kurtosis imaging is easy to implement, but it needs further validation in hepatic steatosis assessment. Ultrasound entropy imaging can use a small sliding window, thus allowing a small-window, high-resolution imaging, but the dynamic range of the Shannon entropy value is limited [22]. The limitations may be overcome in future developments. There are some studies comparing the performance of different ultrasound envelope statistics parametric imaging techniques in assessing hepatic steatosis. [PERSON] et al. [25] compared Nakagami with ASQ imaging and showed that ASQ may be more stable and provides higher sensitivity than Nakagami imaging. [PERSON] et al. [22] demonstrated that entropy imaging yields a better performance than Nakagami imaging, for both liver donors and patients. [PERSON] et al. [60] compared entropy imaging with ASQ imaging and showed that entropy imaging outperforms ASQ imaging. More comparisons  
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