Ultrasound echo envelope statistics have been widely studied for quantitative tissue characterization. In ultrasound measurements, the size of the region-of-interest (ROI) is limited by several factors, such as the locality of the tissue characteristics and the depth dependence of the acoustic field of the ultrasound beam. In this case, the evaluated echo envelope statistics vary even when the envelope amplitudes follow the same population without any noise. In this study, the statistical variance of the moments caused by this finite number of samples was quantified based on the central limit theorem and the law of error propagation. The proposed principles were validated by random number simulation and used to quantify the statistical variance of Nakagami parameter estimation. Finally, the effective number of independent samples in an ultrasonic measurement was quantified based on the relationship between the ROI size and the ultrasound spatial resolution.

We compared the evaluation accuracy of amplitude envelope statistics under the transmission and reception conditions of compounded plane wave imaging (CPWI) and focused beam imaging (FBI). In a basic study using a homogeneous phantom, we found that the amplitude gradient in the depth direction and the PSF in the lateral direction spread in the FBI reduced the accuracy of evaluation in amplitude envelope statistics. On the other hand, CPWI showed a more stable evaluation than FBI because of the elimination of sound field characteristics. In CPWI, the multi-Rayleigh model discriminated signals from two types of scatterer with high accuracy in the evaluation using phantoms mimicking fatty liver. It was confirmed that the combination of CPWI and the multi-Rayleigh model is effective for detecting early fatty liver disease. The results show that CPWI is effective for improving the robustness of amplitude envelope statistics.

Here, the blood vessel shape was accurately measured to evaluate the viscoelastic properties of the radial artery. For a robust measurement, the shape parameters of the elliptical blood vessel were determined by integrating brightness over the entire vessel instead of using the brightness gradient at the vessel boundary. The usefulness of the proposed method was demonstrated via simulations and in vivo experiments. The obtained results exhibited considerable potential for estimating the viscoelastic properties of elliptically deformed blood vessels.

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