A Consideration on System Factors for Spectral Shift Method in Estimating Ultrasonic Attenuation Coefficients

Ultrasonic system, the widely used in clinical diagnosis, is providing the relatively convenient, instantaneity, non-invasion and low cost than MRI, CT, etc. Besides, the assessment of backscattering signal processing on tissue parameters by quantitative analysis have become increasingly popular recently. Based on the above reasons, the spectral shift method is proposed by using the short-time Fourier transform to estimate the ultrasonic attenuation coefficients which is an important issue for clinical diagnosis. Investigating the influence of system factors by proposed method is the main concept of this study, e.g. evaluate the patient’s fatty liver condition by observing the variation tendency of the ultrasonic attenuation coefficients. There are two parts in this study: 1. the graphite phantom experiment and 2. the clinical experiment based on the results of phantom experiment. In the former experiment, the measured standard attenuation coefficients were compared with the results of the spectral shift method in three system factors adjustments, the center frequency of ultrasonic transducer, diffraction compensation, and Time Gain Compensation (TGC). Measurement results showed that the attenuation coefficient increased with high fatty concentration. The characteristics of high frequency with high attenuation was verified because the failing slope of center frequency were increased with larger center frequency of transducer. To precisely improve the estimated attenuation coefficients, the diffraction calibration of the beam focal effects caused by the transducer in acoustic transfer is involved. Experimental results showed that no matter the convex transducer or the linear transducer, the accuracy of modified attenuation coefficients have significantly improved. To have better image visualization of B-Mode, the TGC will be opened to identify the deeper region in the clinical. It was frequently investigated that the attenuation coefficients of the N-TGC estimated have better accuracy than the TGC. Clinical experiment on fatty liver is the second part of this study. It was discovered that the attenuation coefficients of TGC estimated is higher than N-TGC; however, in the ROC curve analysis, the results of TGC estimated is lower than N-TGC. To sum up, lower center frequency of ultrasonic transducer, diffraction calibration, and N-TGC are the three beset system factors to identify the mild and moderate fatty liver in clinical applications.