Characterization of High Intensity Ultrasound Exposures by Quantitative Ultrasound

Abstract

High Intensity Ultrasound (HIU) is of proven value in the treatment of many medical disorders by using US energy without incisions or radiation. The development of an effective method for guiding HIU therapies remains necessary. Changes in spectral-based Quantitative UltraSound (QUS) parameters have been correlated with the temperature changes during HIU exposure (Ghoshal et al. IEEE UFFC 2016). The aim of this study is to quantify the changes in QUS parameters due to temperature elevation after HIU exposure of ex vivo bovine livers. The QUS experiments are conducted in high frequencies (12-38 MHz) to provide insight into the specific changes in liver tissues that cause the changes of backscatter properties for fixed temperature ranging from 37°C up to 80°C after HIU exposure. Samples of porcine livers are exposed to HIU using a plane 3 MHz transducer with aperture diameter of 4 cm. Samples were heated up to 37, 50, 60, 65, 70, 75, 80°C. After reaching the desired temperature, the sample is cut in half at the center of the lesion. US backscatter signals were recorded from the cut samples by using the Visualsonics Vevo 770 imaging system with the RMV710 probe of center frequency 25 MHz. The backscatter coefficients (BSCs) were estimated using the reference phantom technique and the attenuation coefficients were determined using a standard substitution method. Structural scatterer properties were estimated by fitting the measured BSCs with the spherical Gaussian model (SGM) or with the Structure Factor Model (SFM) to estimate the effective scatterer diameter (ESD) and acoustic concentration (EAC). The QUS parameters estimated by the sparse SGM and the concentrated SFM differ strongly from each other for HIU treated samples and suggests that the treated livers can be considered as concentrated media. Results showed that both integrated BSC and ESD estimated by the SFM increased as a function of temperature.