An acoustic backscatter-based method for estimating attenuation towards monitoring lesion formation in high intensity focused ultrasound

This work investigated the transient characteristics of tissue attenuation coefficient before, during and after HIFU treatment at different total acoustic powers (TAP) in ex vivo porcine muscle tissues. Dynamic changes of attenuation coefficient parameters were correlated with conventional B-mode ultrasound images over the whole HIFU treatment process. Two-dimensional pulse-echo radiofrequency (RF) data were acquired to estimate the changes of least squares attenuation coefficient slope (Δβ) and attenuation coefficient intercept (Δα0) averaged in the region of interest, and to construct Δβ, Δα0, and B-mode images simultaneously. During HIFU treatment, bubble activities were visible as strong hyperechoic regions in the B-mode images, causing fluctuations in Δβ and Δα0 estimations during treatment. Δβ and Δα0 increased with the appearance of bubble clouds in the B-mode images to values in the range of 1.5-2.5 [dB/(MHz.cm)] and 4-5 [dB/cm], respectively. After the treatment, Δβ and Δα0 gradually decreased, accompanied by fadeout of hyperechoic spot in the B-mode images, until they were stable at ranges of 0.75-1 [dB/(MHz.cm)] and 1-1.5 [dB/cm], respectively. In conclusion, Δβ and Δα0 images outperformed B-mode images in detecting HIFU thermal lesions by having significantly higher contrast to speckle ratios at all investigated TAP values.