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A new approach for detecting attenuation changes during high-intensity focused ultrasound

The acoustic attenuation coefficient provides extremely valuable information for tissue characterization in general, and is needed for planning of High-Intensity Focused Ultrasound (HIFU) therapy in particular. The attenuation coefficient in tissue is sensitive to changes in temperature and to damage resulting from heating. To date, parametric images of attenuation coefficient from backscattered ultrasound have poor spatial resolution, and are thus inadequate for monitoring and assessment of HIFU. A novel method for detecting changes in attenuation using backscattered ultrasound was developed that could potentially be used for both monitoring and assessment of HIFU therapy. This approach compares the signal energy in an untreated region of sample located behind a region treated with HIFU. The technique was tested by using HIFU to treat a liver sample placed on top of a well-characterized tissue-mimicking phantom. The signal energy in the phantom was monitored using a clinical ultrasound scanner before, during, and after HIFU exposure of the liver. Results strongly depended on the presence of increased brightness in the sample. For cases without sample brightening, detected signal energy trended in a manner similar to the temperature. Also, a decrease in signal energy in the phantom after return to baseline temperature compared to initial conditions was detected in all cases, suggesting that the technique was sensitive to permanent changes induced by HIFU in the liver sample.