The acoustic attenuation coefficient provides extremely valuableinformation for tissue characterization in general, and is needed for planningof High-Intensity Focused Ultrasound (HIFU) therapy in particular. Theattenuation coefficient in tissue is sensitive to changes in temperature and todamage resulting from heating. To date, parametric images of attenuationcoefficient from backscattered ultrasound have poor spatial resolution, and arethus inadequate for monitoring and assessment of HIFU. A novel method fordetecting changes in attenuation using backscattered ultrasound was developedthat could potentially be used for both monitoring and assessment of HIFUtherapy. This approach compares the signal energy in an untreated region ofsample located behind a region treated with HIFU. The technique was tested byusing HIFU to treat a liver sample placed on top of a well-characterizedtissue-mimicking phantom. The signal energy in the phantom was monitored using aclinical ultrasound scanner before, during, and after HIFU exposure of theliver. Results strongly depended on the presence of increased brightness in thesample. For cases without sample brightening, detected signal energy trended ina manner similar to the temperature. Also, a decrease in signal energy in thephantom after return to baseline temperature compared to initial conditions wasdetected in all cases, suggesting that the technique was sensitive to permanentchanges induced by HIFU in the liver sample.