Transmission Ultrasound Imaging to Guide Thermal Therapy

Abstract

Thermal therapy is a technique that can locally destroy tumours by heating them to 55 °C and higher. Transmission ultrasound can detect the treatment induced changes in tissue attenuation. During thermal therapies, an increase in temperature reduces tissue attenuation while changes in tissue structure due to coagulation increases tissue attenuation. In this paper, we investigate the use of a transmission ultrasound camera (AcoustoCam, Imperium Inc., Silver Spring, MD) to quantitatively measure changes in attenuation and monitor in real-time the effect of thermal therapy. To calibrate the ultrasound camera, tissue-mimicking phantoms (10) with different attenuation values ranging from 3 dB to 30 dB were constructed. Frequency-dependent attenuation coefficients were calculated over a frequency range of 3.5 MHz to 7.5 MHz in steps of 0.5 MHz using an independent technique. These phantoms were then imaged using the AcoustoCam and the corresponding mean pixel intensities were obtained. The relationship between average pixel intensity and attenuation value was established. A temperature sensitive phantom was made and heated using a laser fiber. The laser fiber was a 2- cm long cylindrical diffuser (Photoglow, Yarmouth, MA). One image per second was taken during the heating procedure for 20 minutes. The difference in pixel intensity prior to and after heating due to the formation of a lesion was measured and related to the attenuation value change from the calibration curves. It was found that the attenuation changed by 4 dB (0.33 dB/cm) close to the phantom. The results obtained show the reliability of using transmission ultrasound in monitoring thermal therapy which was sensitive to changes as small as 0.08 dB in the PVCP phantom experiment.