Noninvasive determination of in situ heating rate using kHz acoustic emissions and focused ultrasound

Abstract

For high-intensity focused ultrasound (HIFU) to be widely applicable in the clinic, robust methods of treatment planning, guidance and delivery need to be developed. These technologies would greatly benefit if patient specific tissue parameters could be provided as inputs so that the treatment planning and monitoring schemes are customized and tailored on a case by case basis. A noninvasive method of estimating the local in situ acoustic heating rate using the heat transfer equation (HTE) and applying novel signal processing techniques is presented in this article. The heating rate is obtained by experimentally measuring the time required to raise the temperature of the therapeutic focus from a baseline temperature to boiling (here assumed to be 100 degrees C for aqueous media) and then solving the heat transfer equation iteratively to find the heating rate that results in the onset of boiling. The onset of boiling is noninvasively detected by measuring the time instant of onset of acoustic emissions in the audible frequency range due to violent collapse of bubbles. In vitro experiments performed in a tissue mimicking alginate phantom and excised turkey breast muscle tissue demonstrate that the noninvasive estimates of heating rate are in good agreement with those obtained independently using established methods. The results show potential for the applicability of these techniques in therapy planning and monitoring for therapeutic dose optimization using real-time acoustic feedback.

Fig. 1

Schematic of experimental setup for noninvasive heat source estimation.

Fig. 2

(a) Time domain output of stethoscope recorded during a HIFU exposure lasting 42 seconds (b) Spectrogram of the time domain signal shown in (a) computed using the Short Time Fourier Transform. White arrow indicates the onset of broad band signatures corresponding to boiling (c) Power as function of time. The marked increase at t=30 s represents the onset of boiling.

Fig. 3

(a) Time domain output of stethoscope recorded during a HIFU exposure in excised turkey breast muscle (b) Spectrogram of the time domain signal shown in (a) computed using the Short Time Fourier Transform. White arrow indicates the broad band signatures corresponding to the onset of boiling at 21 s (c) Power as function of time.