Image-guided refocusing of dual-mode ultrasound arrays (DMUAs)

Abstract

A major advantage of imaging with dual-mode ultrasound arrays (DMUAs) is the inherent registration between imaging and therapeutic coordinate systems during image-guided surgery which allows for image-based feedback for refocusing the therapeutic beam. Specifically, this capability is critical in image-guided thoracic surgeries where the target is partially obstructed by the rib cage, thus limiting the access and distorting the geometrically focused high-intensity focused ultrasound (HIFU) therapeutic beam. Images obtained with single-transmit focus (STF), in which the therapeutic beam is used at diagnostic levels, allow the user to select target and critical locations for optimizing the power deposition. We have developed an optimal refocusing method that takes advantage of the acoustic window of the intercostals spacing in order to minimize the power deposition over the critical regions (ribs) while maintaining or improving the power deposition at the target location (tumor). The algorithm is verified experimentally with a 64-element 1 MHz DMUA, in an attenuating tissue mimicking phantom (0.5 dB/cm/MHz) with embedded Plexiglas ribs. Thermocouples are used to measure sub-therapeutic temperatures across the ribs and at the target location before, during and after 4 seconds of HIFU exposure for both the geometric focusing and the optimized refocusing while normalizing the driving power for both cases. An increase of normalized temperature (per watt of input power) greater than 50% was observed at the target after refocusing. At the same time, a reduction in normalized temperature rise across the ribs was greater than 70%. Statistics showed that the maximum variance between measurements when the experiment was rerun a minimum of 5 times for each case was approximately 5%. In addition, STF images taken with the refocused HIFU beam showed increased echogenicity at the target and reduced echogenicity at the ribs. This can be quantified by the intensity of the grayscale images. These images show a typical reduction of 11dB across the ribs. These results show that imaging of the therapeutic beam at diagnostic levels in STF mode correlates with the energy deposition during HIFU treatment.