Image-based refocusing of dual-mode ultrasound arrays (DMUAs) in the presence of strongly scattering objects

An advantage of imaging with DMUAs is the potential for identifying target and critical regions in the treatment field for avoidance. Refocusing of the therapeutic beam in the presence of strongly scattering objects, such as the ribs, while targeting liver tumors is of particular importance due to limited access and distortion of the HIFU beam. An image-based refocusing algorithm utilizing gray-scaled images obtained with single-transmit focus imaging allows for selection of control points to be taken from the visible target and ribs in the image. Using a two-step virtual array method to take advantage of the intercostal spacing of the ribs, the algorithm minimizes the power deposition over the critical regions while maintaining or improving the power deposition at the target location. The algorithm is verified experimentally with a 64-element 1MHz array, in an attenuating tissue mimicking phantom ( .5 dB/cm/MHz) with Plexiglas ribs. Thermocouples are used to measure sub-therapeutic temperatures across the ribs and at the target location. An increase of temperature at the target location of 20% is measured, with a decrease of 30% across the ribs. In addition, the intensity of the gray-scaled images showed an improvement of 5 dB at the focus with a reduction of 2dB across the ribs. Image-based refocusing is shown to improve distortions of the HIFU beam experienced by strongly scattering objects that partially obstruct the target.