Close-loop lesion formation control using multiple-focus dual mode ultrasound array

Bubble activity during the high intensity focused ultrasound (HIFU) application could create overexposed lesion in very short period of time due to the increase in acoustic absorption. This process, however, can be utilized to drastically reduce the lesion formation time if the bubble activities can be reliably monitored and controlled. Furthermore, a phased array system could be used to reduce the volume lesion formation time by simultaneously invoking bubble activity at multiple focus locations, and independently adjust the intensity at each location based on the monitoring feedback. We present the results from an ultrasound-guided focused ultrasound platform designed to perform real-time monitoring and control of lesion formation using multiple-focus patterns. A dual-mode ultrasound array (DMUA) is used for both lesion formation and bubble activity monitoring with single transmit focusing (STF) imaging. The beam sequencing is designed to maintain spatial and temporal synchronization between therapy and imaging pulse, this ensures optimum STF imaging performance during the respective therapy application for reliable bubble activity detection. The DMUA is driven by a custom designed 32-channel arbitrary waveform generator and linear power amplifier. Use of arbitrary waveform and linear output allows optimum synthesis of multiple-focus pattern with minimum pre-focal artifacts. The beamformed RF data has been shown to be very sensitive to cavitation activity in response to HIFU in a variety of modes (e.g. boiling cavitation), which is characterized by sudden increase in echogenicity that could occur within milliseconds of the HIFU application. The STF beamforming and the signal processing chain are implemented on a multi-GPU platform and frame rate in excess of 500 fps can be achieved. We present results from a series of experiments in bovine cardiac tissue demonstrating the robustness and increased speed of volumetric lesion formation. Results from our experiments demonstrate the feasibility of producing spatial and temporal modulations of multiple-focus DMUA patterns based on STF imaging feedback with millisecond resolution. This fine temporal and spatial control allows for achieving significant acceleration of volumetric coagulation rates within the target volume without overexposure to the tissue in the prefocal region.