High frame rate ultrasound monitoring of high intensity focused ultrasound-induced temperature changes: A novel asynchronous approach

Abstract

When applying diagnostic ultrasound to guide focused ultrasound (FUS) thermal therapy, high frame rate ultrasonic temperature monitoring is valuable in better treatment control and dose monitoring. However, one of the potential problems encountered when performing ultrasonic temperature monitoring of a FUS procedure is interference between the FUS and imaging systems. Potential means of overcoming this problem include the switch between the FUS system and the imaging system (limited by a reduced frame rate of thermal imaging) or the development of complex synchronization protocols between the FUS therapeutic system and the ultrasonic imaging apparatus (limited by implementation efforts both for software and hardware designs, and low potential for widespread diffusion). In this paper, we apply an asynchronous idea to retrieving high frame rate and FUS-interference-free thermal imaging during FUS thermal therapy. Tone-burst delivery mode of the FUS energy is employed in our method, and the imaging and FUS systems are purposely operated in an asynchronous manner. Such asynchronous operation causes FUS interference to saturate sequential image frames at different A-lines; thus clean A-lines from several image frames can be extracted by a total energy-thresholding technique and then combined to reconstruct interference-free B-mode images at a high frame rate for temperature estimation. The performance of the proposed method is demonstrated by phantom experiments. Relationships of the FUS duty-cycle with the maximum reconstructed frame rate of thermal imaging and the corresponding maximum temperature increase are also studied. Its performance was also evaluated and compared with the existing manually synchronous and synchronous approaches. By proper selection of the FUS duty-cycle, using our method, the frame rate of thermal imaging can be increased up to tenfold compared with that provided by the manually synchronous approach. Our method is capable of pushing the frame rate of thermal images to the same order as that of the synchronous approach while avoiding sacrificing the observable field of view (FOV) of temperature mapping. The asynchronous method can be easily implemented and allows thermal imaging at an improved frame rate, without the need for complex synchronization protocols between the FUS therapeutic system and the ultrasonic imaging apparatus and without sacrifice of observable FOV. This technology may provide an effective alternative for real-time temperature measuring during thermal ablation procedures and can be easily integrated into current high intensity focused ultrasound systems.