Real-time RNN-based acoustic thermometry with feedback control

A major obstacle to the widespread adoption of HIFU therapy is the development of a suitable method of monitoring the a blation therapy in real-time. While MR-thermometry has emerged as a promising method for HIFU therapy monitoring, acoustic guidance has continuously been sought for reasons of cost and practicality. We have previously demonstrated the potential of acoustic thermometry, by using a recurrent neural network (RNN) to estimate changes in tissue temperature during HIFU ablation therapies. A limitation of this method is that an excessive therapeutic dose can cause multiple, non-linear changes within the ultrasound data, resulting in unreliable temperature estimates from the RNN. Accordingly, we propose a revised method of dosing wherein closed loop feedback is used to provide a controlled and specific dose; not only to ensure an efficacious lesion, but also to preserve the integrity of the ultrasound image, thereby producing accurate temperature estimates from the RNN. This investigation of controlling the thermal dose using feedback was performed on ex vivo bovine liver. The acoustic parameters used as inputs to the RNN were: changes in integrated backscatter intensity, thermal strain, and decorrelation. The therapeutic dose was delivered using a 1.1 MHz, 2D-array HIFU transducer transmitting at regular intervals during a 40-second dose. Interleaved between these regular HIFU dose intervals, volumetric ultrasound images were acquired on a Siemens ACUSON SC2000, with a 4Zlc probe. Feedback was introduced to the system by varying the HIFU duty cycle, in order to minimize the difference between a desired temperature curve (assigned a priori) and the estimated focal temperature values. Two methods were used for obtaining the focal temperature: the first was direct measurement using a 75-micron copper-constantan thermocouple embedded within the liver sample, and the second was temperature estimation as calculated from the RNN-based output temperatures estimated from the ultrasound data. The ability of each method to deliver a controlled, therapeutic dose was compared. The feedback controlled dosing both created effective lesions (an indicator of assured cell death), and also sustained the integrity of the ultrasound image signal features, producing acceptably accurate temperature estimates. While thermocouple measured focal temperatures were more precise than the temperature estimates derived from RNN-based acoustic thermometry, effective therapeuti