MR thermometry-guided ultrasound hyperthermia of user-defined regions using the ExAblate prostate ablation array.

Background: Hyperthermia therapy (HT) has shown to be an effective adjuvant to radiation, chemotherapy, and immunotherapy. In order to be safe and effective, delivery of HT requires maintenance of target tissue temperature within a narrow range (40-44 °C) for 30-60 min, which necessitates conformal heat delivery and accurate temperature monitoring. The goal of this project was to develop an MR thermometry-guided hyperthermia delivery platform based upon the ExAblate prostate array that would achieve uniform stable heating over large volumes within the prostate, while allowing the user to precisely control the power deposition patterns and shape of the region of treatment (ROT).

Methods: The HT platform incorporates an accelerated multi-slice real time MR thermometry pulse sequence and reconstruction pipeline. Temperature uniformity over a large contiguous area was achieved by multi-point temperature sampling with multi-focal feedback power control. The hyperthermia delivery system was based on an InSightec ExAblate 2100 prostate focused ultrasound ablation system, and HeartVista's RTHawk real-time MRI system integrated with a 3 T MRI scanner. The integrated system was evaluated in experiments with a tissue-mimicking phantom for prolonged exposures with a target temperature increase of 7 °C from baseline.

Results: Five various shapes of the region of treatment, defined on a 5 × 5 grid (35 × 35 mm, 11-25 focal spots per shape), were implemented to evaluate the performance of the system. MR temperature images, acquired after steady state was reached, showed different patterns of heating that closely matched the prescribed regions. Temperature uncertainty of the thermometry acquisition was 0.5 °C. The time to reach the target temperature (2:58-7:44 min) depended on the chosen ROT shape and on the distance from transducer to focal plane. Pre-cooling with circulating water helped to reduce near-field heating.

Conclusions: We have implemented a real-time MR thermometry-guided system for hyperthermia delivery within user-defined regions with the ExAblate prostate array and evaluated it in phantom experiments for different shapes and focal depths. Our results demonstrate the feasibility of using a commercially available endorectal FUS transducer to perform spatially-conformal hyperthermia therapy and could lead to a new set of exciting applications for these devices.