Breath-hold MR-HIFU hyperthermia: phantom and in vivo feasibility

Background: The use of magnetic resonance imaging-guided high-intensity focused ultrasound (MR-HIFU) to deliver mild hyperthermia requires stable temperature mapping for long durations. This study evaluates the effects of respiratory motion on MR thermometry precision in pediatric subjects and determines the in vivo feasibility of circumventing breathing-related motion artifacts by delivering MR thermometry-controlled HIFU mild hyperthermia during repeated forced breath holds. Materials and methods: Clinical and preclinical studies were conducted. Clinical studies were conducted without breath-holds. In phantoms, breathing motion was simulated by moving an aluminum block towards the phantom along a sinusoidal trajectory using an MR-compatible motion platform. In vivo experiments were performed in ventilated pigs. MR thermometry accuracy and stability were evaluated. Results: Clinical data confirmed acceptable MR thermometry accuracy (0.12-0.44 degrees C) in extremity tumors, but not in the tumors in the chest/spine and pelvis. In phantom studies, MR thermometry accuracy and stability improved to 0.37 +/- 0.08 and 0.55 +/- 0.18 degrees C during simulated breath-holds. In vivo MR thermometry accuracy and stability in porcine back muscle improved to 0.64 +/- 0.22 and 0.71 +/- 0.25 degrees C during breath-holds. MR-HIFU hyperthermia delivered during intermittent forced breath holds over 10 min duration heated an 18-mm diameter target region above 41 degrees C for 10.0 +/- 1.0 min, without significant overheating. For a 10-min mild hyperthermia treatment, an optimal treatment effect (TIR > 9 min) could be achieved when combining 36-60 s periods of forced apnea with 60-155.5 s free-breathing. Conclusion: MR-HIFU delivery during forced breath holds enables stable control of mild hyperthermia in targets adjacent to moving anatomical structures.