MRI-controlled rapidly scanned focused ultrasound hyperthermia for temperature sensitive localized drug delivery

Temperature sensitive drug delivery systems have been limited by a lack of versatile, noninvasive methods for applying uniform non-ablative heating. The objective of this study was to characterize and demonstrate an MRI-controlled scanned focused ultrasound system capable of maintaining temporally and spatially uniform target temperatures ex vivo. Degassed turkey breast was heated in a clinical 3T MRI using a single-element focused transducer rapidly scanned along a circular trajectory by an MRI-compatible transducer positioning system. Spatial temperature distribution was measured every 5s using the proton resonance frequency shift. Temperature at the center of the scan trajectory was used as input for proportional-integral control of applied acoustic power. Uniform temperature elevation of 10°C was maintained for several minutes in a 5 mm target diameter using controller gain values identified by numerical simulations. Simultaneous scanning and imaging caused a correctable periodic drift in baseline phase. Temporally and spatially uniform MRI-controlled scanned focused ultrasound hyperthermia was demonstrated ex vivo with a simple feedback control system.