Ultrasonography-based 2D motion-compensated HIFU sonication integrated with reference-free MR temperature monitoring: a feasibility studyex vivo

Abstract

Magnetic resonance imaging (MRI) and ultrasonography have been used simultaneously in this ex vivo study for the image-guidance of high intensity focused ultrasound (HIFU) treatment in moving tissue. A ventilator-driven balloon produced periodic and non-rigid (i.e. breathing-like) motion patterns in phantoms. MR-compatible ultrasound (US) imaging enabled near real-time 2D motion tracking based on optical flow detection, while near-harmonic reference-free proton resonance frequency shift (PRFS) MR thermometry (MRT) was used to monitor the thermal buildup on line. Reference-free MRT was applied to gradient-echo echo-planar imaging phase maps acquired at the frame rate of 250 to 300 ms/slice with voxel size 1.25×1.25×5 mm(3). The MR-US simultaneous imaging was completely free of mutual interferences while minor RF interferences from the HIFU device were detected in the far field of the US images. The effective duty-cycle of the HIFU sonication was close to 100 % and no off-interval was required to temporally decouple it from the ultrasonography. The motion compensation of the HIFU sonication was achieved with an 8 Hz frame rate and sub-millimeter spatial accuracy, both for single-focus mode and for an iterated multi-foci line scan. Near harmonic reference-less PRFS MRT delivered motion-robust thermal maps perpendicular or parallel to the HIFU beam (0.7 °C precision, 0.5 °C absolute accuracy). Out-of-plane motion compensation was not addressed in this study.

Hybrid Ultrasound/Magnetic Resonance Simultaneous Acquisition and Image Fusion for Motion Monitoring in the Upper Abdomen

“…The temperature-sensitive MR images 22,23 are not optimal to track abdominal organ motion because the temporal resolution is traded off over the SNR and the T2* inherent contrast of GRE sequences is low. The ex vivo feasibility of 2D near-real-time (8 Hz) motion-compensated HIFU treatment, integrated with near-harmonic reference-free MRT, 44 was recently reported ex vivo, 27 the motion compensation being achieved based on 2D US data jointly with high-resolution multiplanar proton resonance frequency shift MRT. Advantageously, this approach does not require any underlying predictive model and is compatible with nonperiodic motion.…”

“…24Y26 Motion robust near real-time MR thermometry (ie, reference-free MRT) integrated with high temporal resolution US-based motion tracking would be a near-ideal image guidance environment for HIFU ablation. 27 The technical challenges arising from the mutual interaction of the 2 devices US and MRI (potentially generating static or dynamic artifacts) are not trivial. 9,10 Ultrasound and MR image coregistration was first reported using separate acquisitions at different times.…”

Magnetic Resonance–Guided Shielding of Prefocal Acoustic Obstacles in Focused Ultrasound Therapy

“…10 Volumetric sonication with interleaved foci inherently decreases the thermal contrast between the focal area and the near field and will proportionally decrease the effectiveness of any rib-sparing method. These effects, together with motion-related issues when using active tracking of the respiratory motion, 28 require further experimental investigations and were considered beyond the purpose of this proofof-concept study.…”

Spatio-temporal quantitative thermography of pre-focal interactions between high intensity focused ultrasound and the rib cage

“…HIFU treatment planning and hardware control was achieved using ThermoguideÔ software (Image Guided Therapy). Further details concerning the HIFU platform are provided in the study by Auboiroux et al [32].…”