Background: Transcranial focus ultrasound applications applied under MRI-guidance benefit from unrivaled monitoring capabilities, allowing the recording of real-time anatomical information and biomarkers like the temperature rise and/or displacement induced by the acoustic radiation force. Having both of these measurements could allow for better targeting of brain structures, with improved therapy monitoring and safety.
Method: We investigated the use of a novel MRI-pulse sequence described previously in Bour et al., (2017) to quantify both the displacement and temperature changes under various ultrasound sonication conditions and in different regions of the brain. The method was evaluated in vivo in a non-human primate under anesthesia using a single-element transducer (f = 850 kHz) in a setting that could mimic clinical applications. Acquisition was performed at 3 T on a clinical imaging system using a modified single-shot gradient echo EPI sequence integrating a bipolar motion-sensitive encoding gradient. Four slices were acquired sequentially perpendicularly or axially to the direction of the ultrasound beam with a 1-Hz update frequency and an isotropic spatial resolution of 2-mm. A total of twenty-four acquisitions were performed in three different sets of experiments. Measurement uncertainty of the sequence was investigated under different acoustic power deposition and in different regions of the brain. Acoustic simulation and thermal modeling were performed and compared to experimental data.
Results: The sequence simultaneously provides relevant information about the focal spot location and visualization of heating of brain structures: 1) The sequence localized the acoustic focus both along as well as perpendicular to the ultrasound direction. Tissue displacements ranged from 1 to 2 μm. 2) Thermal rise was only observed at the vicinity of the skull. Temperature increase ranged between 1 and 2 °C and was observed delayed relative the sonication due to thermal diffusion. 3) The fast frame rate imaging was able to highlight magnetic susceptibility artifacts related to breathing, for the most caudal slices. We demonstrated that respiratory triggering successfully restored the sensitivity of the method (from 0.7 μm to 0.2 μm). 4) These results were corroborated by acoustic simulations.
Conclusions: The current rapid, multi-slice acquisition and real-time implementation of temperature and displacement visualization may be useful in clinical practices. It may help defining operational safety margins, improving therapy precision and efficacy. Simulations were in good agreement with experimental data and may thus be used prior treatment for procedure planning.
Keywords: Acoustic radiation force imaging; Brain imaging; Magnetic resonance guided high intensity focused ultrasound; Magnetic resonance thermometry; Proton resonance frequency shift.
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