Predicting variation in subject thermal response during transcranial magnetic resonance guided focused ultrasound surgery: Comparison in seventeen subject datasets

Abstract

Purpose: In transcranial magnetic resonance-guided focused ultrasound (tcMRgFUS) treatments, the acoustic and spatial heterogeneity of the skull cause reflection, absorption, and scattering of the acoustic beams. These effects depend on skull-specific parameters and can lead to patient-specific thermal responses to the same transducer power. In this work, the authors develop a simulation tool to help predict these different experimental responses using 3D heterogeneous tissue models based on the subject CT images. The authors then validate and compare the predicted skull efficiencies to an experimental metric based on the subject thermal responses during tcMRgFUS treatments in a dataset of seventeen human subjects. Methods: Seventeen human head CT scans were used to create tissue acoustic models, simulating the effects of reflection, absorption, and scattering of the acoustic beam as it propagates through a heterogeneous skull. The hybrid angular spectrum technique was used to model the acoustic beam propagation of the InSightec ExAblate 4000 head transducer for each subject, yielding maps of the specific absorption rate (SAR). The simulation assumed the transducer was geometrically focused to the thalamus of each subject, and the focal SAR at the target was used as a measure of the simulated skull efficiency. Experimental skull efficiency for each subject was calculated using the thermal temperature maps from the tcMRgFUS treatments. Axial temperature images (with no artifacts) were reconstructed with a single baseline, corrected using a referenceless algorithm. The experimental skull efficiency was calculated by dividing the reconstructed temperature rise 8.8 s after sonication by the applied acoustic power. Results: The simulated skull efficiency using individual-specific heterogeneous models predicts well (R 2 = 0.84) the experimental energy efficiency. Conclusions: This paper presents a simulation model to predict the variation in thermal responses measured in clinical ctMRGFYS treatments while being computationally feasible. C 2016 American Association of Physicists in Medicine. [http://dx

Histologic safety of transcranial focused ultrasound neuromodulation and magnetic resonance acoustic radiation force imaging in rhesus macaques and sheep

“…Skull bone absorbs and dephases ultrasound which introduces a risk of cortical heating, and has been demonstrated to contribute to variations in FUS treatment across patients (Vyas et al, 2016). In our study, hydrophone measurements through ex vivo sheep skull caps resulted in a range of estimated in situ intensities, even when similar acoustic power levels were applied (Fig 4).…”

Focused Ultrasound for Noninvasive, Focal Pharmacologic Neurointervention

“…(B) Simulation of energy deposited by a commercial MRI-guided FUS system (Exablate 4000, Insightec, Haifa, Israel) through the skull. Adapted from Vyas et al (2016). Reprinted with permission from the American Association of Physicists in Medicine.…”

“…Attenuation and phase distortions are introduced in the propagating ultrasound waves by the acoustically heterogenous skull bone ( Clement and Hynynen, 2002 ), which undermine the focus quality and increase the risk for nonspecific energy deposition in off target brain areas. Moreover, the inherent acoustic propagation properties of the skull are patient-specific and vary significantly between subjects ( Vyas et al, 2016 ). However, if multielement FUS arrays are available, phase correction and adaptive focusing techniques can be applied to restore the desired focusing accuracy, given that the transcranial pressure field can be visualized reliably and noninvasively.…”

Histologic evaluation of activation of acute inflammatory response in a mouse model following ultrasound-mediated blood-brain barrier using different acoustic pressures and microbubble doses

“…Even when using the same treatment parameters, the in situ acoustic pressures can differ greatly across human patients due to high inter-subject variability of skull geometry and composition 21 , 22 . Therefore, a secondary aim of our study was to assess whether the degree of glial activation or histopathology were correlated with outcome metrics that do not require knowledge of the in situ acoustic pressure, such as relative signal enhancement on contrast-enhanced magnetic resonance imaging (CE-MRI) or acoustic emissions (AEs) from passive cavitation detection.…”