A rapid beam simulation framework for transcranial focused ultrasound.
Authors: Leung SA, Webb TD, Bitton RR, Ghanouni P, Butts Pauly K
Transcranial focused ultrasound is a non-invasive therapeutic modality that can be used to treat essential tremor. Beams of energy are focused into a small spot in the thalamus, resulting in tissue heating and ablation. Here, we report on a rapid 3D numeric simulation framework that can be used to predict focal spot characteristics prior to the application of ultrasound. By comparing with magnetic resonance proton resonance frequency shift thermometry (MR thermometry) data acquired during treatments of essential tremor, we verified that our simulation framework can be used to predict focal spot position, and with patient-specific calibration, predict focal spot temperature rise. Preliminary data suggests that lateral smearing of the focal spot can be simulated. The framework may also be relevant for other therapeutic ultrasound applications such as blood brain barrier opening and neuromodulation.
Introduction
Purpose
Thermal ablation
Study Objective
Develop and validate a rapid, treatment-specific 3D numeric simulation framework using the hybrid angular spectrum method to predict transcranial focused ultrasound focal spot position and temperature rise prior to therapy.
Animal model / Human subject
Homo sapiens (human); strain: N/A; age: None; sex: None
Disease model
Essential tremor
MRI or image guidance method
MR-guided (MR thermometry during treatment; alignment sonications with axial/coronal/sagittal imaging; registration matrices from InSightec ExAblate treatment logs/CT-based registration)
Targeted brain region(s)
Thalamus
Outcomes and Safety
Summary of Outcomes
The hybrid angular spectrum (HAS) simulation accurately predicted transcranial focused ultrasound focal spot position (sub-millimeter errors) and showed a consistent, patient-calibratable linear relationship to MR-measured focal spot temperature rise, and could preliminarily reproduce lateral smearing in a thin temporal bone case. Successful tested parameters included electronically steered sonications (mean steering 1.566 mm, max 2.588 mm), treatments at ~670–680 kHz with acoustic pressures typically 2–3 MPa (up to 4.5 MPa), and comparable performance using single-echo and multi-echo MR thermometry.
Safety-related matter
No adverse effects or patient safety issues were reported in the paper; no clinical adverse events were described. The authors noted exclusion of later sonications showing temperature roll-off consistent with changes in tissue or skull properties over the course of treatment, but did not report this as an adverse effect.
Brain Region
Ultrasound Parameters
Ultrasound instrument
InSightec ExAblate 4000 (InSightec, Tirat Carmel, Israel); hemispherical phased-array transducer, diameter 30 cm; radius of curvature 15 cm; 1024 elements; fundamental frequency 680 kHz
FUS Frequency
680 kHz
Focal Characteristics
Focal depth: None; Focal length: 15 cm; Aperture size: 30 cm
Treatment frequency
Single
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