Transcranial focused ultrasound phase correction using the hybrid angular spectrum method

Abstract

The InSightec Exablate system is the standard of care used for transcranial focused ultrasound ablation treatments in the United States. The system calculates phase corrections that account for aberrations caused by the human skull. This work investigates whether skull aberration correction can be improved by comparing the standard of care InSightec ray tracing method with the hybrid angular spectrum (HAS) method and the gold standard hydrophone method. Three degassed ex vivo human skulls were sonicated with a 670 kHz hemispherical phased array transducer (InSightec Exablate 4000). Phase corrections were calculated using four different methods (straight ray tracing, InSightec ray tracing, HAS, and hydrophone) and were used to drive the transducer. 3D raster scans of the beam profiles were acquired using a hydrophone mounted on a 3-axis positioner system. Focal spots were evaluated using six metrics: pressure at the target, peak pressure, intensity at the target, peak intensity, positioning error, and focal spot volume. For three skulls, the InSightec ray tracing method achieved 52 ± 21% normalized target intensity (normalized to hydrophone), 76 ± 17% normalized peak intensity, and 0.72 ± 0.47 mm positioning error. The HAS method achieved 74 ± 9% normalized target intensity, 81 ± 9% normalized peak intensity, and 0.35 ± 0.09 mm positioning error. The InSightec-to-HAS improvement in focal spot targeting provides promise in improving treatment outcomes. These improvements to skull aberration correction are also highly relevant for the applications of focused ultrasound neuromodulation and blood brain barrier opening, which are currently being translated for human use.

Comparison between MR and CT imaging used to correct for skull-induced phase aberrations during transcranial focused ultrasound

“…The centroid of each bead was used as its position in image space. A singular value decomposition-based least squares registration 15,37,48 was performed to achieve point-wise registration between fiducial marker positions in image and transducer space. The transformation matrix that registers the two coordinate systems is:…”

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Differences between skulls result in a broad range of transcranial ultrasound efficiencies across patients, such that the same applied power can lead to a four-fold range in temperature rise at the focal spot 12 .Many methods have been proposed to account for the aberrating effects of the skull in order to refocus the focal spot. These methods include ray tracing [13][14][15] , finite difference time domain (FDTD) 14,[16][17][18][19][20][21][22][23][24][25][26] , pseudo-spectral time domain (PSTD) [27][28][29][30][31] , and hybrid angular spectrum (HAS) 12,[32][33][34][35][36][37] . Although these methods are different in theory and implementation, they have one similarity: they need to computationally model the skull to estimate and correct for its aberrating effects.

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Precise Ultrasound Neuromodulation in a Deep Brain Region Using Nano Gas Vesicles as Actuators

“…One solution to this has been to use phased‐arrays, creating better focus through the skull by employing the phase aberration correction method. [ 48 , 49 ] Alternatively, the plane and low‐frequency ultrasound generated by a single element ultrasound transducer can achieve deep and precise stimulation with the help of GVs. Our findings provide a demonstration of one method to non‐invasively stimulate specific brain regions with LILFU.…”

“…Implementing ultrasound neuromodulation with high spatiotemporal resolution can be challenging, and involves trade‐offs between different concerns. [ 48 ] Higher frequencies of ultrasound can be used to create smaller focal spots, but also have reduced tissue penetration capabilities, and vice versa for lower frequencies. The significant variation between skulls may also have unpredictable effects on the efficacy of transcranial ultrasound.…”

Binary acoustic metasurfaces for dynamic focusing of transcranial ultrasound

“…Several tFUS technologies have been developed to overcome the skull-induced beam aberration. The most commonly used approach is to utilize phased arrays, which focus the ultrasound beam at the desired target by modulating the phase delay of each independent transducer element ( Deng et al, 2020 ; Sukovich et al, 2020 ; Adams et al, 2021 ; Bawiec et al, 2021 ; Leung et al, 2021 ). Although phased arrays have the advantage of being dynamically programmable, these arrays need a large number of transducer elements and complex electronics ( Melde et al, 2016 ).…”