Evaluation of polyvinyl alcohol cryogel as an acoustic coupling medium for low‐intensity transcranial focused ultrasound

Abstract

Transcranial focused ultrasound (FUS) has emerged as a noninvasive neuromodulatory modality with exquisite depth penetration and spatial selectivity. Liquids, such as degassed water or mineral oil, are used as acoustic coupling media between the ultrasound‐generating transducer and the brain; however, they require a separate container that limits the spatial orientation of the transducers with respect to the sonication target. Nonliquid, gel‐like materials that do not require a housing container have been sought after as coupling media to overcome such limitations. Polyvinyl alcohol (PVA), when dissolved in water and undergone freeze–thaw cycle(s), forms a flexible hydrogel having a high level of acoustic transmission. To examine the feasibility of the PVA cryogel as the coupling material for transcranial FUS, the mechanical properties (in terms of its Young's modulus) and acoustic attenuation of the PVA cryogel were examined using different concentrations and number of freeze–thaw cycles. The cryogel with 6 or 7% (w/v) concentrations and two freeze–thaw cycles showed minimum pressure attenuation (on the order of 1%) across the different ultrasound frequencies (250–650 kHz). The cryogel was molded to fit around a single‐element FUS transducer and was applied to a head phantom, showing the flexibility in orienting the sonication paths at different angles and depths. The use of the cryogel did not alter the location and shape of acoustic focal profile compared to the one measured in the degassed water. The present work suggests that PVA cryogel may be used as an alternative acoustic coupling medium for low‐intensity FUS applications.

Transcranial focused ultrasound stimulation of human primary visual cortex

“…After aligning the acoustic focus to the V1 in reference to the individual-specific functional data (from fMRI) and skull neuroanatomy (from CT) (example shown in Fig. 1c ), FUS was applied to the V1 through a polyvinyl alcohol (PVA) hydrogel 21 , which was compressed to ~1 cm thickness around the contour of the skin (from its original thickness of 15 mm), achieved tight acoustic coupling while maintaining the orientation of the sonication entry as perpendicular as possible to the scalp ( Fig. 1d ).…”

“…The subject’s anatomical features with respect to the inion ( i.e. , external occipital protuberance) were used to initially position the head with respect to the transducer, and subsequent T1–weighted MR imaging (GRAPPA sequence, acceleration factor = 2, TR/TE = 1,900/2.52 ms, flip angle = 9°, slice thickness = 1 mm, FOV = 25.6 × 25.6 cm 2 , image matrix = 256 × 256, voxel size = 1 × 1 × 1 mm 3 ) was performed to visualize the profile of the FUS transducer (represented in Figs 1 d, 2 and 5 a; visible as a negative contour surrounded by the water-rich hydrogel 21 ) and the spatial distortion in the image was minimal. The use of the high-resolution (1 mm 3 isotropic voxel dimension), T1-weighted anatomical images using a short echo time (2.52 ms) effectively revealed the 3D geometry of the transducer for guidance.…”

Transcranial focused ultrasound stimulation of motor cortical areas in freely-moving awake rats

“…A cone-shaped, polyvinyl alcohol (PVA) hydrogel (7–9% weight per volume; two freeze–thaw cycles, U228-08; Avantor, Center Valley, PA) was manufactured in-house for acoustic coupling between the transducer and scalp (Fig. 1 c, right) (the detailed method can be found elsewhere [ 46 ]). The hydrogel showed negligible pressure attenuation on the order of 1%.…”

Simultaneous acoustic stimulation of human primary and secondary somatosensory cortices using transcranial focused ultrasound

“…The gap between the scalp and the transducer surface was filled with a polyvinyl alcohol (PVA) hydrogel for acoustic coupling. The compressible PVA hydrogel (having a thickness of ~10 mm) which was fitted around the transducer allowed for adjustment of acoustic focal depth in the range of 5–20 mm (detailed implementation was described elsewhere [30]). The subject’s hair was parted in the middle of each sonication entry point, and a generic ultrasound hydrogel (Aquasonics, Parker Laboratories, Fairfield, NJ) was applied onto the exposed scalp.

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