Treatment of near-skull brain tissue with a focused device using shear-mode conversion: a numerical study

Abstract

Shear mode transmission through the skull has been previously proposed as a new trans-skull propagation technique for noninvasive therapeutic ultrasound (Clement 2004 J. Acoust. Soc. Am. 115 1356-64). The main advantage of choosing shear over longitudinal mode resides on the fact that there is less wavefront distortion with the former. In the present study, the regions of the brain suitable for shear-mode transmission were established for a simple focused ultrasound device. The device consists of a spherically curved transducer that has a focal length of 10 cm, an aperture between 30 degrees and 60 degrees and operates at 0.74 MHz. The regions suitable for shear-mode transmission were determined by the shear wave acoustic windows that matched the shape of the device acoustic field. The acoustic windows were calculated using segmentation and triangulation of outer and inner faces of skull from 3D-MRI head datasets. Nine heads of healthy adults were analyzed. The surface considered for the calculations was the head region found above the supra-orbital margin. For every inspected point in the brain volume, the axis of the device was determined by the vector between this inspection point and a point located in the center of the brain. Numerical predictions of the acoustic field, where shear-mode conversion through the skull was considered, were obtained and compared to the case of water-only conditions. The brain tissue that is close to the skull showed suitable acoustic windows for shear waves. The central region of the brain seems to be unreachable using shear-mode. Analysis of the acoustic fields showed a proportional relation between the acoustic window for shear mode and the effective degree of focusing. However, this relation showed significant differences among specimens. In general, highly focused fields were obtained when the acoustic window for shear waves (A(SW)) intersected more than 67% of the entering acoustic window (A(TX)) of the device. The average depth from the inner surface of the skull showing this intersection value was 13 +/- 10 mm (mean +/- SD). The differences of the degree of focusing observed among patients suggest that the intersection A(SW) intersection A(TX) can be used as a preliminary criterion for screening and calculation of the acoustic fields should confirm the degree of focusing patient by patient. In conclusion, shear waves provide a useful method for trans-cranial focusing in regions close to the skull surface.

Catheter Hydrophone Aberration Correction for Transcranial Histotripsy Treatment of Intracerebral Hemorrhage: Proof-of-Concept

“…Similar effects to these were seen by Vignon et al where the improvement in corrective delays acquired from a single point became worse than the uncorrected case at steering angles greater than 30° [74]. However, these effects were attributed to a conversion to shear waves at incidence angles greater than 30°, which the correction did not account for [76]. Due to the hemispherical shape of the array used here, the max incidence angle was on the order of about 8° and shear wave conversion at larger steering angles was likely not a factor.…”

Transcranial passive acoustic mapping with hemispherical sparse arrays using CT-based skull-specific aberration corrections: a simulation study

“…A previously reported 3-layered ray acoustic model that includes shear wave propagation within the skull bone (Pichardo & Hynynen 2007) was augmented by including corrections that take into account the skull’s spatially dependent material properties. This was achieved by calculating spatially averaged acoustical properties independently for each ray path traversing the skull.…”

A viscoelastic model for the prediction of transcranial ultrasound propagation: application for the estimation of shear acoustic properties in the human skull

“…The shear wave, which is often much slower than longitudinal waves but also much more attenuated (White et al 2006), is converted back into a longitudinal wave when it reaches the skull cavity. Because the average speed of sound of shear waves in the skull is close to the speed of sound in water or soft tissue (Clement et al 2004, White et al 2006), accounting for this shear-mode conversion is of interest for applications targeting the cortex and subcortex brain tissue (Pichardo & Hynynen 2007). Recent studies have proposed transcranial sound transmission models that consider shear-wave propagation (Pichardo & Hynynen 2007, Pulkkinen et al 2014) but the lack of precise correlation between the density and the shear speed of sound often limits the applicability of the models.…”