Numerical Evaluation of the Influence of Skull Heterogeneity on Transcranial Ultrasonic Focusing

Abstract

In transcranial penetration, ultrasound undergoes refraction, diffraction, multi-reflection, and mode conversion. These factors lead to phase aberration and waveform distortion, which impede the realization of transcranial ultrasonic imaging and therapy. Ray tracing has been used to correct the phase aberration and is computationally more efficient than traditional full-wave simulation. However, when ray tracing has been used for transcranial investigation, it has generally been on the premise that the skull medium is homogeneous. To find suitable homogeneity that balances computational speed and accuracy, the present work investigates how the focus deviates after phaseaberration compensation with ray tracing using time-reversal theory. The waveforms are synthetized with ray tracing for phase aberration, by which the properties of the skull bone are simplified for refraction calculation as those of either (i) the cortical bone or (ii) the mean of the entire skull bone, and the focusing accuracy is evaluated for each hypothesis. The propagation of ultrasound for transcranial focusing is simulated with the elastic model using the k-space pseudospectral method. Unlike the fluid model, the elastic model does not omit shear waves in the skull bones, and the influence of that omission is investigated, with the fluid model resulting in a focal deflection of 0.5 mm. The focusing deviations are huge when the properties of the skull bone are idealized with ray tracing as those of the mean of the entire skull bone. The focusing accuracy improves when the properties of the skull bone are idealized as those of the cortical bone. The results reveal minimal deviation (8.6, 3.9, and 3.2% in the three Cartesian coordinates) in the focal region and suggest that transcranial focusing deflections are caused mostly by ultrasonic refraction on the surface of the skull bone. A heterogeneous skull bone causes wave bending but minimal focusing deflection. The proposed simplification of a homogeneous skull bone is more accurate for transcranial ultrasonic path estimation and offers promising applications in transcranial ultrasonic focusing and imaging.

Transcranial ultrasound simulations: A review

“…This method is used in Refs. [11–13, 15, 19–21, 26, 48–51]. This relationship is similar to Equations (16) and () as it implies a linear fit between speed of sound and HU.…”

“…In Jiang et al., 51 the velocity on the skull surface is taken to be that in cortical bone (which influences refraction computations) and the velocity inside the skull is taken as being the averaged value on the ray paths (which influences the time‐of‐flight computations). The density is taken as being the maximum density as it influences the refraction computations.…”

Numerical and experimental evaluation of low‐intensity transcranial focused ultrasound wave propagation using human skulls for brain neuromodulation

“…As the aperture size of the 500 kHz ultrasound transducer is 30 mm but the 350 kHz ultrasound transducer has an aperture size of 50 mm, we speculated the acoustic refraction difference induced by disparate transducer aperture size might be the reason behind this effect. 54 Detailed study and discussion are presented in Sections 3.3 and 4 .…”

Transcranial Focused Ultrasound Neuromodulation of Voluntary Movement-Related Cortical Activity in Humans

“…No linear relationship was found either between the skull thickness and the transcranial maximal acoustic pressure or between the skull thickness and the ultrasound full-width at half magnitude volume behind the skull [58]. To address this issue, tFUS dose and target planning before the neuromodulation experiment would be required and should be individualized; such a planning may rely on a priori computer simulations for predicting acoustic fields within individual skull cavity based on respective skull model established from CT scans [58,59]. Moreover, given the "cigar" shaped ultrasound beam, it is inevitably to cover other functional brain areas beyond the primary motor cortex.…”