Non-invasive transcranial ultrasound therapy based on a 3D CT scan: protocol validation andin vitroresults

Abstract

A non-invasive protocol for transcranial brain tissue ablation with ultrasound is studied and validated in vitro. The skull induces strong aberrations both in phase and in amplitude, resulting in a severe degradation of the beam shape. Adaptive corrections of the distortions induced by the skull bone are performed using a previous 3D computational tomography scan acquisition (CT) of the skull bone structure. These CT scan data are used as entry parameters in a FDTD (finite differences time domain) simulation of the full wave propagation equation. A numerical computation is used to deduce the impulse response relating the targeted location and the ultrasound therapeutic array, thus providing a virtual time-reversal mirror. This impulse response is then time-reversed and transmitted experimentally by a therapeutic array positioned exactly in the same referential frame as the one used during CT scan acquisitions. In vitro experiments are conducted on monkey and human skull specimens using an array of 300 transmit elements working at a central frequency of 1 MHz. These experiments show a precise refocusing of the ultrasonic beam at the targeted location with a positioning error lower than 0.7 mm. The complete validation of this transcranial adaptive focusing procedure paves the way to in vivo animal and human transcranial HIFU investigations.

Accurate simulation of transcranial ultrasound propagation for ultrasonic neuromodulation and stimulation

“…[9][10][11][12] In finite difference methods, partial derivatives are calculated using a linear combination of function values at neighboring grid points. The finite difference approximations are derived by combining local Taylor series expansions truncated to a fixed number of terms.…”

“…Chauvet et al 11 confirmed the potential for model-driven TR-based focusing inside the human head to millimeter precision, verified by MRI thermometry. Marquet et al 12 showed that model-driven TR is capable of restoring 90% of the peak pressure that can be obtained with gold-standard hydrophone based methods when focusing through an ex vivo skull bone. However, model-driven TR remains subject to systematic errors and uncertainties with a resulting loss in focusing quality or efficiency.…”

“…11,12 Recently a k-space corrected, pseudospectral time domain (PSTD) numerical scheme was used in modeldriven TR and shown to give comparable accuracy. 13 Both FDTD and PSTD schemes use consistent approximations to TABLE II.…”

Sensitivity of simulated transcranial ultrasound fields to acoustic medium property maps

“…Absorption values are inflated to imitate the effect of scattering & reflective losses in the skull layer, which are the dominant attenuating effects [11]. This technique has been used in multiple studies, including a modification using a homogeneous absorption map [12,13]. Several other models based on empirical relationships between CT derived density and acoustic sound speed and absorption have been also proposed [14].…”

A magnetic resonance imaging, histological, and dose modeling comparison of focused ultrasound, radiofrequency, and Gamma Knife radiosurgery lesions in swine thalamus

“…Recent advances in ultrasound transducer technology 3,6,7,27,28,36,37,51,52,60 have enabled transcranial sonication to be precisely focused to deep targets in the brain with MR guidance and temperature monitoring, 8,15,30,40,42 avoiding the superficial burns that were noted in earlier studies. 35 In fact, the first human trials for treatment of glioma and neuropathic pain have proven that FUS can be delivered through the intact human skull to heat targets 39 and even ablate them 38 with precision.…”