Experimental validation of a finite-difference model for the prediction of transcranial ultrasound fields based on CT images

Abstract

The prevalence of stroke worldwide and the paucity of effective therapies have triggered interest in the use of transcranial ultrasound as an adjuvant to thrombolytic therapy. Previous studies have shown that 120-kHz ultrasound enhanced thrombolysis and allowed efficient penetration through the temporal bone. The objective of our study was to develop an accurate finite-difference model of acoustic propagation through the skull based on computed tomography (CT) images. The computational approach, which neglected shear waves, was compared with a simple analytical model including shear waves. Acoustic pressure fields from a two-element annular array (120 kHz and 60 kHz) were acquired in vitro in four human skulls. Simulations were performed using registered CT scans and a source term determined by acoustic holography. Mean errors below 14% were found between simulated pressure fields and corresponding measurements. Intracranial peak pressures were systematically underestimated and reflections from the contralateral bone were overestimated. Determination of the acoustic impedance of the bone from the CT images was the likely source of error. High correlation between predictions and measurements (R2=0.93 and R2=0.88 for transmitted and reflected waves amplitude, respectively) demonstrated that this model is suitable for quantitative estimation of acoustic fields generated during 40-200 kHz ultrasound-enhanced ischemic stroke treatment.

In silico Study of Low-Frequency Transcranial Ultrasound Fields in Acute Ischemic Stroke Patients

“…Transcranial ultrasound propagation was simulated using a 3D finite-difference acoustic propagation model described in detail and validated previously (Bouchoux et al, 2012). Briefly, acoustomechanical parameter maps of heads were obtained from the CT images.…”

“…Hence, the M1 segment of the middle cerebral artery was considered as the targeted region of interest (ROI). The acoustic model used in this study was previously validated (Bouchoux et al, 2012). Good agreement was found between simulations based on CT scans and in vitro measurements in human skulls (R 2 =0.93 for transmitted wave amplitude).…”

“…First, a 120-kHz unfocused transducer producing a broad, naturally-focused beam was considered (Bouchoux et al, 2012). Efficient thrombolysis enhancement has been shown in vitro and ex vivo using this frequency (Datta et al, 2006, 2008; Hitchcock et al, 2011).…”

Sonothrombolysis

“…The transcranial ultrasound field produced by this device was evaluated in-vitro and the safety of this approach was demonstrated in a healthy primate model (Shimizu et al 2012). Bouchoux et al (2014) simulated 120 and 500-kHz transcranial ultrasound fields from the head CT scans of 20 ischemic stroke patients using a validated acoustic propagation numerical model (Bouchoux et al 2012). Consistent and homogeneous insonation of the Ml segment of the MCA was achieved at both 120 and 500 kHz.…”

Microfluidic manufacture of rt-PA -loaded echogenic liposomes

“…It is noteworthy that this frequency may not be able to penetrate the skull for nearly 15 % of patients (Wijnhoud et al 2008). For such patients, sub-megahertz frequencies may be employed (Bouchoux et al 2012, 2014) at which microbubbles in the 20–50 µm range are resonant. However, microbubbles larger 7 µm are filtered by the lungs and thus for cavitation nucleation for enhancement of rt-PA thrombolysis.…”