Elementwise approach for simulating transcranial MRI-guided focused ultrasound thermal ablation

Abstract

This work explored an element-wise approach to model transcranial MRI-guided focused ultrasound (TcMRgFUS) thermal ablation, a noninvasive approach to neurosurgery. Each element of the phased array transducer was simulated individually and could be simultaneously loaded into computer memory, allowing for rapid calculation of the pressure field for different phase offsets used for beam steering and aberration correction. We simulated the pressure distribution for 431 sonications in 32 patients, applied the phase and magnitude values used during treatment, and estimated the resulting temperature rise. We systematically varied the relationship between CTderived skull density and the acoustic attenuation and sound speed to obtain the best agreement between the predictions and MR temperature imaging (MRTI). The optimization was validated with simulations of 396 sonications from 40 additional treatments. After optimization, the predicted and measured heating agreed well (R 2 : 0.74 patients 1-32; 0.71 patients 33-72). The dimensions and obliquity of the heating in the simulated temperature maps correlated well with the MRTI (R 2 : 0.62, 0.74 respectively), but the measured heating was more spatially diffuse. The energy needed to achieve ablation varied by an order of magnitude (3.3-36.1 kJ). While this element-wise approach requires more computation time up front, it can be performed in parallel. It allows for rapid calculation of the three-dimensional heating at the focus for different phase and magnitude values on the array. We also show how this approach can be used to optimize the relationship between CTderived skull density and acoustic properties. While the relationships found here need further validation in a larger patient population, these results demonstrate the promise of this approach to model TcMRgFUS.

Comparison Between Ray-Tracing and Full-Wave Simulation for Transcranial Ultrasound Focusing on a Clinical System Using the Transfer Matrix Formalism

“…On average, the correction from Insightec improves the restored pressure by more than 10% as compared to Kranion. Since the Insightec ray-tracing algorithm is proprietary, it is difficult to compare it to other ray-tracing techniques previously published [18], [19], [76], and similarly, comparing different full-wave simulation algorithms [42], [48], [57], [76] without having access to the details of the code is limited. Nevertheless, the manufacturer ray-tracing simulation performed beyond our expectations.…”

“…Again, values used for ( = 1500 / ) and ( = 3100 / ) were taken from [57] and and are minimal and maximal value of the density inside the skull. The second mapping ( ) was derived from using McDannold's 4 th order polynomial relationship between and described in [42]. The third mapping ( ℎ ) was derived from the multi-frequency characterization of the speed of sound in human skulls by Pichardo et al (2011) [40].…”

“…acoustic stars [16], [17]) to non-invasive based on numerical simulations of the ultrasonic propagation through the skull bone with either ray-tracing [10], [18]- [20] angular spectrum [21]- [24], finite difference [25]- [32] or pseudo-spectral schemes [33]- [37]; or a combination of numerical phase estimation adjusted with echoes from injected micro-bubbles [38]. For all numerical methods, the acoustic properties such as density, speed of sound and attenuation are derived from CT or MR imaging of the skull bone [25], [39]- [42].…”

Technical Principles and Clinical Workflow of Transcranial MR-Guided Focused Ultrasound

“…These regions, along with sensitive neurovascular structures, can be blocked as “no-pass zones” using the MRgFUS system software, which turns off the number of active transducer elements to minimize energy deposition in these regions. Numerical simulations of transskull ultrasound propagation may provide the opportunity to investigate treatment feasibility preoperatively [37-39].…”

Predicting Bone Marrow Damage in the Skull After Clinical Transcranial MRI-Guided Focused Ultrasound With Acoustic and Thermal Simulations

“…To avoid overheating the skull, scalp, and brain surface, a low acoustic frequency is used to minimize absorption in the bone, a hemispherical transducer design is employed to distribute the acoustic energy over a large surface area, and the head is actively cooled with chilled and circulating water. The acoustic energy needed to achieve an effective thermal dose at the focus varies substantially [12], Furthermore, if the symptoms are not improved, sonications are repeated at different focal targets. Thus, both the peak acoustic energy as well as the total acoustic energy delivered over the course of a treatment can vary substantially.…”