Full-Wave Acoustic and Thermal Modeling of Transcranial Ultrasound Propagation and Investigation of Skull-Induced Aberration Correction Techniques: A Feasibility Study

Adamos Kyriakou, Esra Neufeld, Beat Werner, Gábor Székely, and Niels Kuster, Journal of Therapeutic Ultrasound, Volume 3, Issue 11, July 2015, online July 31, 2015

Transcranial focused ultrasound (tcFUS) is an attractive noninvasive modality for neurosurgical interventions. The efficacy of tcFUS is adversely affected by the presence of the skull, the heterogeneous nature and acoustic characteristics of which induce significant distortion of the acoustic energy deposition, focal shifts, and decreases in thermal gain. An integrated numerical framework that allows for 3D full-wave nonlinear acoustic and thermal simulations, integrated into ZMT Zurich MedTech’s simulation platform SIM4LIFE, has been developed and applied to tcFUS. Simulations were performed to investigate the impact of skull aberrations, the possible extension of the treatment envelope, and potential adverse secondary effects. The simulation setup included an idealized model of the ExAblate®Neuro, a commercially available tcFUS system produced by InSightec (Israel) and a detailed anatomical head model based on Duke of the IT’IS Foundation’s Virtual Population suite of high-resolution MRI-based computational human phantoms. Four different approaches, ranging from semi-analytical to simulation-based, were employed to calculate aberration corrections and compensate for the presence of the skull, and the performance of these approaches was evaluated for 22 cerebral targets. The analytical and semi-analytical approaches induced high pressures and/or ablative temperatures only in targets close to the transducer’s geometric focus but were far less efficient for evaluating effects on targets in more remote brain regions. Conversely, simulation-based approaches yielded increased targeting accuracy with more sharply demarcated focal regions, producing lesions at targets centimeters away from the geometric focus, indicating that it may be possible to considerably extend the treatment envelope, including to targets below focal plane of the transducer.

The scientific and technical impact of the study can be summarized as:

• Development, implementation, and validation of a novel and practical computational framework that enables numerical modeling within minutes of complex and realistic focused ultrasound treatment setups
• Derivation of an augmented time-reversal approach that yields very flexible visualization for evaluation of focused ultrasound therapies
• Direct comparison of analytical and semi-analytical approaches currently employed in clinical environments against simulation-based techniques for a wide variety of cerebral targets in a realistic setup
• Presentation of an approach that may be applied to tcFUS to allow for increased targeting accuracy, generation of sharper foci, and the extension of the treatment envelope

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