Full-wave acoustic and thermal modeling of transcranial ultrasound propagation and investigation of skull-induced aberration correction techniques: a feasibility study

Background: Transcranial focused ultrasound (tcFUS) is an attractive noninvasive modality for neurosurgical interventions. The presence of the skull, however, compromises the efficiency of tcFUS therapy, as its heterogeneous nature and acoustic characteristics induce significant distortion of the acoustic energy deposition, focal shifts, and thermal gain decrease. Phased-array transducers featuring hundreds of elements allow for the partial compensation of skull-induced aberrations by calculation and application of precalculated appropriate phase and amplitude corrections. Precise focusing, however, remains a necessity. An integrated numerical framework allowing for 3D full-wave, linear and nonlinear acoustic, and thermal simulations has been developed and applied to transcranial sonicationtcFUS. Simulations were performed to investigate the impact of skull aberrations, compare different aberration correction approaches to achieve refocusing, investigate the possibility of extending the tcFUS treatment envelope, and explore acoustic and thermal secondary effects of the treatment. Methods: The simulated setup comprised an idealized model of the ExAblate Neuro and a detailed MR-based anatomical head model segmented from MR data. Four different approaches were employed to calculate aberration corrections including (analytical calculation of the aberration corrections disregarding tissue heterogeneities,; a semi-analytical ray-tracing approach compensating for the presence of the skull while ignoring soft tissues, and; two simulation-based time-reversal approaches with and without pressure amplitude corrections which taking into account account for the entire anatomy). These impact of these approaches on the pressure and temperature distributions were evaluated for 22 brain-targets in the brain, and their impact on the resulting pressure and temperature distributions was compared. Results: While (semi-) analytical approaches failed to induced high pressure values or ablative temperatures in any but the targets in the close vicinity of the geometric focus, simulation-based approaches indicate the possibility of considerably extending the treatment envelope (including targets below the transducer level and to the locations several centimeters off the geometric focus), generation of sharper foci, and increased targeting accuracy. While the prediction of achievable simulation-based aberration correction appears to be unaffected by the detailed nature of the bone-structure, proper consideration of inhomogeneity is required to correctly predict the pressure level and distribution for a given set of given steering parameters. Conclusions: Utilizing Simulation-based approaches to calculate aberration corrections for tcFUS therapies may aid in the extension of the tcFUS treatment envelope as well as predict and avoid possible secondary effects (standing waves, skull heating...) of these procedures, e.g., standing waves and skull heating. Thus, dDue to their superior performance, simulation-based techniques may prove invaluable in the amelioration of the undesirable skull-induced aberration effects in tcFUS therapy. The next steps are (i) to investigate shear-wave-induced effects which must be considered to reliably exclude secondary hot-spots and permit a clinical extension the treatment envelope, and (ii) to develop a comprehensive uncertainty assessment and validation procedures to enable these techniques to be used in treatment planning.