title: Full-wave acoustic and thermal modeling of transcranial ultrasound propagation and investigation of skull-induced aberration correction techniques: a feasibility study.
abstract: 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 allow for partial compensation of skull-induced aberrations by application of precalculated phase and amplitude corrections.
  
  
  METHODS
  An integrated numerical framework allowing for 3D full-wave, nonlinear acoustic and thermal simulations has been developed and applied to tcFUS. Simulations were performed to investigate the impact of skull aberrations, the possibility of extending the treatment envelope, and adverse secondary effects. The simulated setup comprised an idealized model of the ExAblate Neuro and a detailed MR-based anatomical head model. Four different approaches were employed to calculate aberration corrections (analytical calculation of the aberration corrections disregarding tissue heterogeneities; a semi-analytical ray-tracing approach compensating for the presence of the skull; two simulation-based time-reversal approaches with and without pressure amplitude corrections which account for the entire anatomy). These impact of these approaches on the pressure and temperature distributions were evaluated for 22 brain-targets.
  
  
  RESULTS
  While (semi-)analytical approaches failed to induced high pressure 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 locations several centimeters off the geometric focus), generation of sharper foci, and increased targeting accuracy. While the prediction of achievable aberration correction appears to be unaffected by the detailed bone-structure, proper consideration of inhomogeneity is required to predict the pressure distribution for given steering parameters.
  
  
  CONCLUSIONS
  Simulation-based approaches to calculate aberration corrections may aid in the extension of the tcFUS treatment envelope as well as predict and avoid secondary effects (standing waves, skull heating). Due to their superior performance, simulationbased techniques may prove invaluable in the amelioration of skull-induced aberration effects in tcFUS therapy. The next steps are to investigate shear-wave-induced effects in order to reliably exclude secondary hot-spots, and to develop comprehensive uncertainty assessment and validation procedures.
authors.affiliation: Foundation for Research on Information Technologies in Society (IT'IS), Zeughausstrasse 43, Zürich, 8004 Switzerland ; Swiss Federal Institute of Technology (ETH) Zürich, Rämistrasse 101, Zürich, 8092 Switzerland.
authors.forename: Adamos
authors.initials: A
authors.lastname: Kyriakou
authors.affiliation: Foundation for Research on Information Technologies in Society (IT'IS), Zeughausstrasse 43, Zürich, 8004 Switzerland.
authors.forename: Esra
authors.initials: E
authors.lastname: Neufeld
authors.affiliation: Center for MR-Research, University Children's Hospital, Steinwiesstrasse 75, Zürich, 8032 Switzerland.
authors.forename: Beat
authors.initials: B
authors.lastname: Werner
authors.affiliation: Swiss Federal Institute of Technology (ETH) Zürich, Rämistrasse 101, Zürich, 8092 Switzerland.
authors.forename: Gábor
authors.initials: G
authors.lastname: Székely
authors.affiliation: Foundation for Research on Information Technologies in Society (IT'IS), Zeughausstrasse 43, Zürich, 8004 Switzerland ; Swiss Federal Institute of Technology (ETH) Zürich, Rämistrasse 101, Zürich, 8092 Switzerland.
authors.forename: Niels
authors.initials: N
authors.lastname: Kuster
keywords: Aberration correction; Acoustic modeling; Focusing; Thermal modeling; Transcranial focused ultrasound; Treatment envelope; Treatment planning framework