Hanson, Lars G.

Kuster, Niels

Lee, Hyunjoo Jenny

Montanaro, Hazael

Neufeld, Esra

Pasquinelli, Cristina

Thielscher, Axel

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Mar 29, 2019Investigation of fallacies in focused ultrasound transducer acoustic modelingMontanaro, Hazael; Neufeld, Esra; Kuster, Niels; Pasquinelli, Cristina; Hanson, Lars G. ; Thielscher,Axel; Lee, Hyunjoo JennyPublication date:2018Document VersionEarly version, also known as pre-printLink back to DTU OrbitCitation (APA):Montanaro, H., Neufeld, E., Kuster, N., Pasquinelli, C., Hanson, L. G., Thielscher, A., & Lee, H. J. (2018).Investigation of fallacies in focused ultrasound transducer acoustic modeling. Abstract from The 18thInternational Symposium for Therapeutic Ultrasound , Nashville, United States.CONTROL ID:  2951876TITLE:  INVESTIGATION OF FALLACIES IN FOCUSED ULTRASOUND TRANSDUCER ACOUSTIC MODELINGAUTHORS/INSTITUTIONS:  H. Montanaro, E. Neufeld, N. Kuster, ITIS, Zurich, SWITZERLAND|H. Montanaro, N.Kuster, ETH Zurich, Zurich, SWITZERLAND|C. Pasquinelli, L. Hanson, A. Thielscher, Technical University ofDenmark, Kgs. Lyngby, DENMARK|H. Lee, KAIST, Daejeon, KOREA (THE REPUBLIC OF)|A. Thielscher, DanishResearch Center for Magnetic Resonance, Hvidovre, DENMARK|CURRENT TOPIC:  Modeling and PhysicsPRESENTATION TYPE:  OralOBJECTIVES:  When simulating therapeutic applications of focused ultrasound (FUS), the transducer is typicallymodelled by constructing the transducer surface geometry and imposing a pressure boundary condition (e.g., whenemploying an acoustic pressure wave solver) or a velocity boundary condition (e.g., in combination with an elasticwave solver). However, during experimental validation of transcranial FUS modeling, dramatic deviations betweensimulated and measured pressure distributions (focus location, focus shape, side-foci) that were shown to originatefrom transducer modeling were observed. Here we report the results of a systematic study performed to identify andinvestigate the impact of factors to be considered to obtain realistic models of acoustic exposure by FUS.METHODS:  Acoustic pressure fields generated by curved single-element focused transducers (0.5 MHz) in thepresence and absence of skull obstacles (pig, sheep, and lamb; characterized by computed tomography (CT) andprecisely positioned) were measured with a 3D-scannable, calibrated hydrophone and compared to acousticsimulations of corresponding setup models. The source was initially modeled as a pressure boundary conditionimposed on the transducer surface, which was shaped according to the manufacturer specifications. Subsequently,the model was adapted to reflect the actually measured transducer geometry, which was found to deviate significantlyfrom the specifications. Instead of assuming that the pressure wave originates from the transducer surface, wemodeled the internal structure of the transducer, which features a planar piezoelectric disk below a shaped matchingmaterial. Uncertainties about the internal transducer geometry (primarily the depth of piezo-disk) and materialproperties (speed-of-sound, attenuation, density of the matching material) were considered, and an aperture functionwas introduced and varied to account for the mechanical impact of the transducer wall or for mechanical vibrationmodes.RESULTS:  The sensitivity of the pressure field to the above factors was investigated and, after careful modeladaptation, good agreement between the simulated and measured fields was obtained in the absence of skulls. Focuslocation, shape, and the overall pressure distribution were also in agreement. Particularly, the depth of the piezoelement and the matching material speed-of-sound were found to strongly affect the pressure distribution, whileattenuation primarily impacts the overall intensity. The aperture function influences the occurrence and exact shape ofsecondary foci. Significant deviations are still observed in the presence of skulls and current work focuses on establishing whether thedeviations originate from transducer modeling or from the approach employed to map CT data to acoustic propertydistributions. Literature reveals large uncertainty about the relationship between CT Hounsfield units and acousticproperties, which is further exacerbated by the lack of transferability between different CT scanners (and even morewhen going to MicroCT). The lack of equivalence between pressure and velocity boundary conditions should also be taken into consideration.CONCLUSIONS:  Careful transducer modeling and experimental validation is crucial for reliable simulation of FUSfields, and the current approaches commonly used are found to be unsuitable for extended, curved, or complextransducers. An optimal, but highly demanding and typically infeasible, approach would include complete mechanicalmodeling of the transducer with its housing and fixation. However, a compromise combining 1) improved acousticmodeling of the transducer and its internal structure with 2) aperture functions to account for the missing mechanicalmodeling can be acceptable. Comprehensive uncertainty assessment should typically be performed along withcomputational modeling. Measurement setup and computational model Illustrative measured and simulated pressure fieldsIMAGE CAPTION:  Measurement setup and computational model Illustrative measured and simulated pressure fields AWARDS:  Student Award Competition|Student Travel Awards

Investigation of fallacies in focused ultrasound transducer acoustic modeling

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Investigation of fallacies in focused ultrasound transducer acoustic modeling

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