3D Printed skull bone phantoms for experimental validation of simulated transcranial ultrasound propagation

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1 1 3D Printed skull bone phantoms for experimental validation of simulated transcranial ultrasound propagation James Robertson, Elly Martin, Daniil Nikitichev, Bradley Treeby Biomedical Ultrasound Group (BUG) Department of Medical Physics and Biomedical Engineering University College London

2 2 Rationale Why bone mimics? Transcranial focused ultrasound High intensity ablation Opening the blood-brain barrier Modulation of neurons stimulate or supress activity Focusing through skull complicated by aberration of field Accurate simulation of transcranial ultrasound necessary for time-reversal focusing and prediction of intracranial fields This process must be tested and validated experimentally

3 Mesh based phantom manufacture pipeline 3 Find appropriate materials Characterise acoustic properties Segmentation of MR images Create and refine mesh Print phantom Perform experiments Mount in registered geometry Print holders

4 4 Finding a suitable bone mimicking material: Acoustic properties should be similar to bone: Material Sound-Speed [m/s] Density [kg/m3] 1 MHz [db/cm] Cortical Bone Trabecular Bone Measurement frequency 1 ~ 1 db/cm ~ 5 db/cm ~ 21 db/cm ~9 26 db/cm Chosen materials should be characterised by measurements if properties variable

5 5 Available 3D printing techniques At UCL easy access to: Fused Deposition Modelling (FDM) Ultimaker 2.0 in PolyLactic acid (PLA) Rapid, cheap, good for prototyping, making mounts/holders Low quality print, contains air, not suitable as US phantom Poly Jet printing Stratasys Objet printer in VeroBlack Slower, more expensive High quality print (smooth/high resolution, solid, rigid)

6 6 Our chosen bone phantom materials VeroBlack 3D printing substrate for Stratasys Objet printer Prints cost ~ 40 apiece & take ~24 hrs Araldite 1302 (cast phantoms) Two part resin 35 for 900g, which will make several phantoms + plus cost of positive/mold Material Sound-Speed [m/s] Density [kg/m3] 1 MHz [db/cm] Cortical Bone Trabecular Bone VeroBlack Araldite Clarke et al, PMB, 1994

7 Medical image to mesh 7 T1 weighted MR scan Segment skull bone e.g. Seg3D, FSL Convert to mesh: iso2mesh 3 toolbox (Matlab) Imperial College brain development dataset Final mesh as.stl Manipulate in CAD software: cut, smooth, repair 3 Fang and Boas, IEEE ISBI 2009

8 Computer Aided Design: Holders 8 Printable designs Designed in FreeCAD, solidworks etc. Subtraction of phantom mesh from geometric shape Printed on Ultimaker 2.0 in PolyLactic acid (PLA)

9 9 Computer Aided Design: Holders Side note: Laser Cutting Designed in Adobe Illustrator, 2D drawing Laser cut from Perspex 2D or construct 3D shapes as necessary

10 10 Producing phantoms from 3D printed molds Phantoms can be cast in other materials: Directly print mold Use printed mesh based phantom to create mold Fill with other material

11 Field measurement after propagation through phantom 11 Replicate measurement set up in simulation Breadboard, optical mounts, CAD holders for stereotaxis Source: 1 MHz spherically focused HIFU transducer Detector: PVDF Needle hydrophone Simulations: k-wave Matlab toolbox on supercomputer cluster

12 VeroBlack Araldite Comparison of measurement and simulation 12 Measurement Simulation Focal pressure Difference: focal pressure 0.8% focal position 0.6 mm focal volume 11.6% Difference: focal pressure 0.8% focal position 0.9 mm focal volume 2.1%

13 Summary 13 3D printing materials useful as bone mimics for ultrasound applications Realistic anatomical phantoms can be produced Properties and geometry can be well characterised Excellent tools for validating ultrasound simulations in biologically relevant media For more info:

14 14 Questions? Thanks: Eve Hatten, UCL Jiri Jaros, Brno Uni. Technology Nishant Ravikumar, Imperial College Zeike Taylor, Imperial College