Dr. Simone A Winkler - Ultra High-Field MRI Open Questions in Engineering and Multiphysics

1. STANFORD UHF MRI PROGRAM ULTRA HIGH-FIELD MRI OPEN QUESTIONS IN ENGINEERING AND MULTIPHYSICS SIMONE A. WINKLER, PH.D. MCGILL UNIVERSITY | 2014/04/16 Department of Radiology Stanford University ETH Zrich | 2014/12/19S. A. WINKLER

2. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION OUTLINE Introduction to UHF MRI Engineering-related challenges in UHF MRI Image shading due to B1 inhomogeneities Image distortion and artefacts due to B0 inhomogeneities MR Safety MR Acoustics ETH ZRICH | 2014/12/19S. A. WINKLER

3. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION STANFORD RADIOLOGICAL SCIENCES LABORATORY JOHANNES KEPLER UNIVERSITY LINZ | 2014/05/08S. A. WINKLER Three 3T GE whole-body MR systems (one being the first GE PET-MR) One 7T GE whole-body MR system Gary Glover Brian Hargreaves Daniel Spielman Brian Rutt Rebecca Fahrig Michael Moseley Norbert Pelc Jennifer McNab Roland Bammer Kim Butts Pauly

4. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION PH.D.STUDENTS JasonSu MihirPendse MatthewMarzelli PI Prof.BrianRutt POSTDOCS SimoneWinkler SCIENTIFICSTAFF ManojSaranathan RonWatkins FORMER MEMBERS: - Kimberley Brewer - James Rioux - Ives Levesque - Thomas Tourdias - Prachi Pandit - Mehdi Khalighi - Sui Seng Tee - Jonathan Lu THE RUTT LABORATORY FIELDS OF RESEARCH: - Molecular imaging - Pulse sequence development - Parallel transmit techniques - Hardware development - Multiphysics modeling - MR safety EXTERNAL MEMBERS: - Andrew Alejski - Trevor Wade - Janos Bartha THE RUTT LABORATORY! ETH ZRICH | 2014/12/19S. A. WINKLER LeoTam

5. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION MR BASICS NUCLEAR MAGNETIC RESONANCE Isotopes in the body with odd number of protons possess nonzero spin moving electrons generate small magnetic field Hydrogen atoms most abundant in human body (tissue water content) JOHANNES KEPLER UNIVERSITY LINZ | 2014/05/08S. A. WINKLER TWO-STEP PROCEDURE: 1. Alignment (polarization) of spins with an external static magnetic field B0 2. Perturbation of alignment by use of an RF (electromagnetic) field magnetization of protons can be measured

6. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION MR BASICS STEP 1: SPIN ALIGNMENT JOHANNES KEPLER UNIVERSITY LINZ | 2014/05/08S. A. WINKLER Without external B0-field With external B0-field Larmor frequency depends on magnetic properties of proton, and is proportional to static magnetic field strength Spin precession at Larmor frequency Alignment Precession No net magnetization

7. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION Transverse component can be detected with an RF coil! MR BASICS STEP 2: PERTURBATION/MEASUREMENT JOHANNES KEPLER UNIVERSITY LINZ | 2014/05/08S. A. WINKLER excitationrelaxation RF excitation: Additive B1 pulse in transverse plane at Larmor frequency T2 T1 Precession at Larmor Frequency Added RF pulse at Larmor frequency Tissue contrast: RF coil transceive s transverse magnetic field B1 B0 Idea: measure magnetic field that spin generates! B0 After RF pulse is turned off:

8. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION MR BASICS SPATIAL LOCALIZATION Larmor frequency depends on static magnetic field strength Small magnetic field variation is overlaid depending on location of imaged voxel frequency of received signal contains spatial information! JOHANNES KEPLER UNIVERSITY LINZ | 2014/05/08S. A. WINKLER With linear additive gradient: Spatial information can be recovered from Inverse Fourier Transform!! 3Dlocalization Gradient coils DC current generates linear B0 variation

9. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION MR BASICS - SYSTEM Main system components: 1. Magnet (generates B0 field aligns spins net magnetization) 2. RF coil (excites and receives RF magnetic field at Larmor frequency) 3. Gradient coils (localization by small variation of B0 field) ETH ZRICH | 2014/12/19S. A. WINKLER Typical B0 field strengths (human): Clinical: 1.5T-3T Research: 7T-11.7T

10. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION WHY UHF MRI? Increased sensitivity for proton imaging applications (SNR increases approx. proportionally with B0) Allows for anatomical imaging with higher spatial resolution Use higher sensitivity to examine dynamic and functional aspects (BOLD, other brain activity) ETH ZRICH | 2014/12/19S. A. WINKLER 7T Potential for detection of new physiology of healthy and diseased tissue New forms of contrast Energy gap:

11. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION HIGH-FIELD MRI AT STANFORD GE Discovery MR950 7T scanner 2-ch Tx / 32-ch receive modality 8-ch pTx / 32-ch receive modality ETH ZRICH | 2014/12/19S. A. WINKLER Approximately 50 7T scanners throughout the world only! Not FDA approved

12. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION HARDWARE RELATED UHF MRI CHALLENGES Global SAR increases with B0 2 Local SAR becomes more inhomogeneous due to wave effects Local SAR is strongly position- and coil- dependent ETH ZRICH | 2014/12/19 RF heating in tissue (SAR) B1 inhomogeneities Lorentz ForcesB0 inhomogeneities 128MHz 512MHz Larmor frequency -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 -0.2 -0.15 -0.1 -0.05 0 0.05 W/kg 0.57 1.14 1.71 2.28 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 -0.2 -0.15 -0.1 -0.05 0 0.05 W/kg 0.57 1.14 1.71 2.28 2.25 0 W/kg Image shading Patient safety Acoustics & Vibrations Geometric distortion & artefacts Lorentz forces increase with B0 Higher sound pressure levels Stronger vibrations Spatial encoding depends on B0 homogeneity Signal loss

13. STANFORD UHF MRI PROGRAM IMAGE SHADING B1 INHOMOGENEITY OPTIMIZED DIELECTRIC SHIMMING SIMONE WINKLER, ALESSANDRO SBRIZZI, ET AL. 16-CHANNEL PTX COIL DESIGN RICCARDO STARA, SIMONE WINKLER, ET AL. ETH Zrich | 2014/12/19S. A. WINKLER

14. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION B1 INHOMOGENEITY BASICS Parallel Transmission (pTx): excitation from multiple independent channels with different pulse signal shapes at each channel RF shimming: excitation from multiple independent channels; only amplitude and phase are varied Dielectric shimming: passive perturbation of E- and B-fields by insertion of dielectric material that focuses field lines ETH ZRICH | 2014/12/19S. A. WINKLER 128MHz 512MHz Larmor frequency Remedy:Challenge:

15. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION 16-CHANNEL PTX COIL DESIGN 1. Novel coil element 2. Modular configuration 1. 8ch single row 2. 8ch checkerboard 3. 16ch with two Butler matrices 4. 8ch with Butler matrix 5. 2ch with Butler matrix 6. etc. Inherent inter-element decoupling B1 mapsZ-segmentation 1 2 3 4COAX4 ID=CX2 EL=90 Deg Fo=0.1 GHz Z=50 IND ID=L1 L=580 nH CAPQ ID=C1 C=24 pF Q=0 FQ=0 GHz ALPH=1 CAPQ ID=C2 C=10 pF Q=0 FQ=0 GHz ALPH=1 T 1 2 REST ID=IN1 R=1-5 Ohm T=-273.15 DegC IND ID=L2 L=80 nH DIODE1 ID=Microsemi HUM2015 Nu=0 T=21.85 DegC Io=0 mA CAPQ ID=C3 C=10 pF Q=0 FQ=0 GHz ALPH=1 CAP ID=C4 C=0-1 pF PORT P=1 Z=50 Ohm PORT P=2 Z=50 Ohm CT1 CT2 Coil CM TuningMatching RF choke Decoupling Illumination of lower brain regions Better acceleration Better pTx flexibility ETH ZRICH | 2014/12/19S. A. WINKLER Inter-element coupling 3. 3D segmentation Results

16. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION OPTIMIZED DIELECTRIC SHIMMING Electromagnetic scattering is ANALYTICALLY described for dielectric- absorptive spheres! [1] model dielectric pad as a sphere! Model head as a dielectric sphere Model dielectric shimming pads as additional spheres Optimization to determine optimized position of the sphere(s) Requirement: Electromagnetic fields of unloaded RF coil expanded into vector spherical harmonics S. A. WINKLER [1] Gustav Mie, Beitrge zur Optik trber Medien, speziell kolloidaler Metallsungen, Annalen der Physik, Vierte Folge, Band 25, 1908, No. 3, p 377-445. [2] M. I. Mishchenko, L. D. Travis, and D. W. Mackowski, T-matrix computations of light scattering by nonspherical particles: A review, J. Quant. Spectrosc. ETH ZRICH | 2014/12/19 Improvement: 41.25% Dielectric shimming also increases average B1+ field! Applications: 3T breast MR 7T neuroimaging

17. STANFORD UHF MRI PROGRAM B0 INHOMOGENEITIES RF SHIM COIL AT 7T SIMONE WINKLER, JASON STOCKMANN, ET AL. ETH Zrich | 2014/12/19S. A. WINKLER

18. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION B0 INHOMOGENEITY BASICS B0 field has to be extremely homogeneous (order of ppm, for 7T in the 0.25T range) Spatial encoding depends on B0 homogeneity Geometric distortions and/or signal loss Sources of B0 inhomogeneity: 1. Magnet construction 2. Magnetic susceptibility differences in tissues; in particular: air cavities (ears, sinuses, etc.) ETH ZRICH | 2014/12/19S. A. WINKLER Remedy:Challenge: 7.00001 6.99999 7.00000 B0 shimming: Air cavities water Passive Active Spherical harmonic s Matrix shim Iron inserts

19. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION HISTORY High-order B0 shimming is essential for modern MR neuroimaging Conventional approach: spherical harmonic shimming ISMRM 2015 TORONTO, CANADA | 2015/06/04S. A. WINKLER Multi-Coil Shimming Multi-Coil + RF shimming Alternative to spherical harmonic shimming + Efficient + Switch shim dynamically without eddy currents - Restricted space for RF arrays - Interactions with RF coils Uses RF chokes to bridge DC shim current into RF loop + Larger efficiency due to single-turn loop + Dynamic switching - Construction complexity Juchem C, JMR 212:280288 (2011) Truong TK, Neuroscience 103:235-240 (2014) 7T 3TDrastic B0 homogeneity improvements

20. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION RF-SHIM HELMET - CONCEPT ISMRM 2015 TORONTO, CANADA | 2015/06/04S. A. WINKLER RF-Shim Helmet - Concept: 32chRF-shimcoil 32chRF-onlycoil Conventional RF coil Shim-RF coil Coilelement Combining Rx (RF) and B0 (DC) shimming functions on the same conductor Array of single-turn loops Helmet shape for closer proximity (increased Rx SNR and B0 shim efficiency) + Stockmann, MRM early view (2015) Chokes to bridge capacitor breaks RF-path only Added DC path 3T

21. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION RF-SHIM HELMET - RESULTS ISMRM 2015 TORONTO, CANADA | 2015/06/04S. A. WINKLER z Rx-Shim Helmet 3T Results: B0 shimming results: 32ch commercial c32ch RF-shim coil SNRCoilcorrelation Precursor 32ch coil a.u. 32ch commercial coil32ch RF-shim coil 0 100 200 300 400 SNRCoilcorrelation Precursor 32ch coil 0 1 0.5 RF - SNR results: RF-shim coil exhibits modest SNR loss as compared to precursor RF-only coil Stockmann, MRM early view (2015) + Reduced distortion and closer alignment for blip-up and blip- down + Predicted and measured field maps agree + Low current requiremnts (2.5 A max per coil)

22. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION RF-SHIM CONCEPT AT 7T Using the RF-shim concept at 7T appears compelling: 1. Increased in-vivo B0 distortion at 7T greater impact for EPI (fMRI, Diffusion) QSM Inversion pulses 2. SNR impact at 7T might be lower A well designed receive array is body noise dominated We hypothesize that at 7T added copper noise will have less impact However: Conversion from 3T RF-only to RF-shim array caused 10- 15% SNR loss We seek to minimize SNR loss at 7T ISMRM 2015 TORONTO, CANADA | 2015/06/04S. A. WINKLER Optimized loop design becomes essential The resulting 7T RF- shim array would offer: - Helmet design for SNR and shim efficiency - Minimal interference of RF and shim functions Optimized compact tool for modern high-field MR neuroimaging Here: Exploratory work to choose optimized loop design

23. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION MOTIVATION ISMRM 2015 TORONTO, CANADA | 2015/06/04S. A. WINKLER Larger number of capacitors required for 7T larger number of toroidal bridging chokes Challenges: Coaxial loop element Novel combined conductor concepts: Concentric loop element - Inner conductor: DC shim current - Outer conductor (RF shield): RF current - Inner loop: DC shim current - Outer loop: RF current Reduced number of capacitors No chokes required Additional loss Heating RF field perturbation Construction complexity in confined space Magnetic coupling between conductors lowers self-inductance of RF loopChokes DC path DC path

24. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION THEORY Magnetic coupling between inner and outer conductor Two conjugate poles Two conjugate zeroes ISMRM 2015 TORONTO, CANADA | 2015/06/04S. A. WINKLER Resonance + anti-resonance ,shim ,standalone = 1 1 2 k = inter-loop coupling factor ,shim ,standalone = 1 1 + 2 Resonance: Anti-resonance: = 2 LS = shim loop inductance LRF = RF loop inductance Lm = mutual inductance Trade-off between Q-factor reduction and number of capacitors to be found Coaxial loop element Concentric loop element Effects of added loop: 1. Resonance frequency increases because of lowered self inductance 2. Added loop acts as a resistive load in time-quadrature to the original loop + Fewer capacitors are required (and therefore bridging chokes) - Reduction in Q-factor (and thus SNR) by insertion of additional loop

25. STANFORD UHF MRI PROGRAM MR SAFETY THERMOACOUSTIC SAR MAPPING SIMONE WINKLER, PAUL PICOT, MICHAEL THORNTON, BRIAN RUTT SAR-CONSTRAINED PTX PULSE DESIGN MIHIR RAJENDRA PENDSE, SIMONE WINKLER, BRIAN RUTT ETH Zrich | 2014/12/19S. A. WINKLER

26. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION MR SAFETY BASICS ETH ZRICH | 2014/12/19S. A. WINKLER Main concern: RF power deposition in tissue (metric: specific absorption rate (SAR)) Low field MRI: Wave effects are not prominent and local SAR is therefore easier to predict High-field MRI: 1. SAR increases with B02 2. variation in tissue properties AND fields strong variations in local SAR SAR is strongly dependent on: o Patient o Coil o Position 3. pTx techniques may cause worst-case constructive interference of individual local SAR patterns great concerns! To date there is no experimental procedure to measure local SAR! RF power deposition over entire anatomy of interest Global SAR local variation of SAR that leads to prominent hotspots Local SAR IEC regulation: -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 -0.2 -0.15 -0.1 -0.05 0 0.05 W/kg 0.57 1.14 1.71 2.28 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 -0.2 -0.15 -0.1 -0.05 0 0.05 W/kg 0.57 1.14 1.71 2.28 2.25 0 W/kg 3.2 W/kg 10 W/kg Alternative metric: temperature in tissue (max. 1C heating) SAR-minimized pTx pulse design Find methods to experimentally determine SAR Find alternative metric (temperature) Remedy:Challenge:

27. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION SAR-MINIMIZED PTX PULSE DESIGN ETH ZRICH | 2014/12/19S. A. WINKLER Pendse, Winkler and Rutt, ISMRM 2014 B1 mapping MR scan Localizer Query database for body model matching patient Anatomical parameters minSAR pTx scan minSAR RF Pulse Optimize pulse sequence parameters minSAR MLS Local SAR dependent Local SAR independent RF Pulse Design Non pTx scanning E-field maps VOPs Body models SAR database (computed offline) EM simulationofflinereal-time Compressed dataset

28. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION THERMOACOUSTIC SAR CONCEPT ETH ZRICH | 2014/12/19S. A. WINKLER IDEA: absorbed RF energy SAR! MR RF coil as RF transmitter generates same local SAR pattern as in an MR scan RF-induced thermoacoustic signal is proportional to local SAR use MR coil with short bursts of RF energy 2 1 2 2 2 = Pulsed RF absorptionAcoustic pressure wave US patent: Simone Winkler, Brian Rutt, Paul Picot, Michael Thornton. In-Vivo Specific Absorption Rate Mapping using the Thermoacoustic Effect", May 7, 2014

29. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION MODELING Delay-and- Sum reconstruction ETH ZRICH | 2014/12/19S. A. WINKLER IEC/CENELEC SAM head model with registered SAR pattern from Ella. 10 0 5 2 01 10 05 4 02 2D SAR pattern in W/kg originalreconstructed SEMCAD Matlab COMSOL 2 1 2 2 2 = Pulsed RF absorptionAcoustic pressure wave Winkler, Picot, Thornton and Rutt, ISMRM 2014, ISMRM 2015

30. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION TEST PLATFORM Proof of concept with thermoacoustic imaging platform at Stanford (collaboration with Endra, Inc.) S. A. WINKLER By varying 1. Conductivity 2. Electric Field in a phantom, we can emulate the local SAR variation in tissue in UHF MRI 1) OriginalTan k A B 0 2) One excitation channel off Tan k A X Agar ( = 0) Saline 1.25% 2.25% 3.75% 5% 1 conductivity variation 2 E-field variation 5% 3% 2.25% 1.25% Point source experiments E-Fieldvariation RF horns Ultrasound transducer

31. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION EXPERIMENTS FIRST ATTEMPT ETH ZRICH | 2014/12/19S. A. WINKLER Agar ( = 0) Saline 1.25% 2.25% 3.75% 5% Varying conductivity: Concen- tration Image value 1.25% 53691 2% 876125 2.25% 96065 3.75% 1768353 5% 2423254 ThermoacousticImage 0 500 1000 1500 2000 2500 3000 0.00% 2.00% 4.00% 6.00% Image value over saline concentration

32. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION POINT SOURCE EXPERIMENTS OVERALL SAR VARIATION S. A. WINKLER R = 0.5224 R = 0.8458 0 2 4 6 8 10 12 0 2 4 6 8 10 Horizontal SAR Vertical SAR Linear (Horizontal SAR) Linear (Vertical SAR) Tan k A X Tan k A B 0 5% 3% 2.25% 1.25% 2.25% 2.25% 2.25% 2.25% Conductivity variation Constant conductivity Varied conductivity E-Field variation + rotated ultrasound transducer to average over perceived E- field Without rotation to maximize E- field variation E-field was measured with a coaxial probe Reference test SAR variation E-Fieldvariation Conduc- tivity variation Conductivity variation Conductivity variation E-Fieldvariation UStransducer RF horn Image values fitted to SAR equation:

33. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION MR SYSTEM INTEGRATION Fast rise-time pulse shape required MR system might have to be upgraded with a larger bandwidth auxiliary signal generator It is estimated that the power amplifier of the MR system will be delivering sufficient peak power for the experiment An ultrasound bandwidth of 200 kHz will be used for penetrating skull bone Ultrasound transducers will be mounted at two lateral acoustic windows in the skull S. A. WINKLER Power amplifier MR signal generato rAuxiliary signal generato r Control room workstatio n Data acquisition system Patient head Ultrasoun d transduce r RF coil

34. STANFORD UHF MRI PROGRAM MR ACOUSTICS UHF-MRI ACOUSTICS IN HEAD AND BODY GRADIENTS SIMONE WINKLER, TREVOR WADE, ANDREW ALEJSKI, CHARLES MCKENZIE, BRIAN RUTT ETH Zrich | 2014/12/19S. A. WINKLER

35. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION HEAD GRADIENT INSERT ETH ZRICH | 2014/12/19 Smaller field of linear region (approx. 20 cm) Higher slew rate Higher gradient strength (100 300 mT/m) Inserted into bore 190 170 220 450 490 338 Body gradient coil Head gradient insert coil Faster or higher resolution imaging Large linear region (60 cm) Low slew rate Low gradient strength (50 80 mT/m) Built into MR system Attractive at high field Louder acoustics

36. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION FUNDAMENTALS OF VIBROACOUSTICS ETH ZRICH | 2014/12/19S. A. WINKLER Time- varying pressure inside bore Structural vibration of gradient coil B0 . F(t) i(t) Fundamentals: Time-varying current in gradient conductor, in presence of strong B0 field, causes Lorentz forces Structur e Flui d (air) vibrations Pressure change = sound wave Structural vibrations: n=0 n=1 n=2 n=3 Breathing Bending Ovaling Acoustic coupling: structure modes in the coil structure acoustic modes in the coil bore Interaction MUTUAL COUPLING! Avg displacement (3T, 50A): 1m Total displacement is weighted sum of different modes

37. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION MODELING ISMRM 2015 TORONTO, CANADA | 2015/06/05S. A. WINKLER No displacement in vertical direction Hard acoustic boundary 1 2 2 1. Structural vibration of coil 2. Acoustic wave propagation in air Infinite half space of air on both bore ends Perfectly Matched Layer 2 Insert gradient modeling: Surrounding air modeling: Acousticwavepropagation:LorentzForces: 150 0 N/m Fx Fy YX Z 820 Hz Highly accurate prediction of SPL in gradient coils acoustic-based design strategies for SPL reduction become possible New features in gradient vibroacoustic modeling: as compared to existing published modalities ----------------------------------------------------------------------- 1. Accurate wire patterns 2. Realistic bore shape with patient bridge 3. Acoustic propagation outside bore 4. Full coupling of vibrations and acoustics 5. Lorentz damping

38. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION RESULTS REALISTIC HEAD GRADIENT 50A,3T X-gradient Y-gradient Z-gradient isocenter 20.6 g 26.8 g 8.7 g 3.7 g 15.6 g 12.4 g Innerborewall,+16cm 98.7 dB 99.2 dB 95.4 dB 104.3 dB 97.6 dB 96.9 dB 50A,3TAccelerationsSoundpressurelevel ETH ZRICH | 2014/12/19S. A. WINKLER

39. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION Modes are shifted to higher frequencies and out of the band of interest for RF pulses Extremely short axial length of our head gradient design pushes circumferential modes to much higher frequencies These are not excited by the z- gradient short is good! S. A. WINKLER (0,0,1) (0,0,1) (0,1,1) (0,0,2) (0,1,2) (0,0,3) (0,1,3) (1,0,2) (coupled) (0,5,4) (0,0,2) (0,1,5) (1,0,3) (2,0,3) (1,0,3) (2,0,3) (2,2,1) (3,0,3) (4,0,0) axial modes radial modes COMPARISON HEAD-BODY GRADIENTS ETH ZRICH | 2014/12/19

40. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION LORENTZ DAMPING ISMRM 2015 TORONTO, CANADA | 2015/06/05S. A. WINKLER B0 FL v FL = I x B0 JCL = (v x B0) ICL I ICL = JCL * A FCL = ICL x B0 Total force F = FL + FCL Total force explicit for B0 F = FL - AB0 2v I B0 . F(t) i(t)

41. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION EDDY CURRENT DAMPING ISMRM 2015 TORONTO, CANADA | 2015/06/05S. A. WINKLER Eddy current density induced in moving conductor JCL = *(v x B0) Lorentz force induced due to Eddy current FCL = JCL*A x B0 Total force F = FL + FCL Total force explicit for B0 in z-direction: F = FL - AB0 2*v Equation of motion This damping term is dependent on B0 2 nonlinear increase in SPLs with field strength

42. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION NOISE REDUCTION METHODS Moderate noise reduction achieved Current efforts for sound reduction are ongoing ETH ZRICH | 2014/12/19S. A. WINKLER Bore acts as acoustic waveguide Terminations to outside air are highly reflective bore acts as resonator 1. impedance matching of waveguide to free space impedance lets acoustic energy travel outward (horn) 2. absorbing endcap reduces resonances Traveling wave concept using a horn. Proposed noise reduction method Winkler, Alejski, Wade, McKenzie, Rutt, ISMRM 2014

43. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION GRADIENT COIL ACOUSTICS FIELD DEPENDENCE Lorentz damping includedNo Lorentz damping ISMRM 2015 TORONTO, CANADA | 2015/06/05S. A. WINKLER 7.5 dB from 3T to 7T 10.8 dB from 3T to 10.5 T 91.2 dB 97.5 dB 100.8 dB 3T: 7T: 10.5T: 92.1 dB 89.8 dB 90.5 dB 3T: 7T: 10.5T: Linear scaling with main field strength Nonlinear scaling with main field strength

44. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION GRADIENT COIL ACOUSTICS FIELD DEPENDENCE Lorentz damping includedNo Lorentz damping ISMRM 2015 TORONTO, CANADA | 2015/06/05S. A. WINKLER 91.2 dB 97.5 dB 100.8 dB 3T: 7T: 10.5T: 92.1 dB 89.8 dB 90.5 dB 3T: 7T: 10.5T: Coupled resonanc e Linear scaling with main field strength Nonlinear scaling with main field strength

45. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION CONCLUSION UHF MRI increases SNR and resolution Many technical challenges are faced before clinical adoption This talk showed innovative approaches on: B1 inhomogeneities / image shading B0 inhomogeneities MR Safety Acoustics ETH ZRICH | 2014/12/19S. A. WINKLER

46. STANFORD UHF MRI PROGRAM B0SHIMMINGACOUSTICSSAFETYIMAGESHADINGINTRODUCTION OTHER RESPONSIBILITIES AND THINGS I DO Management of RF and electronics lab 3D printer and mechanical manufacturing facility management ETH ZRICH | 2014/12/19S. A. WINKLER Peripheral nerve stimulation Transcranial magnetic stimulation Molecular imaging magnetogenetics Genetic activation for cancer treatment by alternating magnetic field induced heating and many more!

47. THANK YOU! ACKNOWLEDGEMENTS: o ALL ANONYMOUS VOLUNTEERS o MOHAMMAD MEHDI KHALIGHI o KEVIN & KARLA EPPERSON o IVES LEVESQUE o THOMAS TOURDIAS o ANNE SAWYER o BRIAN HARGREAVES o BRUCE DANIEL o THOMAS WRIEDT o JUERG FROEHLICH o ZMT/SPEAG GMBH COLLABORATORS: o LARRY WALD o JASON STOCKMANN o BORIS KEIL o MIHIR RAJENDRA PENDSE o RICCARDO STARA o ALESSANDRO SBRIZZI o PAUL PICOT o MICHAEL THORNTON o TREVOR WADE o ANDREW ALEJSKI o CHARLES MCKENZIE o JANOS BARTHA ETH Zrich | 2014/12/19S. A. WINKLER