FDTD Simulations In MRI

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FDTD Simulations In MRI

  1. 1. FDTD Simulations in MRI Bart van de Bank Department of Radiology University Medical Center Utrecht [email_address] +31 (0)88 – 75 51386 MSc student @ Life Sciences Biomedical Image Sciences
  2. 2. Overview <ul><li>Introduction </li></ul><ul><ul><ul><ul><li>Larmor equation </li></ul></ul></ul></ul><ul><ul><ul><ul><li>B 0 & B 1 -field </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Radio frequent wavelength </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Dielectric properties </li></ul></ul></ul></ul><ul><ul><ul><ul><li>SAR </li></ul></ul></ul></ul><ul><li>FDTD simulation </li></ul><ul><ul><ul><ul><li>Abbreviation & Objective </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Maxwell’s Equations </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Yee Algorithm </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Model </li></ul></ul></ul></ul><ul><li>Application </li></ul><ul><ul><ul><ul><li>Bodycoil excitation </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Challenges </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Travelling-wave </li></ul></ul></ul></ul><ul><li>Conclusion </li></ul>
  3. 3. Introduction: Larmor equation 64MHz 128MHz 298MHz
  4. 4. Introduction: B 0 & B 1 -field B 0 B 1 Coil M z Δθ Z X’ Y’
  5. 5. Introduction: Radio frequent wavelength <ul><li>In Utrecht: </li></ul><ul><ul><li>7 Tesla [T] </li></ul></ul><ul><ul><li>Resonance frequency: ~ 300 MHz </li></ul></ul><ul><li>Wavelength for hydrogen: </li></ul><ul><ul><li>In air/vacuum: ~100 cm </li></ul></ul><ul><ul><li>In tissue: ~10 cm </li></ul></ul>
  6. 6. Introduction: Dielectric properties <ul><li>Relative permeability </li></ul><ul><ul><li>Ability to conduct magnetic flux </li></ul></ul><ul><ul><li>μ r [Hm -1 ] </li></ul></ul><ul><li>Relative permittivity </li></ul><ul><ul><li>Ability to transmit (‘permit’) an electric field </li></ul></ul><ul><ul><li>ε r [Fm -1 ] </li></ul></ul><ul><li>Conductivity </li></ul><ul><ul><li>Ability to conduct electric currents </li></ul></ul><ul><ul><li>σ = ε ’’ ε 0 ω [Sm -1 ] </li></ul></ul><ul><ul><li>ε 0 = 8.85419*10 -12 </li></ul></ul><ul><ul><li>ε ’’ = dielectric loss </li></ul></ul>
  7. 7. Introduction: SAR (Specific Absorbtion Ratio) <ul><li>SAR </li></ul><ul><ul><li>rate absorbed RF power per mass of tissue </li></ul></ul><ul><ul><li>Heating of tissue, due to electric fields </li></ul></ul><ul><ul><li>ρ = sample density </li></ul></ul><ul><li>Safety measurement </li></ul><ul><ul><li>Magnetic Resonance </li></ul></ul><ul><ul><li>Mobile phones </li></ul></ul><ul><li>FCC (Federal Communications Commission) </li></ul><ul><ul><li>Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields </li></ul></ul><ul><ul><li>SAR Head <= 3.2 W/Kg </li></ul></ul>
  8. 8. FDTD simulation: Abbreviation & objective <ul><li>Finite </li></ul><ul><li>Difference </li></ul><ul><li>Time </li></ul><ul><li>Domain </li></ul><ul><li>Objective of FDTD simulation: </li></ul><ul><ul><li>Determination of B 1 and SAR inside the body with a computational method by using dielectric properties. </li></ul></ul><ul><li>F </li></ul><ul><li>D </li></ul><ul><li>T </li></ul><ul><li>D </li></ul>
  9. 9. FDTD simulation: Maxwell’s equations <ul><li>Differential form </li></ul><ul><ul><li>Electric field </li></ul></ul><ul><ul><li>Magnetic field </li></ul></ul><ul><ul><li>In general </li></ul></ul><ul><ul><li>One direction </li></ul></ul>
  10. 10. FDTD simulation: Maxwell’s equations <ul><li>For all directions separate </li></ul>
  11. 11. FDTD simulation: Yee Cell z x y
  12. 12. FDTD simulation: Model Thelonious Ella Billie Duke 1.74 m 70 kg Van den Bergen, B et.al. 7 T body MRI: B 1 shimming with simultaneous SAR reduction; Physics in Medice and Biology 52 (2007) 5429-5441 http://www.itis.ethz.ch/index/index_humanmodels.html
  13. 13. FDTD simulation: Model <ul><li>Calculate B 1 -field: </li></ul><ul><li>Determine flipangle: </li></ul><ul><li>Calculate SAR: </li></ul>
  14. 14. Application: Bodycoil excitation <ul><li>Create homogeneous transmit field (B 1 + ), with simultaneous SAR reduction </li></ul><ul><li>Simulation of bodycoil </li></ul><ul><ul><li>Standard 3T bodycoil </li></ul></ul><ul><ul><li>tuned to 7T </li></ul></ul><ul><ul><li>Find optimal B 1 + </li></ul></ul><ul><ul><li>Reduce SAR hotspots </li></ul></ul>Van den Bergen, B et.al. 7 T body MRI: B 1 shimming with simultaneous SAR reduction; Physics in Medice and Biology 52 (2007) 5429-5441
  15. 15. Application: Bodycoil excitation Van den Bergen, B et.al. 7 T body MRI: B 1 shimming with simultaneous SAR reduction; Physics in Medice and Biology 52 (2007) 5429-5441
  16. 16. Application: Bodycoil excitation <ul><li>Quadrature setup = standard setup </li></ul><ul><ul><li>Same amplitude </li></ul></ul><ul><ul><li>Different Phase setting </li></ul></ul><ul><ul><ul><li>Δ P = 30° </li></ul></ul></ul>Van den Bergen, B et.al. 7 T body MRI: B 1 shimming with simultaneous SAR reduction; Physics in Medice and Biology 52 (2007) 5429-5441 0° 30° 60° 90° 120° 150° 180° 210° 240° 270° 300° 330°
  17. 17. Application: Bodycoil excitation Van den Bergen, B et.al. 7 T body MRI: B 1 shimming with simultaneous SAR reduction; Physics in Medice and Biology 52 (2007) 5429-5441
  18. 18. Application: Challenges <ul><li>No bodycoil available for 7T: </li></ul><ul><ul><li>Only a headcoil </li></ul></ul><ul><li>Need large and complex designs: </li></ul><ul><ul><li>Multi Transmit </li></ul></ul><ul><ul><li>RF shim strategies </li></ul></ul><ul><li>Total RF Power limited </li></ul><ul><ul><li>32 kW @ 3T </li></ul></ul><ul><ul><li>8 kW @ 7T </li></ul></ul><ul><li>Not an efficient coil design for 7T </li></ul>
  19. 19. Application: Travelling-wave <ul><li>New concept </li></ul><ul><ul><li>Travelling-wave </li></ul></ul><ul><ul><li>Use RF-Shield as wave guidance </li></ul></ul>Brunner, D.O et.al. Travelling-wave nuclear magnetic resonance; Nature 457 (2009) 994-998
  20. 20. Application: Travelling-wave Andreychenko, A et.al. Effective delivery of the traveling wave to distant locations in the body at 7T ; Proc. Intl. Soc. Mag. Reson. Med. 17 (2009) p. 500
  21. 21. Application: Travelling-wave Andreychenko, A et.al. Effective delivery of the traveling wave to distant locations in the body at 7T ; Proc. Intl. Soc. Mag. Reson. Med. 17 (2009) p. 500 Conductive shield Pelvic
  22. 22. Application: Travelling-wave Coronal image of the pelvis RF induced signal contrast of the pelvic region Andreychenko, A et.al. Effective delivery of the traveling wave to distant locations in the body at 7T ; Proc. Intl. Soc. Mag. Reson. Med. 17 (2009) p. 500
  23. 23. Conclusion: <ul><li>FDTD simulation is a very good validation method: </li></ul><ul><ul><li>Determination of the B 1 -field </li></ul></ul><ul><ul><li>Calculating the Electric field components and SAR constraints </li></ul></ul>
  24. 24. Acknowledgements <ul><li>Department of Radiotherapy: </li></ul><ul><ul><li>Alexander Raaijmakers </li></ul></ul><ul><ul><li>Anna Andreychenko </li></ul></ul><ul><ul><li>Nico van den Berg </li></ul></ul><ul><ul><li>Ozlem Ipek </li></ul></ul>

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