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FDTD Simulations In MRI
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FDTD Simulations In MRI

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  • 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. Overview
    • Introduction
          • Larmor equation
          • B 0 & B 1 -field
          • Radio frequent wavelength
          • Dielectric properties
          • SAR
    • FDTD simulation
          • Abbreviation & Objective
          • Maxwell’s Equations
          • Yee Algorithm
          • Model
    • Application
          • Bodycoil excitation
          • Challenges
          • Travelling-wave
    • Conclusion
  • 3. Introduction: Larmor equation 64MHz 128MHz 298MHz
  • 4. Introduction: B 0 & B 1 -field B 0 B 1 Coil M z Δθ Z X’ Y’
  • 5. Introduction: Radio frequent wavelength
    • In Utrecht:
      • 7 Tesla [T]
      • Resonance frequency: ~ 300 MHz
    • Wavelength for hydrogen:
      • In air/vacuum: ~100 cm
      • In tissue: ~10 cm
  • 6. Introduction: Dielectric properties
    • Relative permeability
      • Ability to conduct magnetic flux
      • μ r [Hm -1 ]
    • Relative permittivity
      • Ability to transmit (‘permit’) an electric field
      • ε r [Fm -1 ]
    • Conductivity
      • Ability to conduct electric currents
      • σ = ε ’’ ε 0 ω [Sm -1 ]
      • ε 0 = 8.85419*10 -12
      • ε ’’ = dielectric loss
  • 7. Introduction: SAR (Specific Absorbtion Ratio)
    • SAR
      • rate absorbed RF power per mass of tissue
      • Heating of tissue, due to electric fields
      • ρ = sample density
    • Safety measurement
      • Magnetic Resonance
      • Mobile phones
    • FCC (Federal Communications Commission)
      • Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields
      • SAR Head <= 3.2 W/Kg
  • 8. FDTD simulation: Abbreviation & objective
    • Finite
    • Difference
    • Time
    • Domain
    • Objective of FDTD simulation:
      • Determination of B 1 and SAR inside the body with a computational method by using dielectric properties.
    • F
    • D
    • T
    • D
  • 9. FDTD simulation: Maxwell’s equations
    • Differential form
      • Electric field
      • Magnetic field
      • In general
      • One direction
  • 10. FDTD simulation: Maxwell’s equations
    • For all directions separate
  • 11. FDTD simulation: Yee Cell z x y
  • 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. FDTD simulation: Model
    • Calculate B 1 -field:
    • Determine flipangle:
    • Calculate SAR:
  • 14. Application: Bodycoil excitation
    • Create homogeneous transmit field (B 1 + ), with simultaneous SAR reduction
    • Simulation of bodycoil
      • Standard 3T bodycoil
      • tuned to 7T
      • Find optimal B 1 +
      • Reduce SAR hotspots
    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. 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. Application: Bodycoil excitation
    • Quadrature setup = standard setup
      • Same amplitude
      • Different Phase setting
        • Δ P = 30°
    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. 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. Application: Challenges
    • No bodycoil available for 7T:
      • Only a headcoil
    • Need large and complex designs:
      • Multi Transmit
      • RF shim strategies
    • Total RF Power limited
      • 32 kW @ 3T
      • 8 kW @ 7T
    • Not an efficient coil design for 7T
  • 19. Application: Travelling-wave
    • New concept
      • Travelling-wave
      • Use RF-Shield as wave guidance
    Brunner, D.O et.al. Travelling-wave nuclear magnetic resonance; Nature 457 (2009) 994-998
  • 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. 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. 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. Conclusion:
    • FDTD simulation is a very good validation method:
      • Determination of the B 1 -field
      • Calculating the Electric field components and SAR constraints
  • 24. Acknowledgements
    • Department of Radiotherapy:
      • Alexander Raaijmakers
      • Anna Andreychenko
      • Nico van den Berg
      • Ozlem Ipek

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