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# FDTD Simulations In MRI

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### Transcript

• 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