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DALHM Progress Meeting September 2003

This report discusses the quantitative measurement and imaging experiments conducted on a Swiss Roll metamaterial faceplate. The material acts as a magnetic "faceplate" and demonstrates novel magnetic behavior. The study also examines the effectiveness of the effective medium model and explores the potential applications of metamaterial structures.

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DALHM Progress Meeting September 2003

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  1. DALHM Progress MeetingSeptember 2003

  2. People Mike Wiltshire Imaging Sciences Dept, Hammersmith Hospital, Jo Hajnal Imperial College London Ian Young John Pendry Department of Physics, Blackett Laboratory, Wayne Williams Imperial College London Vassilis Yannopapas David EdwardsDepartment of Engineering Sciences, Chris Stevens University of Oxford Laszlo Solymar Katya Shamonina University of Osnabruck

  3. Activities • Quantitative measurement of field distributions transmitted through “faceplate” • Reported (and augmented) MRI experiments on “faceplate” transmission at ISMRM

  4. Very near field conditions • In the very near field, for distances r << l, we can neglect retardation • ThenE and H are decoupled, leading to the … • Electrostatic or Magnetostatic approximation • E is controlled by e • H is controlled by m • Provides an exemplar for LHM

  5. Swiss Roll metamaterial “faceplate” • Construct 300 off, 50 mm long, Swiss Rolls • Tune them all to 21.5 ± 0.1 MHz, using capacitively coupled sleeves overhanging by 10 mm • Hexagonal packing to make a slab of material, 200mm diagonal, 60 mm thick Thanks to John Cobb for making the rolls and to Stephen Wiltshire for help with the tuning

  6. Scanning Experiments • Quantitative measurement of transmitted field distributions using Oxford XY table • 3 mm loop as source • 3 orthogonal 3 mm loop receivers • Scan in xz and xy planes • Sweep frequency 15 – 35 MHz • Record amplitude and phase for all 3 polarisations • Process as movies of Hz distribution vs frequency

  7. Some predictions… • We consider electromagnetic waves propagating through a highly anisotropic, effective medium, and solve Maxwell’s Equations • The condition for solution is • For mx = 1 and mz→∞, kz = ±k0, so kz is independent of kx, • The input magnetic field pattern is transferred unchanged to the output face • The (uniform) material behaves like “magnetic wires” or a “magnetic faceplate”

  8. The prediction for real materials…? • We have a material with finite anisotropy and loss • At 21.3 MHz, l ~ 14m. So in , and , i.e. the material does not transport the image perfectly • Attenuation becomes significant for Im(kzd) ≈ 1, or kx(max) ≈ b / d which limits the resolution to • With b≈ 6 and d = 60 mm, D ≈ 10 mm, about the same size as the rolls

  9. 21.3 MHz,m″ = 36

  10. Further Predictions… • The condition for solution was • If mz→0, then kx → 0, and only the a uniform magnetic field pattern is transferred to the output face.

  11. 29.7 MHz,m′ = 0

  12. Further Predictions… • The condition for solution was • Recall that k0 ~ 2p/14m whereas kx ~ 2p/14mm, so kx >> k0 • For mx = +1 and mz = -1, kz = ±kx, so the magnetic field pattern travels at 45° to the output face (?)

  13. 24.75 MHz,m′ = -1

  14. 22.45 MHz

  15. 23.25 MHz

  16. JBP Analysis for dielectric • For semi-infinite slab, • where

  17. Analysis (2) • Calculate transmission vs e for a given kx • See “fringes” • Need to take into account:- • Range of kx • Finite sample size

  18. Summary • Detailed measurements made of magnetic field distribution from dipole source transmitted through Swiss Roll slab • Rich variety of structure observed for m < 0 • Still needs full analysis: how good is effective medium model?

  19. Antenna Excited spins Excited spins Swiss Roll Slab (60 mm) “Spenco” “Spenco” Antenna MRI Experiments • Build & tune loop & ‘M’ antennae to 21.3 MHz • Use in transmit – receive mode to excite & detect spins in “Spenco” • Control experiment with “Spenco” placed on top of antenna • Set RF amplitude so that only spins close to the source were excited • Multi-slice spin-echo imaging • Repeat experiment with Swiss Roll slab between antenna and “Spenco” • Centre frequency changed by 30ppm (Ni contamination in Espanex) • No change of RF amplitude required to optimise • Compare to lab experiments

  20. Detector 68 mm Plate (60 mm) Source Detector 8 mm Source Reference Amplitude Phase 65 mm diameter single loop antenna Scanning with a 3 mm probe MRI Images through the Swiss Roll faceplate

  21. MRI Scans Lab Scan Antenna Reference Through Swiss Rolls Imaging Results

  22. Conclusion • Imaging was achieved through metamaterial • M pattern was transferred through slab & back to detector • No lateral spreading beyond roll structure • Material acts as magnetic “faceplate” • Metamaterials can provide novel magnetic behaviour and functionality

  23. Next Steps • Finish analysis of field patterns • Model impact of finite sample size • Is effective medium model adequate? • Construct & characterise isotropic log-pile sample • Metamaterial structures as flux manipulators • Yoke • Compressor Key Questions • Negative e materials at RF? • Space – filling isotropic materials with –ve index? • Noise

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