An 8-channel RF Transceive Coil For Parallel Imaging at 7T/298MHz

Transcription

1 CST European User Group Meeting May 2011, Munich, Germany An 8-channel RF Transceive Coil For Parallel Imaging at 7T/298MHz X. Hanus CEA DSM Irfu SACM An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 1

2 Contents NeuroSpin: a high-field MRI facility at CEA Saclay Iseult: a franco/german R&D program for high field MRI RF coils design with numerical codes MWS-TD comparison for an 8-channel RF prototype coil with two anatomical models, Ella (Virtual Family) and Hum (Homemade Unstructured Model based on Ella tissues): mesh & transient simulation parameters tune/match circuit in Design Studio illustration with a reference birdcage mode (rotating B 1+ field) An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 2

3 NeuroSpin: a neuro-imaging MR facility at CEA Saclay Project initiated in 2003 from the CEA Life Sciences Division (DSV) to achieve a unique platform for high field MR imaging Internal collaboration with CEA Physical Sciences Division (DSM) Franco/german 'Iseult' collaboration (CEA/Siemens/partners) Building delivered in 2006 (Vasconi). Actually: 3T & 7T scanners for human (Siemens), 17T scanners for small animals (Bruker), 11.7T/90 cm human MRI scanner (Iseult, 2013) An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 3

4 NeuroSpin: a neuro-imaging MR facility at CEA Saclay Building delivered in 2006 (Vasconi). An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 4

5 Iseult/Inumac partnership goals Iseult/Inumac: a franco-german CEA-Siemens R&D program for human MR imaging at Ultra High Field (11.7T/500 MHz MRI scanner, contrast agents, pulse sequence designs, etc.) Contract Iseult / CEA-DSM-Irfu to design and build a large bore 11.7T/90 cm magnet, cryogenic system and RF coils (SACM) A challenging transmit RF coil for parallel imaging at 500 MHz Maintain B 1+ homogeneity over the brain despite wavelength effects (as frequency increases, RF wavelength is shortened (λ 0 =60 cm at f=500 MHz) and even more in the brain (ε=50) λ brain =8.5 cm Limit RF dissipations and heating in the inhomogeneous and dissipative dielectric human tissues (SAR, hot spots) A test bed with a validated design at 300 MHz (7T) An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 5

6 Iseult/Inumac partnership organization Oséo Franco-German Co-Operation Agreement BMBF Conv. d Aide Consortium français Consortium allemand Guerbet mandat Univ de Fribourg Acc Collab Acc Collab CEA 25 M Acc Collab Siemens MS Luvata Acc Collab Alstom MSA Bruker Biospin MRI Acc Collab An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 6

7 RF coils design with 3D full-wave solvers Various coil designs simulated with MWS-TD since 2004: a 500 MHz TEM/birdcage volume coil with 32 rods, 300MHz array coils based on striplines superposition Curent designs: 8 channels, superposed striplines with Balun feed, and pre-matching system FEM solver with conformal (tetrahedron) meshing to design and optimize the strips assembling MWS-TD validation with accurate anatomical voxel models (HUGO, Virtual family) or home-made models CAD model imported in MWS (.sat/.hfss) Setup for transient calculations (Boolean operations, coaxial connectors (50 Ω) to apply waveguide excitations, additional inactive volumes for local meshing) tune/match circuit in Design Studio maps (E,H,J) x 8 channels basic birdcage mode (combine) B1, SAR An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 7

8 The FEM model and results B1 + (a= 6.8) S_I,I S_I,1 S_I,2 SAR_CP (a= 6.8) An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 8

9 The FIT model (import + coaxial WG ports) (1) Different views of FIT model An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 9

10 The FIT model: Coil without shielding (2) An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 10

11 The FIT model: Balun and coaxial WG port (3) An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 11

12 The FIT model: radiating dipole and capacitor (4) An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 12

13 The FIT model: WG port to stripline transition (5) An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 13

14 Design model and Cartesian grid 2D mesh view of dipole-1 and dipole-2 in midaxial plane (left), and at port plane (right) Mesh grid details for dipole-1 in axial planes (20 Mcells, local Dx,y = 0.7mm) Align all fixpoints to mesh grid is not possible (< 0.3mm) Meshing is not identical for dipoles (diff. angle positions in the grid) Some mesh cells may contain more than 2 materials (PBA) An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 14

15 Inactive volumes to improve local meshing An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 15

16 The anatomical models 'Ella' and 'Hum' (1) 'Ella' from Virtual Family (1mm resolution ) 'Hum' a 9-tissue Home-made Unstructured Model based on Ella dataset An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 16

17 The anatomical models 'Ella' and 'Hum' (2) RF coil loaded with 'Ella_VF' (top) and 'Hum' (bottom) An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 17

18 Meshing parameters Same global & local meshing parameters but Hum has finally more meshcells (fixpoints) An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 18

19 Mesh grid for Ella_VF-1mm An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 19

20 Mesh grid for Hum An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 20

21 Simulation characteristics Dell 690 XP 64 bit (4x Xeon 5140) + 2xGPU (FX Quadro 5600) Mesh parameters: Global: l/λ= 30/10/30 + FPBA + Equilibrate mesh=1.2 Local: Dx=Dy=0.7 mm, Dz= 1mm (inactive volumes) Local: prority=1 (coaxial connectors, critical elements) cells (Ella_VF) and cells (Hum) Transient parameters: WG excitations [ MHz] on coaxial connectors accuracy limit = -50 db Background=PEC and BC='electric' all except zmin ('open') Time duration 15H to 30H / port Tuning/matching of the dipoles with a DS optimized circuit An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 21

22 Which accuracy limit to use? (1) ProtoV3b_(A) coil loaded with a sphere Inactive volumes for local meshing Global meshing Local meshing Coil elements all lossy materials except coaxial connectors, Background = PEC ; BC = all boundaries, Channel-2 single excitation (τ P = ns, τ=1.02 ps [ MHz]), Stop accuracy limit= -40/-50/-60/-80 db An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 22

23 Which accuracy limit to use? (2) Energy: -40/-50/-60/-80 db 1D-probe [0;0;20] 1D-probe [0;0;20] zoom Reflected S 2,2 parameter (db) 1D-probe [0;0;20] 1D-probe [0;0;20] zoom S 2,2 & 1D-probes at sphere center vs accuracy limits An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 23

24 Which accuracy limit (3) P 0 = 0.5 W (rms) -40db -50 db -60db -80db SAR Calculation Results Pacc. (W-rms) Pabs. (W-rms) Ptissue (W-rms) SAR_10g (W/kg) Max SAR (W/kg) Q_Loss Calculation Results W (J) x x x x 10-8 Pmet. (W-rms) Pdiel. (W-rms) Ptot. (W-rms) Table 1: CHANNEL-2 single f= 298 MHz for E=-40db/-50db/-60db/-80db Table: Losses calculations results vs accuracy limits Accuracy limit = -50 db Power balance consistency (effective input power at ports (S-matrix) Vs total losses in the structure over-estimation of metallic losses?) An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 24

25 Boundary condition at z-limits: 'electric' vs 'open' (1) BC: all boundaries BC: xmin,xmax,ymin,ymax,zmax and zmin Scattering S I,2 parameters for 'electric condition Scattering S I,2 parameters for 'open' condition S-parameters for port-2 for BC= 'electric' (left) and 'open' (right), background= PEC An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 25

26 Boundary condition at z-limits: 'electric' vs 'open' (2) Energy decay at port-2 Scattering S i,2 (db) 1D-H probe at sphere center 1D-E probe at sphere center Energy decay, S-parameters at port-2, 1D-H & E probes at center of coil/sphere An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 26

27 Effect of background material: PEC vs vacuum Background= PEC BC = electric S-parameters at port-1 Background= vacuum BC = open S-parameters at port-3 S-parameters at port-1 & port-3 for background=pec & BC= electric (solid lines) and background= vacuum & BC = open (dashed lines) with Ella_VF) RF coil has lateral shielding zmin only An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 27

28 Improve the frequencies consistency Some improvement in the design handling: Larger coaxial (0.9/3 1.5/5), inactive cylinders better adjusted, lines/λ Gives better consistency but there is no unique parameter tuning of the dipoles frequencies can be made in Design Studio An 8-ch RF transmit coil for MRI at 7T/298MHz (X.H.,M.L.) CEA-DSM-Irfu-SACM 28

29 Tuning/matching circuit optimized in Design Studio A parameterized (Z,C) circuit in Design Studio (ELT & Capa) to tune/match the individual channels The tune/match Transmission Line circuit (Z,C) Ella_VF Hum Parameters are optimized in minimizing the reflected S- parameters magnitudes for each channel (single excitation) 3D-field quantities in MWS are extracted with the optimized tuned/matched circuits in DS An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 29

30 Tuning/matching: reflected S-parameters (1) Effect of the tuning/matching on the reflected S-parameters for Ella_VF (top) and Hum (bottom) An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 30

31 Tuning/matching: reflected S-parameters (2) Tuned/matched S-parameters phase and impedances (Smith) for Ella_VF An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 31

32 Tuning/matching : transmission to neighbours (Ella_VF) original After tuning/matching CHANNEL-1 (db, phase) CHANNEL-2 (db, phase) Transmitted S_parameters before (top) and after tuning/matching (bottom) for CH-1 and CH-2 An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 32

33 Tuned/matched CP: birdcage rotating field B 1 + (1) Basic birdcage rotating mode (a=1, phi= angle position) in D.S. Normalization to produce a left handed 2D mean value H 1+ = 4.68 A/m in a target square [±20,±20,20]) (90 1ms square pulse) Combine a birdcage mode with amplitude a in a S-parameters task in DS, calculate left handed polarization H 1+ in [x,y] and evaluate the 2D mean-value in target square CP1_neg (a=8.8) CP1_neg (a=6.8) Normalized left handed H1+ in axial plane [z=20] for 'Ella_VF' (left) and 'Hum' (right) An 8-ch RF transmit coil for MRI at 7T/298MHz (X.H.,M.L.) CEA-DSM-Irfu-SACM 33

34 Tuned/matched CP: birdcage rotating field B 1 + (2) Normalized 90 1ms H 1 + in mid-axial and sagittal planes with 'Ella_VF' (top) and 'Hum' (bottom) An 8-ch RF transmit coil for MRI at 7T/298MHz (X.H.,M.L.) CEA-DSM-Irfu-SACM 34

35 Tuned/matched CP: PLD and SAR_10g (3) PLD (left) and SAR_10g (right) for 'Ella_VF' (top) and 'Hum' (bottom) in axial plane [z=0] An 8-ch RF transmit coil for MRI at 7T/298MHz (X.H.,M.L.) CEA-DSM-Irfu-SACM 35

36 Tuned/matched CP: PLD and SAR_10g (4) PLD (left) and SAR_10g (right) for 'Ella_VF' (top) and 'Hum' (bottom) in sagittal plane An 8-ch RF transmit coil for MRI at 7T/298MHz (X.H.,M.L.) CEA-DSM-Irfu-SACM 36

37 Conclusion Full CAD model simulation for RF coil Dissipative elements Superposition of thin strips elements Integrated physical pre-matching system Above all, no element aligned with Cartesian grids (PBA) dispersion of the dipoles resonant frequencies tuning/matching circuit is essential Small mesh steps, small time steps, low decay, multi-channel prohibitive calculation time GPU option is required Two kind of solver are necessary (FEM, FIT) Convergence of the simulations results FEM/FIT (sphere, Hum) High res. anatomical model only needed for local investigation An 8-ch RF transceive coil for MRI at 7T/298MHz X. Hanus CEA-DSM-Irfu-SACM 37