Magnetic Field Generation for MRI:

Transcription

1 Magnetic Field Generation for MRI: State of the art and future challenges. Pierre Védrine, CEA Saclay, Irfu 1

2 Medical Imaging Superconducting Magnet for MRI Objectives and challenges for high field MRI Iseult 11.7 T MRI Magnet 2

3 MEDICAL IMAGING: A diagnostic and research tool in neuroscience 3

4 Neuro-imaging To study the human brain Neurology / Neurosurgery - Development, aging, rehabilitation surgery - Psychiatry, mental disorders surgery Neurosciences... structures & functions of the brain Health - Social behavior and culture, art,.. - Human-Computer Interaction - Learning, education,... Interaction, society 4

5 Brain functions and neurodegenerative diseases Understand how the brain works Early detection of Alzheimer's disease by MRI using a dedicated contrast agent Proof of concept at 7 Tesla on animal model Objective: To develop the technique in humans Histology 7 Tesla MRI 5

6 The magnetic resonance imaging tomorrow: A great tool for neuroscience 6

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8 MRI system High field magnet Gradient coils RF coils 8

9 SUPERCONDUCTING MAGNET is the heart of the MRI system 9

10 Superconducting magnet is the heart of the MRI system supports a very high current density with zero resistance, generates magnetic fields (1.5 to 12 T) with little or no electrical power. 1 tesla = 10,000 gauss - Earth's magnetic field = 0.5 gauss in Paris... The quality of the MRI image is linked to: Magnetic field strength: Increased Signal to Noise ratio Field Homogeneity usually defined on a spherical volume of 20mm : B/B B/B for high resolution spectroscopy 10-5 coming from the main coil, 10-7 from superconducting shim coils + resistive correction coils Field stability : field decay < 10-8 /hour (10-9 /h high resolution spectroscopy) 10

11 Magnetic Field Generation J = current desnity, t = coil thickness NI/L = ampere turns/unit length z Courtesy of T. Kyoshi NIMS B = µ J t = 0 0 µ NI L Superconducting Magnet Main Coils: Long Coil Length or Multicoils Compensation Coils: z 2, z 4, z 6 0 Superconducting Shim Coils: Field Correction z 1, z 2, x, y, zx, zy, x 2 -y 2,xy Persistent Operation Field Homogeneity Field Stability: Room Temperature Shim Coils Field Correction z 0, z 1, z 2, z 3, z 4, x, y, x 2 -y 2,xy Higher Order Components 11

12 Magnetic Field Shielding 12

13 Winding scheme External Interference Shields Shielding coils Main coil Gradient coils Iron shims x y z Cryo-shims 13

14 Superconducting Materials YBCO B Tape Plane YBCO B Tape Plane (A/mm²) J E MgB 2 Nb-Ti Maximal J E for entire LHC Nb-Ti strand production ( ) CERN-T. Boutboul '07, and (- -) <5 T data from Boutboul et al. MT-19, IEEE- TASC 06) RRP Nb 3 Sn Bronze Nb 3 Sn Complied from ASC'02 and ICMC'03 papers (J. Parrell OI-ST) filament strand with Ag alloy outer sheath tested at NHMFL SuperPower tape used in record breaking NHMFL insert coil 2007 YBCO Insert Tape (B Tape Plane) YBCO Insert Tape (B Tape Plane) MgB 2 19Fil 24% Fill (HyperTech) 2212 OI-ST 28% Ceramic Filaments NbTi LHC Production 38%SC (4.2 K) filament High Sn 18+1 MgB 2 /Nb/Cu/Monel Bronze-16wt.%Sn- Courtesy M. Tomsic, wt%Ti (Miyazaki- MT18-IEEE 04) Nb 3 Sn RRP Internal Sn (OI-ST) Nb 3 Sn High Sn Bronze Cu:Non-Cu Applied Field (T) 14

15 NbTi Conductor NbTi filaments in a copper matrix - Standard wire : 54 filaments Cu/Sc ratio Dimensions 0.4 to 1.6 mm, issulated or not ou non (Formvar, glass fiber) - Wire in channel to increase the copper section NbTi NMR wire : NbTi/Cu 36 filaments Copper 15

16 Nb3Sn Conductor Bronze route Internal Tin Bruker-EAS Oxford OST Nb3Sn Externally stabilized, rectangular NbSn wire. Cross section: 0.8 to 7.0 mm2 Number of filaments: 4000 to > Recommended magnetic field range: 12.0 to 23.5 T 16

17 Cryostat design Coils fit inside cryostats, to keep the coil at low temperature, with moderately low consumption. Thanks to persistent mode and very accurate cryostat design, LHe refilling is required every 6 to 12 months : 1-2 l/day, up to 6-7 l/day for bigger systems. Since a certain amount of LHe is usually present, up to 1000 l, the cryogenic safety has to be seriously considered in case of a quench due to the He boil-off. Quench valve opening 200 mbar, bursting disk 17

18 Safety Magnetic Fields : - Strong magnetic fields - Strong magnetic fields gradients Effect of stray fields on : -Operation of equipments -Implants or prosthetic devices Effect of field gradients : - Large attractive forces induced on ferromagnetic objects (especially for active shielded magnet) displacement of objects could cause injuries Handling of cryogenic substances : Helium & Nitrogen - during a refill - during a quench (rapid He boil-off ) avoid cold burning, suffocation Protective clothing, no smoking, Ventilation (emergency exhausts) Definition of zones: - Exclusion zone (5 Gauss line): no access for individuals with implants - Security zone : no ferromagnetic objects 18 1

19 State of the art Courtesy Agilent MRI for animals MRI for humans 11.7T /680 mm 11.7T /900 mm 9.4T /900 mm 19

20 OBJECTIVES AND CHALLENGES for high field MRI: A global competition 20

21 Why MRI at high field? Gain = 3.8 Gain = 2.8 Gain =? 1.5T 3T 7T 11,7T CNR = 0.6 CNR =2.3 CNR = 7.9 CNR =? Comparison in Contrast to Noise ratio High-field MRI = higher sensitivity, new contrasts 21

22 Spatial and time resolution A few millimeters to better than a tenth of a millimeter From seconds to milliseconds 22

23 MRI for the future: towards the high fields... Push the limits Medical : teslas «Research»: 3 5 teslas «High Field» : 7 teslas and + «Ultra high field» : 11.7 teslas Iseult 11,7 T Aimant 3.0T (Siemens) Aimant 9.4 T GE 600 mm (USA) Aimant 1.5T (GE) SHFJ/CEA 23

24 High Field MRI in the world 11.7T, 65cm, Passive shielding T, 90cm, Active shielding T : 850 systems in operation - 7T : 50 systems in operation - 1 system 8T WB, 4 systems 9.4T WB - 1 funded project 10.5T WB Minneapolis - 2 funded project 11.7T WB : CEA et NIH - 2 potential projects: Tokyo, Boston - 1 potential project 14T Seoul T : 100 systems in operation - 2 systems7t WB - 1 system8t WB T : 700 systems in operation - 7T : 30 systems in operation - 1 system 8T WB, 3 systems 9.4T WB - 4 projects 11.7T WB (NeuroSpin, Tokyo, Berkeley, NIH) 24

25 Neurospin Centre - CEA Saclay Novembre T 7T 11.7 T 3T MEG Séminaire Dautreppe novembre

26 When art meets science: the arches of Neurospin Claude Vasconi, Architect Séminaire Dautreppe novembre

27 ISEULT 11.7 T MRI MAGNET 27

28 Objectives and Challenges OBJECTIVES Development of molecular imaging at high field Three complementary aspects: Construction of an MRI scanner at ultra high field (11.7T) whole body (CEA, Siemens) Development of new technologies (MRI Univ.de Fribourg, Bruker, Siemens) Study of new contrast agents for high field MRI on target following pathologies: Alzheimer's, stroke, brain tumors (Guerbet, CEA) CHALLENGES Obtain a spatial or temporal resolution unequaled in humans, discover new contrasts Completion of the MRI magnet = first world for the field and the technology used Important technological innovations for the other elements of the imager 11.7T MRI Innovative developments for contrast agents: EST, Gd / USPIO, 19F Perspectives for industrial development & support industrial Guerbet 28

29 Iseult 11.7 T MRI Magnet B0 / Aperture Field stability Homogeneity Stray field 5 G 11.75T / 900mm 0.05 ppm/h < 0.5 ppm on 22 cm DSV 13.5 m axial, 10.5 m radial QUANTITY Unit Value Stored Energy MJ 338 Inductance H 308 Current A 1483 Length m 5.2 Diameter m 5 Weight t 132 Observer An innovative design - NbTi conductor 9,2 mm x 4,9 mm -170 double pancakes for the main coil - 2 shielding coils - Cryostat for liquid helium at 1.8 K, 1.25 bars - Voltage +/ V - 3,9 bars maximum pressure in case of quench - Dedicated refrigerator 70 l/h + 40 W à 4.2 K 11.7 T magnet section : in orange the windings, in blue the mechanical structure at 1.8 K and in violet the cryostat 29

30 Winding layout and design Blocks design Homogeneity (cancellation of terms up to the 12 th order included) Cancelation of the straight field 30

31 Double pancake design for Homogeneity Original Double Pancake design The objective is to design a magnet theoretically intrinsically homogeneous m X cos n n mϕ n m m Bz ( r, θ, ϕ) = B0 + r ZnPn (cosθ ) + + W n Pn (cosθ ) = = n 1 m 1 m Yn sin mϕ x z O O O y 2s 2s 31

32 Windings Specific design Main coil made of cryostable double-pancakes Shielding coils vacuum impregnated with epoxy resin Inter-pancake Insulation [ IPI ] (fiberglass) Main coil structure (real size) Inter-turn Insulation [ ITI ] (impr. Kapton) Conductor [ SC ] Double pancake stacking 170 Double-Pancakes 82 turns CC3D 9 juin

33 Design options validated by prototypes Séminaire Dautreppe novembre

34 Conductor manufacturing 160 km of main coil conductor: 1500 A at 11.7 T and 2.8 K, 9.2 mm x 4.9 mm 60 km of shielding coil conductor: 2100 A at 5T, 1.8 K, 9.1 mm x 4.2 mm Main coil conductor : Rutherford cable with 10 Cu/NbTi strands in a copper channel using SnSb solder Shielding coil conductor: Cu/NbTi strand in a copper channel 34

35 Double pancake manufacturing Winding of 10 preseries double pancake coils Start of the serie production of 170 doubles pancakes in december 2011 Winding machine at Alstom Belfort Double pancake coil on 3D measuring bench First double pancake April

36 11.7 T 68 cm MRI magnet for NIH 36

37 See you in 2013!... Séminaire Dautreppe novembre