NMR and MRI. Seppo Vahasalo Philips Medical Systems MR Finland

Transcription

1 NMR and MRI Seppo Vahasalo Philips Medical Systems MR Finland

2 Contents Some local MRI history NMR and MRI Physics Magnet technology and electronics Recent trends in MRI and magnet technology Markets and players Magnetic fields. Safety, EMF directive 2

3 Philips Medical Systems MR Finland 3

4 Company History 30 Years of MRI Expertise 1978 MRI research program at Helsinki University of Technology with Instrumentarium Oy 1981 First 0.17 T superconducting scanner 1980s Instrumentarium Imaging: Acutscan, Magnaview, Mega4, Merit MRI systems (Low Field resistive solenoid magnets) 1993 Joint Venture (50/50) with Picker International Inc.; Picker Nordstar Oy founded 1995 First Open MR scanner: Picker Outlook (0.23T) 1996 Full ownership to Picker International 2000 Marconi Medical Systems Finland Oy 2000 OEM agreement with Philips 2001 PMS MR Technologies Finland Oy 2002 Panorama 0.6T with superconducting magnet 2004 Panorama 1.0T coils 2005 Start Integrated coil development for 1.5T/3T 4

5 Product History 0.02T Acutscan, T Magnaview, Outlook, Panorama 0.23T, T Merit, Proview, Panorama 0.6T,

6 NMR and MRI Nuclear Magnetic Resonance is physical phenomenon, referring to property of atoms with an odd spin to be able to adsorb energy at the specific frequency, depending on the atom type and strength of the external magnetic field NMR spectroscopy is a method to measure chemical composition of material and especially chemical bonds of organic molecules by high resolution RF probing in a strong magnetic field Magnetic Resonance Imaging is used for anatomical imaging, typically capable of whole body scans, using a single atom type and frequency, whereas 900MHz, 21.2 T NMR Magnet at HWB-NMR, Birmingham, UK being loaded with a sample 6

7 Physics of NMR - Polarization Hydrogen nucleus, for example in water behaves like a tiny magnet When H 1 nuclei are put into a strong external magnetic field, they are aligned according to that and precess around it at the Larmor frequency Larmor frequency is directly proportional to the external magnetic field strength by the gyromagnetic ratio Hz/T 7

8 Physics of NMR - Magnetization About half of the nuclei are aligned parallel to the external field and the other half antiparallel There is an excess of about 1 ppm of spins pointing to the direction of external field -> measurable macroscopic net magnetization How much is the excess for a 2mm x 2mm x 5mm voxel water (= 0.02ml = 0.02g)? One mol of water weighs 18g (2H 1 +O 16 ) and carries 2x 6.02x10 23 H 1 nuclei In one voxel there are (0.02/18) x 2x 6.02x10 23 = 1.34x10 21 H 1 nuclei The excess is 3x1.34x10 21 / 2x10 6 = 2.01x10 15 nuclei 8

9 Physics of NMR Excitation of Nuclei It is possible to bring in energy to the system at the resonant frequency (Larmor frequency). For a 21.2T NMR Spectrometer this corresponds to irradiating the sample at RF frequency of MHz In CW spectroscopy the frequency (or B1 field) of excitations is swept, in pulsed NMR a very short RF pulse is applied to excite a broad range of frequencies (Fourier spectrum) Macroscopically this corresponds to: Forcing the phase of individual spins coherent and Tilting the macroscopic magnetization away from B 0 direction 9

10 Physics of NMR - Relaxation When excitation ends, the nuclei start to relax back to the original state. That happens by emitting weak RF signals. There are two different relaxation methods: T1 (spin-matrix or loss of transversal magnetization) and T2 relaxation (spin-spin or loss of phase coherence) An efficient antenna is needed to collect these signals that are digitized and stored into a computer memory as raw data. In case of NMR there are no gradients applied and the received signal is just a FID (Free Induction Decay), 10

11 Physics of NMR Spectral Information In the real world the resonance is much more complicated When the resonance is measured at high resolution for spectroscopy, in the signal can be seen for example: Effect of multiple coupled spins in different bonds Local chemical environment (perturbations to the local magnetic field, caused by neighboring atoms 11

12 MRI Some Additional Physics In MRI linear magnetic field gradients are applied to acquire spatial information By applying a narrow bandwidth excitation pulse with a linear magnetic field gradient on, it is possible to excite only a narrow slice of tissue By manipulating the excited spins during echo formation and signal collection with additional gradients and RF pulses, signal can be identified into a single voxel By using different TE and TR times, it is possible to get tissue contrast 12

13 MRI Technology Extremes in electronics: 400A magnet currents and about 400 kw peak power is used to get μv level signals 13

14 Nobel Prize in

15 Paul C. Lauterburg University of Illinois, Urbana, IL, USA Discovered in 1971 the possibility to create a two-dimensional picture by introducing gradient in the magnetic field. By analysis of the characteristics of the emitted radio waves, he could determine their origin. This made it possible to build up two-dimensional pictures of structures (H 2 O vs D 2 O) that could not be visualized with other methods. 15

16 Sir Peter Mansfield University of Nottingham, School of Physics and Astronomy, Nottingham, United Kingdom Further developed the utilization of fast changing gradients in the magnetic field. He showed how the signals could be mathematically analyzed, which made it possible to develop a useful imaging technique. Mansfield also showed how extremely fast imaging (EPI) could be achievable. This became technically possible within medicine a decade later. 16

17 Key benefits of MRI Safe imaging method with good and selectable contrast between different tissues Diffusion imaging MR Angiography: imaging of arteries to detect abnormal flow, stenosis and aneurysms Functional imaging 17

18 MRI Magnets 18

19 MRI scanners Cylindrical, axial field Open, transverse field 19

20 Open-type magnets Field generators on either side of a gap Field direction transverse to patient 20

21 Cylindrical magnets GE Magnex Oxford/ Siemens IGC/ Philips 21

22 Simple Cylindrical Magnet 22

23 Simple Cylindrical Magnet 23

24 Simple Cylindrical Magnet 24

25 Simple Cylindrical Magnet Region of ±1% deviation from central field On axis: B(z)=B 0 +C 2 z 2 Field is not homogeneous 25

26 Sectioned Magnet Remove turns from center of coil 26

27 Sectioned Magnet Remove turns from center of coil 27

28 Sectioned Magnet Region of ±1% deviation from central field Windings can be concentrated 28

29 Sectioned Magnet Windings can be concentrated Field stays the same if centers of coils don t move 29

30 Sectioned Magnet ± 1 % region Helmholtz Coil length 0.5*diam. B(z)=B 0 +C 4 z 4 Not good enough for MRI! Hermann von Helmholtz ( ) 30

31 Sectioned Magnet 3 sections 31

32 Sectioned Magnet 3 sections 32

33 Sectioned Magnet 3 sections 33

34 Sectioned Magnet 3 sections 34

35 Sectioned Magnet 3 sections 35

36 Sectioned Magnet 3 sections 36

37 Sectioned Magnet 3 sections Maxwell Coil length 0.76*diam B(z)=B 0 +C 6 z 6 Still not good enough for MRI James Clerk Maxwell ( ) 37

38 Sectioned Magnet 4 sections 38

39 Sectioned Magnet 4 sections 39

40 Sectioned Magnet 4 sections 40

41 Sectioned Magnet 4 sections Oxford Instr (Aberdeen) Bruker, 1979 (Philips) 41

42 Sectioned Magnet 5 sections 42

43 Sectioned Magnet 5 sections 43

44 Sectioned Magnet 5 sections 44

45 Sectioned Magnet 6 sections 45

46 Sectioned Magnet 6 sections 46

47 Sectioned Magnet 6 sections ± 1 % region length 1.3*diam. B(z)=B 0 +C 12 z 12 State of the art MRI-magnet 47

48 Cylindrical MRI Magnet ±1000 ppm 48

49 Cylindrical MRI Magnet ± 100 ppm 49

50 Cylindrical MRI Magnet ± 10 ppm 50

51 Cylindrical MRI Magnet ± 1 ppm B(z)=B 0 +C 12 z 12 large gradients at edge of FOV Magnetic force is proportional to field gradient... Safety issue? 51

52 Cylindrical MRI Magnet Full range FOV boundary gradients are invisible. Strong gradients are located outside magnet. 52

53 Cylindrical MRI Magnet FOV boundary gradients are invisible. Strong gradients are located outside magnet. 53

54 Cylindrical MRI Magnet FOV boundary gradients are invisible. Strong gradients are located outside magnet. 54

55 External field Magnetic force hazard Strong gradients are located outside magnet Force on piece of iron: F x = m dip db/dx Typical gradient 10 T/m F mag 50 F gravity 55

56 External field Pacemaker hazard Fields > 0.5 mt reach far outside magnet B(r) 1/r 3 10% of central field 56

57 Active shielding Active shielding Coils at larger diameter with opposing current Shield coil 57

58 Active shielding Active shielding Coils at larger diameter with opposing current 58

59 Active shielding Active shielding Coils at larger diameter with opposing current 59

60 Active shielding Active shielding Coils at larger diameter with opposing current 60

61 Active shielding Typical compact actively shielded magnet configuration 0.5 mt boundary < 4 m axial < 2.5 m radial B(r) 1/r 5 61

62 Shielded Magnet external field Magnetic force hazard Active Shielding does NOT solve the magnetic force problem! 62

63 Magnetic dipole flying into magnetic Field Potential energy = m dip. B converted into kinetic energy Impact velocity independent of field details Comparable to drop from high building 63

64 MRI Magnet external field The magnet trap The stronger the field the deeper the hole 64

65 Compact cylindrical magnet technology 3 tesla built tesla built

66 Magnet technology details Coil Helium tank Radiation shield Vacuum can Helium 66

67 Superconducting magnet technology 4.2 K First generation compact magnet Patient Space 67

68 Operating conditions of superconductor B conductor Quench Ample margin needed between actual current and critical current More field >> more conductor Current 68

69 Superconducting magnet technology 4.2 K First generation compact magnet Patient Space 69

70 Superconducting magnet technology 4.2 K Thinner structures Reduced clearance Increased current density Patient Space Stronger magnet in same envelope 70

71 Superconducting magnet technology 4.2 K NbTi/Cu wire 1-2 mm 200 A/mm2 Patient Space 71

72 Superconducting magnet technology 4.2 K NbTi/Cu wire 1-2 mm 200 A/mm2 Patient Space 72

73 High temperature superconductors and MRI Problems with HTC superconductors in MRI are Maximum current density, maximum critical flux density Limited length of available wires -> At the moment not cost effective We have done a feasibility study for a 0.6T HTC iron core MRI magnet Possible to build With the extrapolated developments (improvements in current/flux density, reductions in price) will be viable soon, but.. Trend is towards higher field strengths, so this design does not make sense commercially 73

74 Ultra-high field magnets Magnex 7T and 8T 74

75 9.5T Whole Body Magnet University of Illinois in Chicago Imaging of Na ions Requires slow motion in magnetic field 75

76 Cost aspect of Ultra-high field magnets Rule of thumb: system cost is about $1 million/t up to 7T systems 1.5T system cost $1.5M The rule applies up to 7T, after that exponential growth of cost A 7T system Has some 420 kilometers of superconducting wire Weighs about 30 tons Requires about 440 tons of steel around magnet for stray field shielding From 7T to 9.4T add about $8M Above 10T the cost is about $10M/T, so a 15T system would cost $150 million In France plans to build a 11.7T whole body magnet 76

77 Better Detectors? One option to higher magnetic field strength is to use more sensitive detectors Ultimately thermal noise from sample and environment sets the limit for SNR in detection Best results with SQUIDS and optical atomic magnetometers, still not competitive with traditional resonant antennas with LNAs 30 mt magnetic field, SQUID detector (Vadim Zotev, LANL,NM) Optical magnetometer IR laser, Cesium filled cell, detector. John Kitching, NIST 77

78 Key properties of cylindrical MRI magnets Field generation: Field shaping: Stray-field containment: Coils Coils Coils Enclosed in cryostat with liquid helium L/D ratio > 1.5 Common architecture up to 3 Tesla Unshielded long coils for 7T and more 78

79 Open-type magnets Technology determined by central field - low field (<0.4T) - mid field ( T) - high field (>0.8T) 79

80 Low-field open magnets (B<0.4T) Iron magnetic circuit Air gap with pole shoes Pole shoe D>2 x gapsize (Field shaping) Yoke (Return flux) 80

81 Low-field open magnets + resistive coils Drive coil More field >> more ampereturns heavier yoke 81

82 Resistive Magnets with iron flux return yoke and pole shoes, B 0.2 T Philips Siemens 82

83 Low-field open magnets + permanent magnet Layer of PM material (Nd B Fe blocks) More field >> more PM material heavier yoke 83

84 Example of permanent magnet Hitachi Aperto 0.4 tesla Yoke Magnet Pole piece Column 84

85 Limits of yoke/pole configuration Power dissipation of resistive magnet too large Permanent magnet material cost explodes Magnetic saturation of pole iron For B > 0.5T use superconducting drive coils Field shaping with iron rings 85

86 Mid-field open magnets + superconducting coils Drive coil (in Cryostat) Rings to profile field in gap More field >> more ampereturns heavier yoke 86

87 Mid-field open magnet Iron flux return yoke Iron ring structure Drive coil Philips 0.6 T 87

88 Examples of Mid-field open magnets with iron flux return and superconducting coils Philips 0.6T Panorama Hitachi 0.7T Airis 88

89 Iron-less superconducting high-field open magnets Weight of yoke becomes prohibitive at >1 tesla ( 40 tons at 0.7 tesla). Fully superconducting coil system for field generation and field shaping External field cancelled by active shield coils 89

90 Fully superconducting 1.0T open magnet (Philips/IGC) Shield coil Main coil 90

91 Fully superconducting 1.0T open magnet (Philips/IGC) Vacuum tank of cryostat Total weight 6 tons zero boil-off 91

92 Open magnet summary B<0.4 tesla 0.5<B<0.8 T B>0.8T Field generation Resistive coil Supercon coils Supercon coils Nd B Fe Field shaping Pole piece Coils + Iron Coils Flux return Iron yoke Iron yoke Coils Coils 92

93 Trends in MRI and Magnets Going for higher field strengths Initially 0.5T magnet In 90 s 1.5T system the majority Now 3.0T magnets are the fastest growing sector (1.5T still biggest) Now biggest frenzy in short wide bore magnets Other fashions: In 90 s gradient strength and slew rate In early 00 s number of receive channels Today magnetic field strength and openness Siemens Magnetom Espree, 1.5T Bore diameter 70cm, length 125cm 93

94 MRI Markets and players Price about k /system About 3000 systems sold annually Biggest market share: Siemens Traditional number one, GE has been trailing, now about the same with Philips All the rest (Hitachi, Toshiba, etc) fairly small 94

95 Magnetic fields and safety Now about 50 years of research and patient scans -> No adverse health effects caused by magnetic fields Movement in strong static field may induce temporary nausea and dizziness, typically affect only engineers and operators, not patients. Biggest concern projectile effects and RF burns 95

96 Concerns with legislation in Europe Directive 2004/40/EC of the European Parliament, originally to be implemented by 30 th April 2008 in member countries. Protection of workers from exposure to electromagnetic fields Exposure limit for low frequency 40 ma/m 2 (rms) Order of Magnitude calculation for a person moving at speed of 1.25 m/s in 1.5T magnetic field: J move = 375 ma/m 2 (peak) Similar problem for gradient field and RF fields Because of fierce opposition from MR community, implementation in law has now been postponed for 4 years Current exposure limits: IEC: 4T for patients ICNIRP: 2T for workers FDA :7T for patients Simulated RF adsorbtion 96

97 Thanks Thanks to J. Overweg and M. Savelainen for simulation/magnet pictures and magnet information Thank you for your attention Questions? 97

98