Advances in Ultra-High Field MRI at 9.4T and Hybrid 3T MR-PET

Transcription

1 Mitglied der Helmholtz-Gemeinschaft Advances in Ultra-High Field MRI at 9.4T and Hybrid 3T MR-PET N. Jon Shah Institute of Neuroscience and Medicine 4 Research Centre Juelich Juelich GERMANY

2 Mitglied der Helmholtz-Gemeinschaft Magnetic Resonance Imaging Introduction to MRI

3 MRI Magnetic Resonance Imaging Anatomy MRI The interaction between magnetic fields and nuclear spins allows for a variety of opportunities to investigate structures and processes in the human body non-invasively. Angiography Quantitative Imaging Diffusion X-Nuclei fmri nstitut/mrt/fmri.jpg Slide 3

4 MR Scanner Components (simplified) 3 Types of magnetic fields: 1. Static field (B 0 z) 2. Dynamic gradients (G i = db z /dr i ) 3. Radio-frequency (B 1 z) Master Controller Amps Gradient Controller x y z RF Controller RF coil Transmit Amp x z y y z x Gradient coils Superconducting magnet (high static field) Receive Amp Image Reconstruction Slide 4

5 MR Scanner Components (simplified) 3 Types of magnetic fields: 1. Static field (B 0 z) 2. Dynamic gradients (G i = db z /dr i ) 3. Radio-frequency (B 1 z) RF coil x y z Gradient coils Superconducting magnet (high static field) Slide 5

6 1 H NMR - Nuclear Zeeman Effect Exposing a sample to an external static magnetic field, B 0, the z- components of the nuclear magnetic moments of the protons align with B 0. Without loss of generality, the direction of B 0 defines the z-axis. Two possible orientations of the magnetic moments: parallel and antiparallel with energies 1 E, z B, 0 B 0 2 B 0 =0 B 0 0 Zeeman splitting E B 0 0 Larmor frequency Slide 6

7 Equilibrium Magnetisation B There is a small excess of spins in the lower energy states. The population fraction is given by Boltzmann statistics which is a very tiny number at room temperature T. n E / k e n B T However, the small excess amounts in a measurable macroscopic magnetisation, M, which is the ensemble average of the nuclear magnetic moments. The equilibrium magnetisation for N atoms with nuclear spin quantum number, S, is given by Curie s law: M 2 2 S( S 1) N B0 3k T 0 B M 0 k B = Boltzmann constant J / K Slide 7

8 Larmor Precession B 0 0 Larmor frequency 0 B 0 Remember, in the equilibrium only the spin s z- components are aligned with B 0. The transverse components perform circular motion All spins precess about B 0 with the Larmor frequency, ω 0! But the resulting magnetisation is static and aligned with B 0! Imagine one can somehow deflect all spins, what happens with the magnetisation? The magnetisation precesses about B 0 with the Larmor frequency, ω 0! Slide 8

9 Excitation Apply a resonant radiofrequency (RF) pulse to disturb the equilibrium and to flip the magnetisation vector into the xy-plane. Resonance frequency of RF pulse = Larmor frequency ω ω 0 0 Slide 9

10 Relaxation If the magnetisation is not in equilibrium, it will approach M 0 with sample specific relaxation times: T 1 (spin-lattice relaxation time) T 2 (spin-spin relaxation time) M M z xy M M e 1 t / T 0 0 e t / T 2 1 T 2 < T 1!!! M const, i.e. funnel-shaped trajectory back to equilibrium e.g. T1=1000ms e.g. T2=100ms Slide 10

11 5 October 2011 Slide 11 Institute of Neuroscience and Medicine Bloch Equation The equation of motion is given by the Bloch equation: )ˆ ( ˆ ˆ T e M M T e M e M B M dt M d z z y y x x

12 MR Signal Reception Faraday s law: A changing magnetic flux induces a voltage in a conductive loop. (electromotors, dynamos, ) Maximise the area of the loop by orienting it perpendicularly to the transverse plane (in-plane would not cause any voltage) 0 U [V] t 0 0 Slide 12

13 Free Induction Decay (FID) T 2 * < T 2!!! Most simple experiment: excitation and subsequent signal reception (FID) The MR signal we receive does not last forever because of the T 2 decay. Local B 0 inhomogeneities cause an even shorter effective decay time, T 2 * ( T-two-star ). Slide 13

14 NMR Spectroscopy Your sample may have protons with a slightly varying precessional frequency. I.e. the Larmor frequency is influenced by the chemical binding of the protons ( chemical shift ). Nuclear Magnetic Resonance (NMR) spectroscopy. Mx (demod.) FFT My (demod.) Artificial example with water (Larmor frequency) and a compartment with a 500Hz chemical shift (Larmor frequency + 0.5kHz) Slide 14

15 The Essence of MRI: Spatial Encoding The basic idea of MRI: Make the precessional frequency a function of space! The spectrum then reflects spatial distribution. Using linear field gradients in x- and y-direction to spatially encode the measured signal in 2 dimensions 2D k-space. B 0 G x ω 0 -dω ω 0 ω 0 +dω ω 0 Slide 15 x

16 k-space received signal reconstructed image s(k x,k y ) m(x,y) k-space (MR acquisition space) 2D FFT image space Slide 16

17 k-space: Finite and Discrete Sampling In the mathematical sense, the k-space is an infinite sized and continuous construct. However, in real MR measurements there are two important restrictions in the way we can obtain information from the k-space: We can only acquire a finite part of the k-space determines image resolution sample a few discrete points of the k-space determines field-of-view Slide 17

18 Resolution - Illustration Slide 18

19 Field-of-View - Illustration Slide 19

20 Example: 2D Gradient Echo Sequence 90º y RF z x G s G p k y G r k x ADC. k y Slide 20

21 Illustration Image Acquisition in k-space Slide 21

22 Mitglied der Helmholtz-Gemeinschaft Hybrid MR-PET

23 Magnetic Resonance Imaging Molecular Imaging Anatomy Function Sensitivity/Specificity Temporal Clinical Utility Spatial Clinical Availability Development Prospects High Very Good Good Low Very Low Technological Maturity Slide 23

24 Positron Emission Tomography Molecular Imaging Anatomy Function Sensitivity/Specificity Temporal Clinical Utility Spatial Clinical Availability Development Prospects High Very Good Good Low Very Low Technological Maturity Slide 24

25 Hybrid MR-PET Anatomy Molecular Imaging Function Sensitivity/Specificity Temporal Clinical Utility Spatial Clinical Availability Development Prospects High Very Good Good Low Very Low Technological Maturity Slide 25

26 Principles of PET γ Detecter ring coincidence lookup and dataprocessing radiotracer γ β + decay and anihilation γ (511 kev) image reconstruction γ (511 kev) Slide 26

27 Simultaneous 3T MR-PET Hybrid Measurements 3T MR-PET hybrid scanner simultaneous acquisition of 18 F-FDG-PET and diverse MR contrasts Slide 27

28 Avalanche photo diodes (APD) vs. photo multiplier tubes (PMT) PMT APD Magnetically PMT sensitive APD insensitive Size mm dia. 5x5 mm Gain Up to 10 6 Up to 200 Risetime ~1 ns ~5 ns Slide 28

29 APD-based PET detector cassette Consists of: Six 12 x 12 arrays of 2.5 x 2.5 x 20 mm LSO crystals read out by 9 APDs (Hamamatsu) Two 10 channel charge-sensitive preamplifiers (Concorde MicroSystems) Temperature stability with compressed air Grazioso et al., 2005/2006 Crystal Identification map of LSO array using 3 x 3 APDs Slide 29

30 The BrainPET with 32 single cassettes Slide 30

31 MR-BrainPET: Components PET insert new integrated detector block PET insert gantry 33 mm x 33 mm x 63 mm phantom head coil Slide 31

32 Jülich 3T-MRI Scanner Slide 32

33 Detector APD A C B D 1) Constant fraction discriminator (CFD) check if photon energy exceeds noise level 2) Position lookup and block profile 512x512 positions 12x12 crystal index 3) Energy discrimination and energy resolution Rejection of scattered photons 12x12 crystals, 3x3 APDs Slide 33

34 Pile up Effect of migration of counts from the radial outer areas to the inner areas of the block lower countrate higher countrate Slide 34

35 Trues and other coincidences true coincidence: random (or delayed) coincidence: scattered coincidence and attenuation: scatter Slide 35

36 Data rebinning 2D Sinogram view φ 1 : R view φ 48 : view φ 96 : Transaxial view φ: Radial offset R R Slide 36

37 Data rebinning 3D Sinogram Sensitivity in 3D mode is much higher than in 2D mode 2D 3D BrainPET: 1399 sinogram planes (intrinsic data compression) 237MB per 3Dsinogram LOR-File (one bin per LOR) 868 MB Slide 37

38 Simultaneous 3T MR-PET Hybrid Measurements PET countrate = f (time, MR sequence) MP-RAGE FLAIR UTE EPI Slide 41

39 Influence of MR Sequences measured PET countrate MP- RAGE UTE -1.1 % -2 % countrate reduction is homogeneously distributed among the PET detector modules Slide 42

40 Sources of Influences relative PET countrate = f (gradient repetition time TR) ms 2.5ms G x 1.2ms G y G z MR sequence consists of gradients only, no MR imaging Slide 43

41 Sources of Influences relative PET countrate = f (gradient superposition) ~ G y G x G z G x +G y ~ G x +G y +G z MR sequence consists of gradients only, no MR imaging Slide 44

42 Correction of MR Influences MP- RAGE UT E MP- RAGE UT E MR sequence dependent correction (lookup tables) PET countrate after correction Slide 45

43 Template Based Attenuation Correction E. Rota Kops, IEEE MIC 2008 Slide 46

44 Data Acquisition and Preprocessing frame length listmode data deadtime correction decay correction correction of MR influences prompt? delayed Delayeds CrystalMAP variance reduction prompts LOR-file or sinogram Variance Reduced Randoms LOR-file or sinogram Slide 47

45 Image Reconstruction randoms- sinogram/lor prompts- sinogram/ LOR normalization attenuation map scatter- sinogram corrected image (flat format, cps) calibration PRESTO: J. Scheins, et.al. IEEE TMI D PET Image reconstruction: I.K.Hong, et al. IEEE TMI 2007 Scatter correction: C.C.Watson, et.al. IEEE NuclSci 2000 quantitative image (kbq/ml, ECAT format) Slide 48

46 Improvement of Image Quality only normalisation applied: Fully corrected: Fully corrected + resolution modeling: Oncology study with FET-PET Image of 20min- 40min p.i. (Herzog, Stoffels, Kaffanke, Rota, Tellmann) Slide 49

47 Our First MR-FDG-PET Images min p.i. 18 FDG-PET reconstructed with PRESTO The PET data are normalized, attenuation corrected, not scatter corrected. Simultaneous T1 MPRAGE Fusion Christoph Weirich Slide 50

48 Combined 3T MR-PET and ECAT HR+ Study Example: human study with 224 MBq FET ([ 18 F]-Fluorethyl-Tyrosin) BrainPET: tracer injection within scanner 55 min listmode acquisition 16 dynamic frames BrainPET image Patient moved to HR+ scanner ECAT HR+: 4 x 5 min frames Slide 54

49 Combined 3T MR-PET and ECAT HR+ Study BrainPET: dynamic frames Tumor Activity (kbq/ml) Activity (kbq) Cortex Whole Brain BrainPET image Time(min) human study with FET ([ 18 F]-Fluorethyl- Tyrosin) Slide 55

50 Combined 3T MR-PET and ECAT HR+ Study Activity (kbq/ml) Activity (kbq) BrainPET: extrapolation Tumor Cortex Whole Brain BrainPET image Time(min) human study with FET ([ 18 F]-Fluorethyl- Tyrosin) Slide 56

51 Combined 3T MR-PET and ECAT HR+ Study BrainPET image ECAT HR+ image human study with FET ([ 18 F]-Fluorethyl- Tyrosin) Slide 57

52 Combined 3T MR-PET and ECAT HR+ Study Tumor Activity (kbq/ml) Activity (kbq) BrainPET image 0 BrainPET (16 frames) Cortex Whole Brain Time(min) human study with FET ([ 18 F]-Fluorethyl- Tyrosin) HR+ (4 frames) ECAT HR+ image Slide 58

53 Mitglied der Helmholtz-Gemeinschaft Applications DTI combined with simultaneous PET

54 Hybrid MR-PET: DTI + resting state fmri + PET Neuner et al. 2010: Proc Intl Soc Mag Res Med Slide 60

55 Hybrid MR-PET: MRI + DTI + PET + CSI CSI T1 FA INS1/NAA PET Slide 61

56 Several MR images acquired during PET BrainPET simultaneous Post-contrast FET T1 T2 T1 Slide 62

57 T1-MRI and fmri simultaneously with FET-PET HR+: min p.i. BrainPET min p.i. T1- MPRAGE T1-MPRAGE + fmri (right finger tapping) Slide 63

58 Presurgical Imaging on a 3T MR-BrainPET T1 MPRAGE (6 min) PET: [ 18 F]-fluor-ethyl-tyrosine min p.i. BOLD imaging: Finger tapping left hand Fusion Slide 64 Dummy

59 Positron Range at 9.4T Polymer brain phantom filled with 120 I and measured at: 0 T (top left), 3 T (top right), 7 T (bottom left) and 9.4 T (bottom right). Slide 65

60 Mitglied der Helmholtz-Gemeinschaft High-Field MRI

61 Ultra High-Field MRI Market Total Systems sold: 30 Germany: seven sites Magdeburg Essen Leipzig Berlin Heidelberg Tübingen Jülich 7 T 7 T 7 T 7 T 7 T 9.4 T 9.4 T rule of thumb: 10 6 per Tesla Slide 67

62 9.4T Whole-Body Scanner in Jülich 60 cm patient bore TQ-engine gradient coil 50 cm FoV Magnet weight: 57 tonnes 870 tonnes of iron shielding 3.70 m length Stored energy: MJ Length of wire: 750 km Complete with Hybrid PET Capability! Slide 68

63 Basal ganglia, 3T, axial exterior globus pallidus putamen interior globus pallidus fornix anterior commissure claustrum 600x600x600μm 3 Slide 69

64 Basal ganglia, 9.4T, axial exterior globus pallidus putamen interior globus pallidus thalamic nuclei internal capsule claustrum fornix mamillary body 120x120x120μm times smaller voxels anterior commissure Slide 70

65 Basal ganglia, 3T, coronal capsula externa globus pallidus putamen claustrum Slide 71

66 Basal ganglia, 9.4T, coronal interior globus pallidus capsula externa exterior globus pallidus capsula interna putamen claustrum optic nerve Slide 72

67 Basal ganglia, axial view exterior globus pallidus putamen interior globus pallidus thalamic nuclei internal capsule claustrum fornix mamillary body anterior commissure 120x120x120 m 3 Slide 73

68 Thalamus putamen fornix and plexus choroideus claustrum capsula interna thalamic nuclei capsula externa Slide 74

69 Hippocampus, thalamus corpus callosum fornix and plexus choroideus external globus pallidus thalamic nuclei putamen internal globus pallidus entorhinal cortex hippocampus Slide 75

70 UHF: the other side of the coin Pronounced Susceptibility Artefacts (e.g. EPI) Wave Effects: High Frequency Artefacts Slide 76

71 New MRI 4T: undistorted sub-cortical DTI with single-shot STEAM STEAM Stöcker, Kaffanke, and Shah. Magnetic Resonance in Medicine, 2009, 61: Slide 77

72 New results: parallel transmit / selective excitation Single channel: Multi channel: True 3D selective pulses: Solving a HUGE linear system Using FZJ supercomputers Examples below need 65 GB of RAM Simulation result: (JEMRIS) 8 Tx channels 3D checkerboard 4 Tesla result: 8 Tx channel pseudo set up 3D selective excitation of a homogenous box Slide 78

73 Transmit (B 1 ) homogeneity: simultaneous excitation with multiple transmit coils I. Homogenization of the transmit field single coil 8 Tx-coils, single RF channel 8 Tx-coils, independent channels II. Zoomed MRI with 3D Gradient-Encoded RF pulses (Generalization of the slice selection process) MGH, Boston High resolution ROI imaging in short scan times! Vahedipour, Shah, Stöcker; FZ Jülich Slide 79

74 Selective Excitation (Zoomed MRI) High resolution ROI imaging in short scan times! Vahedipour, Stöcker, Shah; FZ Jülich Slide 80

75 3D Selective Excitation Slide 81

76 RF Field Distortions Function of Larmor frequency, ω 0 Bird-cage global excitation in 9komma4. b 1 -map of Centre slice of spherical water phantom. [diameter: 180mm, FA: 100% = 90 deg, RMS=48%] Slide 82

77 RF Field Distortions Function of Larmor frequency, ω 0 No way around multiple transmitters Gain additional degrees of freedom through RF shimming ptx 8 TX, 16 RX channels Slide 83

78 RF Field Distortions RF Shimming Same pulse, different complex weights Ampl P inc # # # # # # # # RF shimmed excitation in 9komma4. b 1 -map of Centre slice of spherical water phantom. [diameter: 180mm, FA: 100% = 90 deg, RMS=28%] Slide 84

79 High Field Toolbox Selective Excitation 3D ptx excitation in 9komma4. b 1 -map of Centre slice of spherical water phantom. [diameter: 180mm, FA: 100% = 90 deg, RMS=4%] Slide 85

80 High Field Toolbox Selective Excitation Let us now push the limits Pulse duration: 8ms Trajectory: 2D spiral Field of Exc: 64 circular Resolution: ca. 128x128 Acceleration: R4 Flip angle: 40 deg Slide 86

81 Mitglied der Helmholtz-Gemeinschaft Sodium MRI

82 Tumor Imaging 23Na 4T - FET Tumor Imaging 23Na 4T - FET Slide 88

83 Oligodendroglioma Grade II FLAIR FET-PET 4T Slide 89

84 Tumor patient UTE 1H FLAIR TOF T2 UTE 23Na TQF 23Na Slide 90 Sep 9, 2011 Slide 9

85 Triple-Quantum Filtered Imaging D.Fiege et al. ISMRM 2011 Slide 91

86 First In vivo 9.4 T results Anatomy - 1H MP-RAGE 4T 1 mm isotropic 5 min acq. time Sodium 9.4T TPI 2 mm isotropic 15 min acq. time Sodium 4T TPI 2 mm isotropic 15 min acq. time Slide 92

87 First in vivo 9.4 T results Slide 93

88 Opportunities MRI Patient / volunteer compliance: 2 scans in 1 (at 3T and 9.4T) Higher spatial resolution (structural imaging) Higher functional (BOLD) contrast => columnar resolution fmri? Better image quality (contrast) Non-proton MRI and spectroscopy PET Partial volume correction with MRI Attenuation correction with MRI Motion correction with MRI (navigator echoes) Hybrid MR-PET Metabolic imaging (e.g. FDG + 17O + 31P + 23Na + MP-RAGE) Accurate receptor density mapping Novel paradigms for brain function Slide 94

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106 Mitglied der Helmholtz-Gemeinschaft Acknowledgements Dr. T. Stöcker K. Vahedipour Dr. J. Felder Dr. A.-M. Oros-Peusquens A. Celik I. Neuner D. Brenner Prof. H. Herzog Dr. J. Scheins Dr. E. Rota-Kops C. Weirich L. Tellmann PET Group MR Group SIEMENS / BMBF Prof. K.-J. Langen Brain Tumour Group