Introduction to MRI Acquisition

Transcription

1 Introduction to MRI Acquisition James Meakin FMRIB Physics Group FSL Course, Bristol, September

2 What are we trying to achieve? 2

3 What are we trying to achieve? Informed decision making: Protocols need to be tailored to the problem (Motion? Effect size? Area of activation?) Learning some physics will make this less daunting 2

4 What are we trying to achieve? Informed decision making: Protocols need to be tailored to the problem (Motion? Effect size? Area of activation?) Learning some physics will make this less daunting A common language: Explain your needs to physicists/radiographers Understand their response There is a LOT of jargon, but you can master it! 2

5 MRI Physics Today: Basics of (nuclear) Magnetic Resonance Image Formation Functional MRI The BOLD effect Acquisition and artefacts 3

6 Nuclear Spin spin H 1 Some elementary particles (eg Hydrogen) exhibit spin Appear to rotate about an axis Charge + spin = magnetic moment 4

7 Nuclear Spin spin N H 1 S Some elementary particles (eg Hydrogen) exhibit spin Appear to rotate about an axis Charge + spin = magnetic moment 4

8 Nuclear Spin spin N H 1 S Some elementary particles (eg Hydrogen) exhibit spin Appear to rotate about an axis Charge + spin = magnetic moment 4

9 Magnetic Fields (B) No Field 5

10 Magnetic Fields (B) What happens when you place a bunch of nuclei with spin into a magnetic field? No Field 5

11 Magnetic Fields (B) What happens when you place a bunch of nuclei with spin into a magnetic field? Main B Field 5

12 Magnetic Fields (B) What happens when you place a bunch of nuclei with spin into a magnetic field? Main B Field On average, they ll tend to align with the field (a net magnetic moment) 5

13 Precession A force (Gravity or B Field) tries to tilt the spinning object But because of spin, the axis precesses instead of tilting 6

14 Excitation B 0 courtesy of William Overall ω 0 = γb 0 Energy pulse tips magnetisation away from B0...if energy rotates at resonant frequency: RF pulse! 7

15 Excitation B 0 courtesy of William Overall ω 0 = γb 0 Energy pulse tips magnetisation away from B0...if energy rotates at resonant frequency: RF pulse! 7

16 Precession B 0 courtesy of William Overall ω 0 = γb 0 Once excited, magnetisation precesses at resonance frequency 8

17 Precession B 0 courtesy of William Overall ω 0 = γb 0 Once excited, magnetisation precesses at resonance frequency 8

18 Signal detection B 0 Changing magnetic field induces current in wire Precessing magnetisation detected with coil Can only detect component in transverse (xy) plane courtesy of William Overall 9

19 Signal detection B 0 Changing magnetic field induces current in wire Precessing magnetisation detected with coil Can only detect component in transverse (xy) plane courtesy of William Overall 9

20 Magnetic Resonance ω 0 = γb 0 Magnetic: external field (B0) magnetises sample Usually detect hydrogen protons in water Potentially any element with spin (1 H, 19 F, 31 P...) Resonance: magnetization has characteristic frequency Also called the Larmour frequency For protons, resonance frequency is in RF range Proportional to the strength of the magnetic field the spin is in 10

21 Relaxation B 0 courtesy of William Overall Magnetization relaxes back into alignment with B0 Speed of relaxation has time constants: T1 and T2 11

22 Relaxation B 0 courtesy of William Overall Magnetization relaxes back into alignment with B0 Speed of relaxation has time constants: T1 and T2 11

23 Relaxation: T1 and T2 M xy Time RF pulse Signal decays according to T2 in transverse plane 12

24 Echo time (TE) & T 2 contrast Signal Gray matter White matter Echo time (TE) 13

25 MRI Physics Today: Basics of (nuclear) Magnetic Resonance Image Formation Functional MRI The BOLD effect Acquisition and artefacts 14

26 Making an image B 0 magnet Differentiate between signal from different locations 15

27 Making an image B 0 G magnet Differentiate between signal from different locations Add a spatially varying magnetic field gradient (G) Field varies linearly along one direction Gradient field adds to or subtracts from B0 15

28 Precession B 0 courtesy of William Overall ω 0 = γ(b 0 +ΔB) Resonance frequency is proportional to total field 16

29 Magnetic gradients Higher field Higher frequency B 0 Lower field Lower frequency 17

30 Magnetic gradients Higher field Higher frequency B 0 Lower field Lower frequency Protons at each position precess at different frequencies RF coil hears all of the protons at once Distinguish material at a given position by selectively listening to that frequency 17

31 Decoding Frequency: The Fourier Transform Expresses a function of time as a function of frequency Imagine an orchestra: you differentiate between different instruments based on their frequency 18

32 The Fourier Transform Fourier Amplitude transform Time / s Fourier transform Amplitude Bass Cellos (loudest) Violins Frequency / Hz 19

33 Spatial frequencies Image Space 20

34 Spatial frequencies Image Space Fourier Transform 20

35 Spatial frequencies Image Space Fourier Transform K-Space Brightness = How much of this spatial frequency is in your image 20

36 Spatial frequencies Low Spatial Frequencies Image Space Fourier Transform K-Space Brightness = How much of this spatial frequency is in your image 20

37 Spatial frequencies Low Spatial Frequencies Image Space Fourier Transform K-Space Brightness = How much of this spatial frequency is in your image 20

38 Spatial frequencies Image Space Fourier Transform K-Space Brightness = How much of this spatial frequency is in your image 20

39 Spatial frequencies Image Space Fourier Transform K-Space Brightness = How much of this spatial frequency is in your image High Spatial Frequencies 20

40 Spatial frequencies Image Space Fourier Transform K-Space Brightness = How much of this spatial frequency is in your image High Spatial Frequencies 20

41 Spatial frequencies Image Space Fourier Transform K-Space Brightness = How much of this spatial frequency is in your image 20

42 Spatial frequencies Image Space Fourier Transform K-Space Delete some spatial frequencies in the LR direction Brightness = How much of this spatial frequency is in your image 20

43 Spatial frequencies Image Space Fourier Transform K-Space Delete some spatial frequencies in the LR direction Brightness = How much of this spatial frequency is in your image 20

44 Spatial frequencies Image Space Fourier Transform K-Space Delete some spatial frequencies in the LR direction Brightness = How much of this spatial frequency is in your image 20

45 2D k-space describes contribution of each spatial frequency (8,1) x (0,4) x x ky=0 (2,1) kx=0 21

46 What does this have to do with MRI? Remember, we detect all excited protons in the object at the same time They re resonating at different frequencies due to the gradients We acquire the data in k-space! We then fill k-space & Fourier transform it to get the image 22

47 Simple MRI pulse sequence RF Acq ➀ ➁ ➂ ➃ ➄ TE TR ➀ Excite magnetization (transmit RF pulse) ➁ Wait for time TE ( echo time ) ➂ Acquire signal from transverse magnetization (Mxy) ➃ Wait until time TR ( repetition time ) ➄ Repeat from ➀ 23

48 Linescan (2DFT) Acquisition kx ky Acquire one line after each excitation Useful for structural images (minimal artefacts) 24

49 Linescan (2DFT) Acquisition kx ky Acquire one line after each excitation Useful for structural images (minimal artefacts) 24

50 Echo-planar Imaging (EPI) Acquisition kx ky Acquire all of k-space in a single shot Used for FMRI, diffusion imaging 25

51 Echo-planar Imaging (EPI) Acquisition kx ky Acquire all of k-space in a single shot Used for FMRI, diffusion imaging 25

52 Signal-to-noise ratio (SNR) Signal-to-noise ratio: describes signal robustness All else being equal, we want to maximise SNR!! high SNR low SNR SNR = Signal σ noise 26

53 Signal-to-noise ratio (SNR) 27

54 Protocol choices affecting SNR... RF receive coil & field strength Timing: TE & TR Voxel volume Scan duration Anything affecting signal!!! 28

55 Protocol choices affecting SNR... RF receive coil & field strength Timing: TE & TR Voxel volume Scan duration Anything affecting signal!!! 28

56 What affects noise? Acquisition time σnoise scan time Longer acquisition less noise higher SNR SNR improves with the square root of scan time 29

57 What affects noise? Acquisition time σnoise scan time Longer acquisition less noise higher SNR SNR improves with the square root of scan time 29

58 What affects signal? Voxel volume? 8x SNR Larger voxels have signal from more tissue! Signal proportional to voxel volume 2x2x2mm has 8x higher SNR than 1x1x1mm! 30

59 Averaging to achieve high resolution? 8x SNR 31

60 Averaging to achieve high resolution? 8x SNR Can we recover lost SNR by averaging? Yes! But it requires a 64-fold increase in scan time! 31

61 Contrast-to-noise ratio (CNR) 32

62 MRI Physics Today: Basics of (nuclear) Magnetic Resonance Image Formation Functional MRI The BOLD effect Acquisition and artefacts 33

63 Deoxyhemoglobin is the source of FMRI signal Oxyhemoglobin: diamagnetic (same as tissue) Deoxyhemoglobin: paramagnetic (magnetic) 34

64 The BOLD Effect [ Ogawa et al, 1990 ] Blood Oxygenation Level Dependent (BOLD) effect 35

65 The BOLD Effect [ Ogawa et al, 1990 ] imaging voxel Blood Oxygenation Level Dependent (BOLD) effect Creates a range of frequencies in imaging voxel 35

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68 Vascular Response to Activation neuron capillary HbO dhb HbO 2 2 dhb dhb HbO HbO 2 2 dhb HbO 2 HbO HbO dhb 2 2 HbO 2 HbO 2 HbO 2 = oxyhemoglobin dhb = deoxyhemoglobin 37

69 Vascular Response to Activation neuron capillary HbO dhb HbO 2 2 dhb dhb HbO HbO 2 2 dhb HbO 2 HbO HbO dhb 2 2 HbO 2 HbO 2 HbO 2 = oxyhemoglobin dhb = deoxyhemoglobin 37

70 Vascular Response to Activation neuron capillary HbO dhb HbO 2 2 dhb dhb HbO HbO 2 2 dhb HbO 2 HbO HbO dhb 2 2 HbO 2 HbO 2 O 2 metabolism dhb HbO 2 = oxyhemoglobin dhb = deoxyhemoglobin 37

71 Vascular Response to Activation neuron capillary HbO 2 dhb dhb dhb dhb HbO HbO 2 2 dhb dhb HbO dhb 2 dhb dhb HbO 2 O 2 metabolism dhb HbO 2 = oxyhemoglobin dhb = deoxyhemoglobin 37

72 Vascular Response to Activation neuron capillary HbO 2 HbO 2 HbO 2 HbO 2 HbO dhb 2 HbO 2 dhb dhb dhb dhb HbO 2 HbO 2 HbO 2 HbO HbO HbO 2 2 HbO 2 dhb dhb 2 HbO HbO dhb 2 HbO HbO 2 2 HbO 2 dhb 2 HbO 2 O 2 metabolism blood flow dhb HbO 2 HbO 2 = oxyhemoglobin dhb = deoxyhemoglobin 37

73 Vascular Response to Activation neuron capillary HbO 2 HbO 2 HbO 2 HbO HbO 2 2 HbO dhb 2 HbO 2 dhb dhb dhb dhb HbO 2 HbO 2 HbO 2 HbO HbO HbO 2 2 HbO 2 dhb dhb 2 HbO HbO dhb 2 HbO HbO 2 2 HbO 2 dhb 2 HbO 2 HbO 2 HbO 2 O 2 metabolism blood flow blood volume dhb HbO 2 HbO 2 HbO 2 = oxyhemoglobin dhb = deoxyhemoglobin 37

74 Vascular Response to Activation neuron capillary HbO 2 HbO 2 HbO 2 HbO HbO 2 2 HbO dhb 2 HbO 2 dhb dhb dhb dhb HbO 2 HbO 2 HbO 2 HbO HbO HbO 2 2 HbO 2 dhb dhb 2 HbO HbO dhb 2 HbO HbO 2 2 HbO 2 dhb 2 HbO 2 HbO 2 HbO 2 O 2 metabolism blood flow blood volume dhb HbO 2 HbO 2 HbO 2 = oxyhemoglobin dhb = deoxyhemoglobin [dhb] 37

75 BOLD Contrast O 2 use blood flow blood volume [dhb] BOLD signal rest [dhb] [dhb] active Echo time (T E, ms) Signal increases during activation (less decay) Signal change for longer delay (TE) Typically, 1 5% signal change 38

76 BOLD Contrast O 2 use blood flow blood volume [dhb] BOLD signal rest difference (contrast) active Echo time (T E, ms) Signal increases during activation (less decay) Signal change for longer delay (TE) Typically, 1 5% signal change 38

77 BOLD Contrast 1.0 O 2 use blood flow blood volume [dhb] BOLD signal optimal range rest difference (contrast) active Echo time (T E, ms) Signal increases during activation (less decay) Signal change for longer delay (TE) Typically, 1 5% signal change 38

78 BOLD signal and field strength (B0) SNR and BOLD increase with field strength Image artefacts worse at higher field strength 3T is currently a good tradeoff of signal vs artefacts 39

79 Sources of BOLD Signal Blood flow Neuronal activity Metabolism [dhb] BOLD signal Blood volume Indirect measure of activity (via metabolism!) Subject s physiological state & pathology can change neurovascular coupling, muddying interpretation 40

80 Hemodynamic response function (HRF) on Stimulus timing off time 41

81 Hemodynamic response function (HRF) on Stimulus timing off time Vascular response to activity is delayed & blurred 41

82 Hemodynamic response function (HRF) on Stimulus timing BOLD response off time Vascular response to activity is delayed & blurred 41

83 Hemodynamic response function (HRF) on Stimulus timing BOLD response off time Vascular response to activity is delayed & blurred Described by hemodynamic response function Limits achievable temporal resolution Must be included in signal model 41

84 What is required of the scanner? image 1 23 TR Typical stimulus lasts 1 30 s Rapid imaging: one image every few seconds Anatomical images take minutes to acquire! Acquire single-shot images (e.g., EPI) 42

85 Typical* FMRI Parameters * Typical, not fixed!! Parameter Value Relevant points T E (echo time) T R (repeat time) Matrix size / Resolution Scan duration 1.5T: 60 ms 3.0T: ms 7.0T: ms 1 4 s 64x64 / 2-3 mm 2-60 mins Determines functional contrast, set T2* HRF blurring < 1s; Poor resolution > 6s Limited by distortion, SNR, FOV Lower limit: sensitivity Upper limit: compliance 43

86 Confounds: Noise time Purely random noise (example: thermal ) Structured noise (example: physiological ) Noise: signal fluctuations leading to less robust detection with respect to statistical measures 44

87 Confounds: Artefacts Dropout Distortion Ghosting Artefacts: systematic errors that interfere with interpretability of data/images 45

88 Source of signal dropout BOLD contrast is based on signal dephasing BOLD imaging requires long delay (T E ) for contrast 46

89 Source of signal dropout sinus Dephasing also occurs near air-tissue boundaries Sensitivity to BOLD effect reduces near air-tissue boundaries 47

90 BOLD Signal Dropout Short TE Long TE Dephasing near air-tissue boundaries (e.g., sinuses) BOLD contrast coupled to signal loss ( black holes ) Air-tissue effect is often larger than BOLD effect surrounding vessels! 48

91 Image distortion field offset local warping Field map EPI We think frequency maps to spatial location... So errors in frequency cause spatial mis-localization! 49

92 Non-BOLD fmri BOLD depends on CBF, CBV, CMRO2 Consider looking at these variables separately for longitudinal studies: CBF - Arterial Spin Labeling (ASL) CBV - Vascular Space Occupancy (VASO) CMRO2 - Calibrated BOLD 50

93 Final Thoughts 51

94 Final Thoughts Learn how different experimental parameters affect SNR and image artefacts 51

95 Final Thoughts Learn how different experimental parameters affect SNR and image artefacts Tradeoffs: you can t get something for nothing, but you do have options 51

96 Final Thoughts Learn how different experimental parameters affect SNR and image artefacts Tradeoffs: you can t get something for nothing, but you do have options Get to know a physicist/radiographer: get help setting up study protocols, show them your artefacts 51

97 Final Thoughts Learn how different experimental parameters affect SNR and image artefacts Tradeoffs: you can t get something for nothing, but you do have options Get to know a physicist/radiographer: get help setting up study protocols, show them your artefacts Quality assurance: always look at your data, even if you are running a well-tested protocol 51

98 Acknowledgements Karla Miller for slides Previous years lecture (and more) available at PractiCal fmri (UC Berkeley) Animations: Spinbench 52

99 Thank you! 53