Sequence Overview. Gradient Echo Spin Echo Magnetization Preparation Sampling and Trajectories Parallel Imaging. B.Hargreaves - RAD 229

Transcription

1 Sequence Overview Gradient Echo Spin Echo Magnetization Preparation Sampling and Trajectories Parallel Imaging 75

2 Pulse Sequences and k-space RF k y G z k x G x 3D k-space G y k y k z Acq. k x 76

3 Gradient Echo Pulse Sequence Flip Angle RF TE ~ 1+ ms G z G y G x Signal 77 Slice-Select Gradient Phase- Encode Gradient Dephaser Gradient Refocusing Gradient Gradient Echo Readout Gradient???

4 RF-Spoiled Gradient Echo Contrasts Gradient Spoiled Balanced SSFP

5 Spin Echo: T 2 and T 2 * Decay Gradient Echo Spin Echo Courtesy of Kim Butts Pauly 79

6 Spin Echo Pulse Sequence RF 180º TE ~ 8+ ms G z Slice-Select Gradient Slice-Select Gradient G y G x Signal 80

7 Basic Spin Echo Considerations Pros: Refocusing pulse reverses dephasing Image acquired at spin echo increases signal Cons: RF power deposition (SAR) Longer echo times than gradient echo (GRE) 81

8 Echo Train Imaging RF Signal k y k y k x k x 82 PD-weighted k-space T2-weighted k-space

9 Echo Train Order and Contrast Proton-Density: high signal but blurring T2-weighted: edge enhancement 3D offers more options Proton Density Weighted T2 Weighted 83

10 Single-Shot FSE (SSFSE, HASTE) Entire image acquired in single echo train Lower resolution Significant echo-train blurring Robust to motion RF... 84

11 Fast Recovery (FR) or Driven Equilibrium RF G z G y 180º 180º 180º 90º -90º Fast-Recovery... G x... Signal 85...

12 Preparation Sequences Acquisition method may not give desired contrast Prep block adds contrast MP-RAGE = Magnetization prepared rapid acquisition with gradient echo (Mugler, ~1990) Inversion-recovery (IR) prep Fat saturation T2-prep Diffusion-weighted imaging 86

13 Fat-Saturated FSE RF 90º 180º 180º... G z... G y Fat-Sat... G x... Signal... 87

14 Fat Saturated PD vs T1 FSE Fat-Saturated PD T1 FSE 88

15 RF Inversion-Recovery TI 180º 180º 90º 1 Signal 0-1 Fat suppression based on T 1 Short TI Inversion Recovery (STIR) 89

16 Fat Suppression near B0 Inhomogeneity 90 Fat Sat STIR

17 Fluid Attenuated Inversion-Recovery TI 180º 180º RF 1 Signal 0-1 Fluid suppression based on T 1 FLAIR 91

18 Long Inversion Time (TI) - FLAIR Long TI suppresses fluid signal 92

19 Mag-Prep: Inflow-enhanced MRA Preparation: Background Suppression Fat Suppression 93

20 T2-Prep (Enhance T2 contrast) RF G z 180º 180º 90º -90º Regular Imaging Sequence 94 T2-prep + Fat-Sat Renal Artery

21 T2-Prep: Flow-Independent Angiography Inversion: Suppress synovial fluid T2-prep: Arterial-venous contrast No Prep T2-prep + IR prep 95

22 Diffusion-Weighted Imaging (DWI) RF 180º G z No Diffusion 96

23 Diffusion-Weighted Imaging (DWI) RF 180º G z Diffusing Spins 97

24 Diffusion-Weighted Imaging (DWI) Low b-value High b-value ADC T2 FSE 98

25 Phase Contrast RF G z Frequency Position = (x Z Phase is not zero! (any position) G x dt + x 0 Z G x tdt) x Zero Moment First Moment 99

26 Flow Encoded Imaging 100 Marcus Alley Krishna Nayak

27 Magnetization Transfer (MT) Saturate very-short-t2 water bound to macromolecules MT effect causes saturation of free water (signal loss) More RF generally causes more MT saturation (adverse) Courtesy of Feliks Kogan 101 Henkelman RM et al. NMR in Biomedicine 2001; 14(2):57-64.

28 Sampling & Point-Spread Functions PSF = Fourier transform of sampling pattern Also just 1 s at all sample points Simple, mostly a matter of scaling in both domains k-space Sampling Point-Spread Function Fourier Transform Extent Spacing Width FOV 102

29 Partial Fourier Acquisition/Reconstruction k y k y k x k x k y k x 103

30 Alternate k-space Trajectories k y k y k y k x k x k x Cartesian EPI Spiral k y k y k x k x 104 Radial Projection

31 Parallel Imaging Coils have limited sensitivity Unalias based on known sensitivities (SENSE) Limited sensitivity results in k-space correlations Fill in missing k-space (GRAPPA) Build up FOV with coil arrays 105

32 SENSE: Unalias Image Sensitivity 1 (S 1 ) Sensitivitiy 2 (S 2 ) Pruessmann 1999 SENSE Image A A A B B B Coil 1 Signal (C 1 ) Coil 2 Signal (C 2 ) When it fails A A B B 106

33 SENSE: Brief Mathematics At each pixel Using Coil 1: Using Coil 2: S1 = C1A x A + C1B x B S2 = C2A x A + C2B x B A B If we know C1 and C2 at A,B and signals S1 and S2, A = C1B S2 - C2B S1 C2AC1B - C2BC1A B = C2A S1 - C1A S2 C2AC1B - C2BC1A More complicated with more than 2 coils If denominator is small, noise amplification 107

34 SENSE Calibration Low-resolution images from each coil Divide images by RMS image or body coil image Challenge: coil sensitivity in area of low signal k phase k read Low Resolution Image 108

35 GRAPPA: Coil Sensitivities and k-space Reduced Image Extent Blurred Image k y k y k y k x k x k x Blurred k-space Reduced k-space Extent 109

36 GRAPPA Calibration Fully-sampled central k-space Find data correlation between lines/coils Note: data-driven vs model (SENSE) Not just image vs k-space! k phase Coil 2 Coil 1 Coil 3 Griswold 2002 k read 110 Repeat for all calibration points and all coils

37 GRAPPA Synthesis Use kernel information to synthesize data Repeat for all coils Combine coils and reconstruct Coil 3 Coil 2 Coil 1 k phase Griswold 2002 k read 111

38 Summary of Sequence Overview Gradient Echo Sequence Spin Echo sequences Magnetization Preparation Imaging Readouts / Sampling Parallel Imaging 112