Magnetization Preparation Sequences

Transcription

1 Magnetization Preparation Sequences Acquisition method may not give desired contrast Prep block adds contrast (and/or encoding) MP-RAGE = Magnetization prepared rapid acquisition with gradient echo (Mugler, ~1990) Inversion-recovery (IR) prep Fat saturation T2-prep Diffusion-weighted imaging Mag Prep 1... Imaging Sequence Mag Prep

2 (From Previous) Challenge: Diffusion GT 180 Δt GT 1D Gaussian Diffusion: l = p 2D Imagine a sequence with 2 gradients of area GT, with a 180 refocusing pulse between. What is the expected value of the spin echo signal as a function of D, Δt, GT, ignoring T2? t 2

3 Challenge: Diffusion (Solution) GT 180 GT 1 p 2 e x Phase vs x is φ = γgτ x Δt = l = p 2D t Expected value is expected value of cos(φ) Z 1 x cos( GT x) p e 2 4D t dx 4 D t p 4 D t = p e ( GT )2 D t = e ( GT )2 D t 4 D t exampleb2_2.m b =( GT ) 2 t = e bd 3

4 Contrast Review Spin Echo PD, T1, T2 Echo-train effects Gradient Echo bssfp, Gradient Spoiled (T2/T1) RF spoiled (T1) PD (how?) is inefficient, T2 is not possible 4

5 Fast Recovery (FR) or Driven Equilibrium RF G z G y 180º 180º 180º 90º -90º Fast-Recovery... G x... Signal 5...

6 Saturation or Nulling Eliminate the signal from something Chemical species Regions of image Advantages Minimal cost (example, can do short TE) Increase dynamic range for desired signal Disadvantages Exciting unwanted signal - it can come back! 6

7 Fat-Saturated FSE RF 90º 180º 180º... G z... G y Fat-Sat... G x... Signal... 7

8 Fat Suppression for Contrast PD FSE Fat-Sat PD FSE Coronal Wrist Coronal Wrist Radial cyst was otherwise iso-intense with fat 8

9 Fat Saturation RF Bandwidth Excite Fat Only Fat Dephase Fat Signal Water... Sequence (maybe short TE!) Time Fat Suppression Frequency Unsuppressed Fat Signal Chemically-selective excitation Dephaser gradient Normal imaging sequence 9 Water Suppression

10 Effect of Fat Saturation Fat-Saturated (PD) Not Fat-Saturated (T1w) 10

11 Questions: Fat Saturation What happens if the fat-sat flip angle is not 90? Residual fat signal What is the residual fat signal of the fat-sat flip angle is only 60? You get sin(60 ) or 0.5 of the fat signal Why might you use (say) 100 flip angle for fat-sat? T1 recovery between fat-sat RF and sequence How does fat-sat sensitivity to off-resonance compare to water-only excitation? (spectral-spatial) More sensitive - fat peak is wider 11

12 RF Inversion-Recovery TI 180º 180º 90º 1 Signal 0-1 Fat suppression based on T 1 Short TI Inversion Recovery (STIR) 12

13 Fat Suppression near B0 Inhomogeneity 13 Fat Sat STIR

14 RF Fluid Attenuated Inversion-Recovery TI 180º 180º 1 Signal 0-1 Fluid suppression based on T 1 FLAIR 14

15 Long Inversion Time (TI) - FLAIR Long TI suppresses fluid signal 15

16 IR Prep to enhance T1 contrast Often used with GRE (MP-RAGE) Example: Cardiac CINE, IR at start (note septum) IR-Prep RF-Spoiled 16

17 Mag-Prep: Inflow-enhanced MRA Preparation: Background Suppression Fat Suppression 17 Courtesy Pauline Worters

18 Questions: Inversion-Recovery If the inversion is too low (say 160 ) what happens to the recovery curve (shape, null-point time)? Curve shifts up (m0-m(0) decreases) Null occurs earlier How might you address the imperfect flip angle? Model the inversion efficiency in T1 fitting Use adiabatic inversion pulses 18

19 T2-Prep (Enhance T2 contrast) RF G z 180º 180º 90º -90º TE Regular Imaging Sequence 19 T2-prep + Fat-Sat Renal Artery

20 T2-Prep: Flow-Independent Angiography Inversion: Suppress synovial fluid T2-prep: Arterial-venous contrast 20 No Prep T2-prep + IR prep Courtesy Neal Bangerter

21 Questions: T2-prep What are pros/cons of using refocusing pulses in a T2-prep sequence? Reduce sensitivity to B0 Increases RF power / SAR If varying T2-prep lengths are used to measure T2, what is the effect of T2-contrast in the sequence? No effect! T2-contrast just scales the T2-prep contrast 21

22 Spatial Saturation Reduced FOV imaging Saturate bands outside FOV to prevent aliasing SSFSE SSFSE w/ Sp. Sat Product full FOV SSFSE SSFSE w/ Spatial Sat. 22

23 Spatial Saturation Use with arbitrary sequences Save time with reduced FOV Very selective w/o time penalty Cosine modulate (dual-band) Osorio JA, et al. MRM

24 Myocardial Tagging Spatially selective saturation pattern (lines, grid) Often cine acquisition Zerhouni E, 1988 McVeigh ER, MRI 1996 TrueFISP (bssfp) 24 FLASH (RF Spoiled) Courtesy J. Zwanenburg (MRM 2003)

25 Diffusion-Weighted Imaging (DWI) RF 180º G z No Diffusion 25

26 Diffusion-Weighted Imaging (DWI) RF 180º G z Diffusing Spins 26

27 Diffusion-Weighted Imaging (DWI) Low b-value High b-value ADC T2 FSE 27

28 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 28

29 Flow Encoded Imaging 29 Marcus Alley Krishna Nayak

30 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) CEST: Saturation at specific frequency 30 Henkelman RM et al. NMR in Biomedicine 2001; 14(2): Courtesy of Feliks Kogan

31 MT and EPG! EPG-X: An Extended Phase Graph formalism for systems with Magnetization Transfer or Exchange. Shaihan J Malik, Rui PAG Teixeira, Joseph V Hajnal. ISMRM Workshop on MR Fingerprinting Add state for bound Mz 31

32 Other Preparations Double IR: Non-selective, then selective Black Blood Multiple IR: Null multiple species simultaneously Arterial spin labeling (Invert blood, subtract reference) Diffusion preparation (tip-up) Motion-sensitized driven equilibrium (MSDE) Null vessel signal 32

33 Summary of Magnetization Prep Suppression: Spatial, Fat, Blood, Fluid Contrast: Inversion, T2-prep, Diffusion Encoding: Flow/motion, Diffusion, Tagging 33