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 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 2

3 Fast Recovery (FR) or Driven Equilibrium RF G z G y 180º 180º 180º 90º -90º Fast-Recovery... G x... Signal 3...

4 Fat-Saturated FSE RF 90º 180º 180º... G z... G y Fat-Sat... G x... Signal... 4

5 Fat Suppression for Contrast PD FSE Fat-Sat PD FSE Coronal Wrist Coronal Wrist Radial cyst was otherwise iso-intense with fat 5

6 Fat Saturation RF Bandwidth Excite Fat Only Fat Dephase Fat Signal Water... Sequence Time Fat Suppression Frequency Unsuppressed Fat Signal Chemically-selective excitation Dephaser gradient Normal imaging sequence 6 Water Suppression

7 Effect of Fat Saturation Fat-Saturated (PD) Not Fat-Saturated (T1w) 7

8 RF Inversion-Recovery TI 180º 180º 90º 1 Signal 0-1 Fat suppression based on T 1 Short TI Inversion Recovery (STIR) 8

9 Fat Suppression near B0 Inhomogeneity 9 Fat Sat STIR

10 RF Fluid Attenuated Inversion-Recovery TI 180º 180º 1 Signal 0-1 Fluid suppression based on T 1 FLAIR 10

11 Long Inversion Time (TI) - FLAIR Long TI suppresses fluid signal 11

12 Mag-Prep: Inflow-enhanced MRA Preparation: Background Suppression Fat Suppression 12 Courtesy Pauline Worters

13 T2-Prep (Enhance T2 contrast) RF G z 180º 180º 90º -90º Regular Imaging Sequence 13 T2-prep + Fat-Sat Renal Artery

14 T2-Prep: Flow-Independent Angiography Inversion: Suppress synovial fluid T2-prep: Arterial-venous contrast 14 No Prep T2-prep + IR prep Courtesy Neal Bangerter

15 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. 15

16 Myocardial Tagging Spatially selective saturation pattern (lines, grid) Often cine acquisition Zerhouni E, 1988 McVeigh ER, MRI 1996 TrueFISP (bssfp) 16 FLASH (RF Spoiled) Courtesy J. Zwanenburg (MRM 2003)

17 Diffusion-Weighted Imaging (DWI) RF 180º G z No Diffusion 17

18 Diffusion-Weighted Imaging (DWI) RF 180º G z Diffusing Spins 18

19 Diffusion-Weighted Imaging (DWI) Low b-value High b-value ADC T2 FSE 19

20 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 20

21 Flow Encoded Imaging 21 Marcus Alley Krishna Nayak

22 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 Courtesy of Feliks Kogan 22 Henkelman RM Section et al. NMR A2 in Biomedicine 2001; 14(2):57-64.

23 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 23

24 Summary of Magnetization Prep Suppression: Spatial, Fat, Blood, Fluid Contrast: Inversion, T2-prep, Diffusion Encoding: Flow/motion, Diffusion, Tagging 24