Principles of MRI EE225E / BIO265. Lecture 21. Instructor: Miki Lustig UC Berkeley, EECS. M. Lustig, EECS UC Berkeley

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1 Principles of MRI Lecture 21 EE225E / BIO265 Instructor: Miki Lustig UC Berkeley, EECS

3 Question What is the difference between the images?

4 Answer Both T1-weighted spin-echo gradient-echo Lower SNR Meniscus (short TE)

5 Spin-Echo Properties Robust to off-resonance effects Excellent Contrast You -- Get cervical, thoracic and lumbar T2 weighted Fast Spin-Echo MRIs

6 Spin-Echo Properties Robust to off-resonance effects Excellent Contrast but... SAR limitations (high-power RF) Long scan times, and long echo-time Mostly multi-slice 2D Artifacts/long scan-time in 3D Gradient-echo: Fast, short TE, often 3D

7 Spin-Echo Pulse Sequence Excitation Refocusing 180 TE~10+ ms RF Gz Acquisition Gy Gx A/D Spin-Echo+ Gradient-Echo

8 Gradient-Echo Pulse Sequence No Refocusing! RF Gz Gy Gx A/D Gradient-Echo

9 Gradient-Echo Pulse Sequence TE~1+ ms RF Gz Gy Gx A/D Gradient-Echo

10 MRI is all about contrast...

11 Contrast Knobs: GRE Variations TR TE Prep. Spoiling (sequence) RF: Flip / Phase Timing (TR, TE) Preparation Sequence Flip/Phase Spoiling No preparation Fat-saturation

12 Assumptions m(x,y,t) is a function of time Approximaion: when analyzing I(x,y), assume m(x,y,t=te) Consider: T1 > TR > 3T2 Later : TR < 3T2

13 Review Magnetization Dynamics RF Excitation Free-precession/ (gradient induced) Relaxation

14 RF TR

15 Very Long TR: Full Relaxation TR>>T1 M z Mxy decays completely before next RF Mz recovers fully before next RF M xy Full signal after RF

16 Long TR: T1-Weighting TR~T1 >> T2 M z Mxy decays completely before next RF Mz partially recovers before next RF M xy T1-weighted signal after RF

17 RF TR M 0 M 0 1 e TR T 1 Mz Mxy 0 M 0 1 e TR T 1 Mxy 1st TR usually not used

19 RF TR M 0 Mz TE TE Mxy 0 T2* T2 Mxy

21 RF TR Mz - Mz + Mz M 0 Mxy

25 RF TI 90 TR TI 90 M 0 (1) (3) (5) (7) Mz (4) (8) (6) (2)

29 T1 SE

30 T2 FSE T1 SE

31 T2 FSE T1 SE Minor Stroke

32 T2 FSE T2 fluid suppressed

33 T2 FSE fluid suppressed

34 T1 SE T1 FGRE + Contrast

35 T2 FLIR T1 FGRE + Contrast

36 Short TR Steady-State Imaging TR << T1,T2 Mxy persists before next RF May have shifted in phase Adds/Subtracts from next signal makes a HUGE difference on image contrast

37 Balanced SSFP True-FISP, FIESTA, balanced FFE, BASG Do nothing at the end Balanced Gradients Oppelt 1986, Duerk 1997

38 Balanced SSFP On-Resonance 1/4 cycle precession 1/2 cycles precession High steady-state signal T2/T1 mixed contrast Sensitive to off-resonance

39 Signal Magnitude Balanced-SSFP Dark Bands Must limit precession Short TR Limits resolution 1/TR Freeman 1971 Frequency

40 Balanced-SSFP Cardiac Imaging Fast (TR=2-5ms) Good contrast Flow-compensated

41 Gradient Spoiling RF Gz FFE, FISP, GRASS, GRE, FAST, Field Echo Spin distribution across slice Reduce sensitivity to offresonance by Spoiling Mxy before next RF

42 Question RF Gz Does gradient spoiling eliminates transverse signal at the end of TR? Spin distribution across slice Spin distribution across slice First TR Steady State

43 Answer RF Gz No, its an average of balanced-ssfp but... No dark bands Lower signal than balanced-ssfp Reduced contrast

44 Gradient Spoiled vs Balanced SSFP Gradient-Spoiled Balanced SSFP (Courtesy of Krishna Nayak, USC Electrical Engineering)

45 RF Spoiled Imaging Goal: Pure T1 contrast with short TR Fast, 3D T1-Weighted imaging Need to Zero Mxy at the end of TR SPGR, FLASH, T1-FFE, RF-spoiled FAST Frahm 1987, Zur 1991

RF every TR Spoiled magnetization has random phase and

46 RF Spoiling Quadratic-phase increment RF Gz The Trick: Quadratic Phase Increment of RF Effectively Random angle RF every TR Spoiled magnetization has random phase and does not add Low SNR! Real distribution Effective distribution

47 RF Spoiled Contrast Enhanced MR Pre-Contrast SPGR Post-Contrast SPGR

48 RF Spoiled Dynamic Contrast MR Enhancement over Time Repeated 6-second breath holds, 10 seconds apart 32 slices using 3x accelerated imaging

50 Gradient Echo Sequence Comparison Sequence Balanced Gradient RF-Spoiled SSFP Echo Spoiling None Gradient RF + Gradient Transverse Magnetization Retained Averaged Cancelled Contrast T 2 /T 1 T 2 /T 1 T 1 SNR High (but Banding) Moderate Lower

51 Quiz I Here are a balanced SSFP and RF-Spoiled post-contrast image. Which is the image on the left? 1) RF-Spoiled Post Gd 2) Balanced SSFP

52 Quiz I 2) Balanced SSFP Bright Fluid (T2-like) 1) RF Spoiled post Gd T1 contrast, enhanced wall

53 Image Comparison Identify the images shown (Same TR, TE, Flip) 1) 2) 3) RF Spoiled Balanced SSFP Gradient Spoiled Gradient Spoiled RF Spoiled Balanced SSFP RF Spoiled Gradient Spoiled Balanced SSFP

54 Image Comparison Same TR, TE, flip angle Differences: Signal, Contrast, Dark-Bands 3) RF Spoiled Gradient Spoiled Balanced SSFP

55 Contrast Knobs: GRE Variations TR TE Spoiling (sequence) Flip RF: Flip Timing (TR, TE) Preparation Sequence

56 Flip Angle in Gradient Echo Sequences Does increasing the flip angle increase signal? 1) Yes: Signal always increases with flip angle. 2) No: Signal decreases as flip angle increases 3) Sometimes: The signal peaks at a specific flip angle

57 Flip Angle in Gradient Echo Sequences Does increasing the flip angle increase signal? 1) Yes: Signal always increases with flip angle. 2) No: Signal decreases as flip angle increases 3) Sometimes: The signal peaks at a specific flip angle

58 Flip Angle Selection Ernst Angle Buxton 1990

59 Flip Angle Selection? The best flip angle to use is found by: 1) Maximizing the image SNR 2) Maximizing contrast between certain tissues 3) Both 1 and 2

60 Flip Angle Selection? The best flip angle to use is found by: 3) Both 1 and 2: maximizing SNR and contrast (CNR)

61 Flip Angle Examples RF-Spoiled Best? Gradient Spoiled Balanced SSFP Best?

62 Contrast Knobs: GRE Variations TR TE Spoiling (sequence) RF: Flip / Phase Timing (TR, TE) Preparation Sequence

63 Echo Time (TE) Considerations Longer TE: T2* weighting (BOLD, Perfusion) BOLD Imaging for fmri T2*-weighted perfusion Short TE Reduced flow/motion sensitivity Reduced T2* weighting In-phase and Out-of-phase TE Water/Fat cancellation, Dixon Imaging

64 Dixon-Based Imaging RF Signal Water Fat Fat Water

65 Liver Imaging In-Phase Out-of-Phase Water Fat

66 Question Gradient spoiled images - which is opposed phase? 1) Left 2) Right

67 Gradient Spoiling: TE Effects In-Phase 2) Right Opposed-Phase Left adrenal lesion with signal loss on opposed phase imaging Diagnosis Benign Adenoma

68 Contrast Knobs: GRE Variations Prep. Spoiling (sequence) RF: Flip / Phase Timing (TR, TE) Preparation Sequence

69 Preparation Options Fat Saturation Inversion - Recovery Myocardial Tagging T2-prep Magnetization Transfer Mag Prep... Mag Prep Imaging Sequence

70 Fat Saturation Example Not Fat-Sat RF-Spoiled Fat Sat RF-Spoiled

71 Cardiac: bssfp and IR-RF-Spoiled Balanced SSFP IR-Prep RF-Spoiled

72 Summary TR TE Prep. Spoiling (sequence) RF: Flip / Phase Timing (TR, TE) Preparation Sequence Flip/Phase Spoiling No preparation Fat-saturation

73 Summary and Acronyms RF spoiled SPGR, FLASH, T1-FFE, RF-spoiled FAST Balanced SSFP True-FISP, FIESTA, balanced FFE, BASG Gradient spoiled FFE, FISP, GRASS, GRE, FAST, Field Echo Vendor acronyms are confusing -- Demand that they tell you what it really is...! Acronym source: mr-tip.com

74 Slide Acknowledgements Brian Hargreaves Lewis Shin Krishna Nayak Phil Young Robert Herfkens Anne Sawyer Marcus Alley Shreyas Vasanawala Neal Bangerter Pauline Worters Bruce Daniel Misung Han Jiang Du Graeme Bydder Gary Glover

Gradient Spoiling. Average balanced SSFP magnetization Reduce sensitivity to off-resonance. FFE, FISP, GRASS, GRE, FAST, Field Echo

Gradient Spoiling. Average balanced SSFP magnetization Reduce sensitivity to off-resonance. FFE, FISP, GRASS, GRE, FAST, Field Echo Gradient Spoiling Average balanced SSFP magnetization Reduce sensitivity to off-resonance FFE, FISP, GRASS, GRE, FAST, Field Echo 1 Gradient-Spoiled Sequence (GRE, FFE, FISP, GRASS) RF TR G z G y G x Signal