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141 physics of mri
This document provides an overview of the physics of magnetic resonance imaging (MRI) through a series of lectures. It covers topics such as nuclear magnetic resonance, relaxation processes, gradient echo and spin echo pulse sequences, contrast mechanisms, and how different sequence parameters affect tissue contrast. The goal is to understand how MRI generates signal from protons and utilizes magnetic field gradients to spatially encode this signal and form images with varying contrast weightings.
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141 physics of mri
- 1. Physics of MRI •Course syllabus – Lecture 1: Intro to NMR Dr. Lowe – Lecture 2: Imaging Sequences I Dr. Buckwalter – Lecture 3: Imaging Sequences II Dr. Buckwalter – Lecture 4: Spatial encoding I Dr. Buckwalter – Lecture 5: Spatial encoding II Dr. Buckwalter – Lecture 6: Spin prepped imaging Dr. Lowe – Lecture 7: Ultrafast imagingDr. Lowe http://www.indyrad.iupui.edu/public/lectures/mri/iu_lectures/mri_homepage.htm
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- 5. Relaxation and Imaging •FID (free induction decay) is the relaxation behavior following a single RF pulse • most imaging done with repetitive RF energy deposition • the interval between the RF energy pulses is called the TR interval (time to repetition)
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- 7. Equilibrium • after 5or so repetitions, the system reaches equilibrium • similar to water flowing into a leaky bucket relaxation RF in equilibrium
- 8. Differential Relaxation • shortTR • lower absolute ML • marked difference in relative signal • long TR • higher absolute ML • minimal difference in relative signal fat protons water protons
- 9. T1 Relaxation 0 0.2 0.4 0.6 0.8 1 1.2 0 10002000 3000 4000 5000 6000 msec ML long T1 short T1
- 10. Image Contrast andT1 Relaxation • shorter TRs maximize differences in T1 relaxation, generating tissue contrast • longer TRs minimize differences in T1 relaxation, reducing T1 tissue contrast
- 11. Imaging Sequences part I •Gradient Echo • Spin Echo • Fast Spin Echo • Inversion Recovery
- 12. Goals of Imaging Sequences •generate an RF signal perpendicular to β0 • generate tissue contrast • minimize artifacts
- 13. Measuring the MRSignal z y x RF signal from precessing protons RF antenna β0
- 14. Gradient Echo • simplestsequence –alpha flip-gradient recalled echo • 3 parameters –TR –TE –flip angle • reduced SAR • artifact prone
- 15. Gradient Echo FID gradientrecalled echo α RF pulse rephase dephase signal gradient
- 16. z y x z y x α0 RF t=t0t=t0+ Partial Flip α0 ML MXY M MXY = M sin(α) ML = M cos(α)
- 17. Dephasing in thexy-plane view from the top y x z Mxy y x z Mxy≈0 dephase phase coherency phase dispersion
- 18. y x z Mxy phase coherency minust2* decay Rephasing in the xy-plane view from the top rephase y x z Mxy≈0 phase dispersion
- 19. MR Signal During Rephasingz yx RF signal “echo” RF antennaβ0
- 20. T2* decay • occursbetween the dephasing and the rephasing gradients • rephasing incompletely recovers the signal • signal loss is greater with longer TEs • decay generates image contrast
- 21. T2* decay • T2*decay is always faster than T2 decay • gradient echo imaging cannot recover signal losses from –magnetic field inhomogeneity –magnetic susceptibility –water-fat incoherence
- 22. T2 and T2*Relaxation • T2* relaxation influences contrast in gradient echo imaging • T2 relaxation influences contrast in spin echo imaging
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- 24. Gradient Echo advantages • fasterimaging –can use shorter TR and shorter TEs than SE • low flip angle deposits less energy –more slices per TR than SE –decreases SAR • compatible with 3D acquisitions
- 25. Gradient Echo disadvantages • difficultto generate good T2 weighting • magnetic field inhomogeneities cause signal loss –worse with increasing TE times –susceptibility effects –dephasing of water and fat protons
- 26. Gradient Echo changing TE TE9 FA 30 TE 30 FA 30 susceptibility effect T2* weighting
- 27. Gradient Echo magnetic susceptibility post-surgicalchange “blooming” artifact
- 28. Gradient Echo • imagecontrast depends on sequence • conventional GR scan –aka GRASS, FAST –decreased FA causes less T1 weighting –increased TE causes more T2* weighting
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- 30. Gradient Echo • SpoiledGR –aka SPGR, RF-FAST –spoiling destroys accumulated transverse coherence –maximizes T1 contrast
- 31. Gradient Echo • Contrastenhanced GR –aka SSFP, CE-FAST –infrequently used because of poor S/N –generates heavily T2* weighted images
- 32. Gradient Echo • othervarieties –MTC • T2 - like weighting –IR prepped • 180 preparatory pulse –DE (driven equilibrium) prepped • 90-180-90 preparatory pulses • T2 contrast
- 33. MTC GR TE 13,FA 50
- 34. Spin Echo • widelyused sequence –90-180-echo • 2 parameters –TR –TE • generates T1, PD, and T2 weighted images • minimizes artifacts
- 35. Spin Echo FID spin echo 900 RFpulse readoutfrequency encode signal gradient 1800 RF pulse
- 36. Gradient versus SpinEcho Spin Echo FID spin echo 900RF pulse readoutfrequency encode signal gradient 1800RF pulse Gradient Echo FID gradient recalled echo α RF pulse rephase dephase signal gradient
- 37. 900 Flip z y x z y x 900 RF t=t0t=t0+ 900 After ML=0 MXY=M Before ML=M MXY=0
- 38. Dephasing in thexy-plane view from the top y x z Mxy y x z Mxy≈0 phase coherency phase dispersion Dephasing begins immediately after the 900 RF pulse. t=0 t=TE/2 900 RF
- 39. y x z Mxy phase coherency minust2 decay Rephasing in the xy-plane view from the top y x z Mxy≈0 phase dispersion t=TE/2 t=TE 1800 RF
- 40. z y x z y x z yx z y x t=TE/2 t=TE 1800 RF t=0 900 RF dephased rephased 1800 Flip
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- 46. Effects of the1800 Pulse • eliminates signal loss due to field inhomogeneities • eliminates signal loss due to susceptibility effects • eliminates signal loss due to water/fat dephasing • all signal decay is caused by T2 relaxation only
- 47. Spin Echo advantages • highsignal to noise • least artifact prone sequence • contrast mechanisms easier to understand
- 48. Spin Echo disadvantages • highSAR than gradient echo because of 900 and 1800 RF pulses • long TR times are incompatible with 3D acquisitions
- 49. Spin Echo Contrast •T1 weighted –short TR (450-850) –short TE (10-30) • T2 weighted –long TR (2000 +) –long TE (> 60) • PD weighted –long TR, short TE
- 50. Spin Echo Contrast T2Relaxation 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 100 200 300 400 500 msec Mxy long T2 short T2 T1 Relaxation 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 1000 2000 3000 4000 5000 msec ML long T1 short T1 T1 weighted - T1 relaxation predominates •Short TE minimizes differences in T2 relaxation •Short TR maximizes differences in T1 relaxation T2 weighted - T2 relaxation predominates •Long TE maximizes differences in T2 relaxation •Long TR minimizes differences in T1 relaxation
- 51. T1 weighted T2weighted Spin Echo Contrast
- 52. Spin Echo Contrast PDweighted T2 weighted