141 physics of mri
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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
- 2. Loose Ends
- 3. Energy Absorption β0 M0=x M0=z 900 tip 900 RF pulse ω=ω0
- 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)
- 6. Relaxation β0 t=t0 900 RF t=t3 ML=b t=t4 ML<b 900 RF t=t3+ ML=0 900 RF t=t4+ ML=0 t=t5 ML<<b TR TR
- 7. Equilibrium • after 5 or so repetitions, the system reaches equilibrium • similar to water flowing into a leaky bucket relaxation RF in equilibrium
- 8. Differential Relaxation • short TR • 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 1000 2000 3000 4000 5000 6000 msec ML long T1 short T1
- 10. Image Contrast and T1 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 MR Signal z y x RF signal from precessing protons RF antenna β0
- 14. Gradient Echo • simplest sequence –alpha flip-gradient recalled echo • 3 parameters –TR –TE –flip angle • reduced SAR • artifact prone
- 15. Gradient Echo FID gradient recalled echo α RF pulse rephase dephase signal gradient
- 16. z y x z y x α0 RF t=t0 t=t0+ Partial Flip α0 ML MXY M MXY = M sin(α) ML = M cos(α)
- 17. Dephasing in the xy-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 minus t2* decay Rephasing in the xy-plane view from the top rephase y x z Mxy≈0 phase dispersion
- 19. MR Signal During Rephasingz y x RF signal “echo” RF antennaβ0
- 20. T2* decay • occurs between 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
- 24. Gradient Echo advantages • faster imaging –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 • difficult to 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 TE 9 FA 30 TE 30 FA 30 susceptibility effect T2* weighting
- 27. Gradient Echo magnetic susceptibility post-surgical change “blooming” artifact
- 28. Gradient Echo • image contrast depends on sequence • conventional GR scan –aka GRASS, FAST –decreased FA causes less T1 weighting –increased TE causes more T2* weighting
- 29. Conventional GR TE 20, FA 15
- 30. Gradient Echo • Spoiled GR –aka SPGR, RF-FAST –spoiling destroys accumulated transverse coherence –maximizes T1 contrast
- 31. Gradient Echo • Contrast enhanced GR –aka SSFP, CE-FAST –infrequently used because of poor S/N –generates heavily T2* weighted images
- 32. Gradient Echo • other varieties –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 • widely used sequence –90-180-echo • 2 parameters –TR –TE • generates T1, PD, and T2 weighted images • minimizes artifacts
- 35. Spin Echo FID spin echo 900 RF pulse readoutfrequency encode signal gradient 1800 RF pulse
- 36. Gradient versus Spin Echo 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=t0 t=t0+ 900 After ML=0 MXY=M Before ML=M MXY=0
- 38. Dephasing in the xy-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 minus t2 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 y x z y x t=TE/2 t=TE 1800 RF t=0 900 RF dephased rephased 1800 Flip
- 43. WNMR Race
- 45. t=TE WNMR Race
- 46. Effects of the 1800 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 • high signal to noise • least artifact prone sequence • contrast mechanisms easier to understand
- 48. Spin Echo disadvantages • high SAR 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 T2 Relaxation 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 T2 weighted Spin Echo Contrast
- 52. Spin Echo Contrast PD weighted T2 weighted