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141 physics of mri

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141 physics of mri

  1. 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. 2. Loose Ends
  3. 3. Energy Absorption β0 M0=x M0=z 900 tip 900 RF pulse ω=ω0
  4. 4. Relaxation β0 t=t0 RF t=t1 ML=0 t=t2 ML=a t=t3 ML=b t=∞ ML=1 …. t ML t0 t1 t2 t3
  5. 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. 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. 7. Equilibrium • after 5 or so repetitions, the system reaches equilibrium • similar to water flowing into a leaky bucket relaxation RF in equilibrium
  8. 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. 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. 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. 11. Imaging Sequences part I • Gradient Echo • Spin Echo • Fast Spin Echo • Inversion Recovery
  12. 12. Goals of Imaging Sequences • generate an RF signal perpendicular to β0 • generate tissue contrast • minimize artifacts
  13. 13. Measuring the MR Signal z y x RF signal from precessing protons RF antenna β0
  14. 14. Gradient Echo • simplest sequence –alpha flip-gradient recalled echo • 3 parameters –TR –TE –flip angle • reduced SAR • artifact prone
  15. 15. Gradient Echo FID gradient recalled echo α RF pulse rephase dephase signal gradient
  16. 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. 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. 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. 19. MR Signal During Rephasingz y x RF signal “echo” RF antennaβ0
  20. 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. 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. 22. T2 and T2* Relaxation • T2* relaxation influences contrast in gradient echo imaging • T2 relaxation influences contrast in spin echo imaging
  23. 23. Gradient Echo pulse timing echo RF signal readout α0 phase slice TE
  24. 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. 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. 26. Gradient Echo changing TE TE 9 FA 30 TE 30 FA 30 susceptibility effect T2* weighting
  27. 27. Gradient Echo magnetic susceptibility post-surgical change “blooming” artifact
  28. 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. 29. Conventional GR TE 20, FA 15
  30. 30. Gradient Echo • Spoiled GR –aka SPGR, RF-FAST –spoiling destroys accumulated transverse coherence –maximizes T1 contrast
  31. 31. Gradient Echo • Contrast enhanced GR –aka SSFP, CE-FAST –infrequently used because of poor S/N –generates heavily T2* weighted images
  32. 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. 33. MTC GR TE 13, FA 50
  34. 34. Spin Echo • widely used sequence –90-180-echo • 2 parameters –TR –TE • generates T1, PD, and T2 weighted images • minimizes artifacts
  35. 35. Spin Echo FID spin echo 900 RF pulse readoutfrequency encode signal gradient 1800 RF pulse
  36. 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. 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. 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. 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. 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
  41. 41. Spin Echo pulse timing echo RF signal readout 900 phase slice TE 1800
  42. 42. WNMR Race t=0 900 RF
  43. 43. WNMR Race
  44. 44. WNMR Race t=TE/2 1800 RF
  45. 45. t=TE WNMR Race
  46. 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. 47. Spin Echo advantages • high signal to noise • least artifact prone sequence • contrast mechanisms easier to understand
  48. 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. 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. 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. 51. T1 weighted T2 weighted Spin Echo Contrast
  52. 52. Spin Echo Contrast PD weighted T2 weighted

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