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Topics. spatial saturation TOF imaging chemical saturation magnetization transfer. t=t 1 M L =0. t=t 2 M L =a. t=t 3 M L =b. 90 0 RF.  . Review: Relaxation. …. t=  M L =1. t=t 0. M L. t. t 0. t 1. t 2. t 3. Relaxation. t=t 3+ M L =0. t=t 4+ M L =0. 90 0 RF.

george-massey
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  1. Topics • spatial saturation • TOF imaging • chemical saturation • magnetization transfer

  2. t=t1 ML=0 t=t2 ML=a t=t3 ML=b 900 RF  Review: Relaxation …. t= ML=1 t=t0 ML t t0 t1 t2 t3

  3. Relaxation t=t3+ ML=0 t=t4+ ML=0 900 RF 900 RF 900 RF  t=t4 ML<b t=t5 ML<<b t=t3 ML=b t=t0 TR TR

  4. Equilibrium RF in • after 5 or so repetitions, the system reaches equilibrium • similar to water flowing into a leaky bucket equilibrium relaxation

  5. longer TR, more recovery of ML shorter TR, less recovery of ML

  6. TR and ML • prolonged TRs allow for more recovery of ML • shorter TRs allow for less recovery of ML • condition referred to as “partial saturation”

  7. Saturation • “total” magnetization • application of additional RF pulses has no effect on proton orientation • saturation exists only briefly • net magnetization recovers longitudinal relaxation immediately after protons are “saturated”

  8. Types of Saturation • spatial • fat • water • magnetization transfer (1st cousin)

  9. Spatial Saturation • application of an RF pulse immediately prior to the imaging sequence saturates all of the protons under the influence of that pulse

  10. Spatial Saturationpurpose/advantages • reduce motion artifacts in the phase encoding direction • swallowing • CSF pulsation • respiratory motion • reduce signal from flowing blood • facilitate angiography/venography

  11. Spatial Saturationdisadvantages • fewer slices per TR • timing of saturation pulse prolongs effective TR interval • higher SAR

  12. Spin Echo gradient frequency encode readout  RF pulse  RF pulse signal FID spin echo

  13. Saturation  RF pulse  RF pulse signal no echo saturation pulse additional time required for single saturation pulse

  14. z z z 0 RF y x y x y x Saturation Pulse 0 0 sat pulse t=t0 t=t0+ t=t0++ ML=0 SATURATION MXY=0 no signal

  15. SAT pulses 900 RF pulses

  16. Spatial Saturation saturation band within the FOV

  17. arterial venous superior saturation pulse (arterial) Spatial Saturationoutside the FOV stack of slices 2D acquisition inferior saturation pulse (venous)

  18. fully magnetized protons in arteries fully magnetized protons in veins end slices may have bright flow in arteries or veins arterial flow middle slices usually have “flow voids” in vessels partially saturated protons in vessels Entry Slice Phenomenon venous flow

  19. s2 s3 s1 s1 s2 s3 blood moves downstream flow direction vessel saturated spins unsaturated spins s1 900RF s1 T=TE bright flow, entry slice phenom s2 900RF on saturated spins, flow void MR Flow Void

  20. arterial venous superior saturation pulse (arterial) stack of slices 2D acquisition inferior saturation pulse (venous)

  21. Summary: Flow Effects • entry slice phenomenon due to unsaturated spins • flow void due to saturation of previous slice coupled with downstream migration of spins • spatial presaturation bands can reduce (eliminate) signal from flowing blood

  22. Magnetic Resonance Angiography • exploits flow enhancement of GR sequences • saturation of venous flow allows arterial visualization • saturation of arterial flow allows venous visualization • no IV contrast is required

  23. tumor Magnetic Resonance Angiography AP projection Lateral projection right thigh

  24. 2D TOF Angiography • anatomy imaged using a series of gradient echo images • each image is acquired separately • all slices experience entry slice phenomenon • saturation pulse placed proximal for venous imaging, distal for arterial imaging

  25. s1 s1 flow direction vessel presat band unsaturated spins s1 0RF s1 T=TE bright flow, entry slice phenom 2D TOF

  26. 2D TOF Angiography • saturation band is located the same distance from each slice to maximize its effect • “walking presat” • vascular images reconstructed using maximum intensity projection technique

  27. MIP Reconstruction . . . lateral projection AP projection SPIRAL CT ANGIOGRAPHY

  28. 2D TOF • GR images used • short TR (~ 20-40 msec) • very short TE • shortest TE times minimize intravoxel dephasing resulting in maximum flow effects • small to medium flip angles

  29. 2D TOF Carotid Study MIP

  30. Chemical Saturation • similar to spatial saturation • narrow band RF pulse causes selective saturation of water or fat protons • “chem sat” • “fat sat” • compatible with many imaging sequences

  31. Fat Sat fat selective bandwidth fat water frequency 220 Hz 1.5 T

  32. Fat Saturation  RF pulse  RF pulse signal echo from water only fat sat pulse additional time required for saturation pulse

  33. Fat Satexamples

  34. Fat Satadvantages • increase conspicuity of fluid on T2 weighted images • widens dynamic range • addresses FSE fat-fluid isointensity problem • post-gadolinium T1 weighted fat sat • reduced respiratory motion artifact

  35. Fat Sat disadvantages • fewer slices per TR • timing of saturation pulse prolongs effective TR interval • higher SAR • requires homogenous magnet • shimming

  36. Fat Sat disadvantages • requires uniformly shaped body part • doesn’t work well at base of neck, crook of ankle, etc. • not recommended with FOV > 30 cms • unreliable • works poorly at lower fields • S/N ratio drops

  37. Fat Suppression and SNR • non fat-suppressed image • each image pixel comprised of signal from water and fat in the imaging voxel • fat-suppression • reduces total signal by suppression of fat from the voxel • reduces SNR

  38. without fat suppresion high SNR with fat suppression lower SNR SI SI water and fat water only frequency frequency Fat Suppression

  39. Magnetization Transfer with MT without MT TR 550, TE 15.7, 45° TR 450, TE 15.7, 45°

  40. Magnetization Transfer • first cousin of Fat Sat • off-resonance RF pulse applied similar to Fat Sat pulse • “bound water” proton pool absorbs the RF energy • energy is transferred to “unbound” proton pool

  41. Magnetization Transfer • think of as “tissue SAT” • tissues high in proteins (brain, muscle) become darker • MT pulse causes a selective saturation effect • tissues low in proteins relatively unaffected • fat • free fluid/water/edema

  42. Magnetization Transfer saturation effect energy transfer MT pulse ~1000 kHz off-resonance free bound frequency

  43. Magnetization Transfer  RF pulse  RF pulse signal echo MT pulse additional time required for saturation pulse

  44. Magnetization Transferadvantages • generates T2-like weighting with GR images • good cartilage sequence • suppresses background tissues • improved TOF angiography • increased contrast (gadolinium) visualization

  45. Magnetization Transferadvantages • magnetic field homogeneity not critical • generates images with new contrast relationships • compatible with many sequences; also compatible with fat sat

  46. Magnetization Transfer disadvantages • fewer slices per TR • timing of saturation pulse prolongs effective TR interval • higher SAR

  47. Magnetization Transfer with MT without MT TR 550, TE 15.7, 45° TR 450, TE 15.7, 45°

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