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Time of flight mra

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Time of flight mra

  1. 1. TIME OF FLIGHT Nafeesa M A C B Sc (AHS) INTERNEEDepartment of Radiology and Imaging Sciences Sri Ramachandra University, Porur, Chennai
  2. 2. INTRODUCTION• Time of flight is one of the MR Angiography techniques which produces images of the blood vessels with out using any contrast agent
  3. 3. TIME OF FLIGHT MR ANGIOGRAPHY Entry slice phenomenon
  4. 4. TIME OF FLIGHT MR ANGIOGRAPHY• Time of flight produces vascular contrast by manipulating the longitudinal magnetization of the stationary spins• The TR is kept well below T1 time of the stationary tissues so that T1 recovery is prevented
  5. 5. TIME OF FLIGHT MR ANGIOGRAPHY• Time of flight MR angiography uses incoherent gradient echo sequence in combination with gradient moment rephasing• Gradient moment rephasing is used to enhance flow• It uses additional gradients to correct the altered phases back to their original values• The flowing spins travelling along the gradients accumulate zero phase change
  6. 6. TIME OF FLIGHT MR ANGIOGRAPHY Gradient moment nulling
  7. 7. TIME OF FLIGHT MR ANGIOGRAPHYFor evaluation of Cerebral arteries
  8. 8. TIME OF FLIGHT MR ANGIOGRAPHYFor evaluation of Cerebral veins
  9. 9. TIME OF FLIGHT MR ANGIOGRAPHY 2D TOFIn 2D TOF the signals are received sequentially froma series of image slices. MR Cerebral Venogram
  10. 10. TIME OF FLIGHT MR ANGIOGRAPHY 3D TOFIn 3D TOF the signals are received from the entirevolume of interest MR Cerebral Angiogram
  11. 11. TIME OF FLIGHT MR ANGIOGRAPHY 3D TOF MR Neck Angiogram
  12. 12. TIME OF FLIGHT MR ANGIOGRAPHY 2D TOF Advantages Disadvantages Large area of coverage Lower resolution Sensitive to slow flow Venetian blind artifact 3D TOF Advantages DisadvantagesHigh resolution for smaller Saturation of in-plane flow vessels Sensitive to T1 effects Small area of coverage
  13. 13. TIME OF FLIGHT MR ANGIOGRAPHY PARAMETERS 2D TOF 3D TOFTR 40-50ms 40-50msTE Minimum permissible Minimum permissibleFlip angle Approximately 600 Approximately 600Saturation slab Inferior Superiorslice thickness 1.5mm and above 1mm and above
  14. 14. TIME OF FLIGHT MR ANGIOGRAPHYMultiple overlapping thin 3D slab acquisition MOTSAIn MOTSA overlapping thin slabs are usedAdvantages:• Reduce flow signal saturation• High spatial resolution• Good SNR
  15. 15. TIME OF FLIGHT MR ANGIOGRAPHY (MTC) Magnetization Transfer contrastMTC is used to improve the image contrast so as toenhance the conspicuity of blood vessels by effectivelysuppressing the intervening background tissue signals WITHOUT MTC WITH MTC
  16. 16. TIME OF FLIGHT MR ANGIOGRAPHY ANEURYSM
  17. 17. TIME OF FLIGHT MR ANGIOGRAPHY
  18. 18. TIME OF FLIGHT MR ANGIOGRAPHY
  19. 19. TIME OF FLIGHT MR ANGIOGRAPHY
  20. 20. TIME OF FLIGHT MR ANGIOGRAPHY Conclusion
  21. 21. THANKYOU
  22. 22. IMAGING MODETOF MRA can be either acquired in 2d or 3d modeApplications:2D:• Optimal in areas of slower signal velocity flow• Used where larger area of coverage is required.3D:• Used in high areas of velocity flow• For high resolution to visualize small vessels
  23. 23. TIME OF FLIGHT MR ANGIOGRAPHY Gradient moment nulling
  24. 24. ADVANTAGES AND DISADVANTAGESAdvantages:• Reasonable imaging time• Sensitive to slow flow• Reduced sensitivity to intra voxel dephasingDisadvantages:• Sensitive to T1 effects• Saturation of in plane flow• Enhancement is limited to either flow entering the fov or very high velocity flow.
  25. 25. VENETIAN BLIND ARTIFACTvenetian blind artifact occurs due to therespiration and the patient motion that movestissue in and out of the slice that is beingsuppressed.
  26. 26. MOTSA(multiple overlapping thin slab acquisition)Multiple thin slab acquisition reduces the signalsaturation of slowly flowing blood compared toone thick single-slab acquisition, andoverlapping slab acquisition eliminates thesignal void from each slab boundary regionoften called the venetian blind artifact. Theslices located in the overlapped region can beacquired twice with an entry slice from the firstslab and an exit slice from the next slab.
  27. 27. MOTSA(multiple overlapping thin slab acquisition)Increasing the amount of slab can reduce theslab boundary artifact. Increasing the slab by50% completely reduces the slab boundaryartifact.The disadvantage is increasing the overlapincreases the scan time per unit coverage in theslice direction
  28. 28. INTRODUCTIONTime-of-flight techniques derive contrast betweenflowing blood and stationary tissues bymanipulating the magnitude of themagnetization, such that the magnitude of themagnetization from the moving spins is large andthe magnitude of the magnetization from thestationary spins is small. this leads to a large signalfrom moving blood spinsand a diminished signal from stationary tissue spins.
  29. 29. TIME OF FLIGHT MR ANGIOGRAPHY 2D TOF Advantages Disadvantages Large area of coverage Lower resolution Sensitive to slow flow Venetian blind artifact 3D TOF Advantages DisadvantagesHigh resolution for smaller Saturation of in-plane flow vessels Sensitive to T1 effects Small area of coverage

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