University of Wisconsin MRI

1. University of Wisconsin MRI

2. Magnetic Resonance Angiography Walter Block, Ph.D. Charles Mistretta, Ph.D. Thomas Grist, MD Departments of Biomedical Enginering, Radiology and Medical Physics University of Wisconsin - Madison

3. Collaborators Faculty: Chuck Mistretta, Ph.D. Tom Grist, M.D. Kris Pillai, M.D. Funding: Whitaker Foundation NIH HBL Institute Students and Graduates: Dana Peters, Ph.D. Andrew Barger, Ph.D. Oliver Wieben, Ph.D. Ethan Brodsky Tian Liang Gu Aiming Lu

4. MRA Methods Time-of-Flight Phase Contrast Contrast-Enhanced

5. Acquisition Theory – Our Strength

6. ky kx k-Space Signal Image

7. ky kx k-Space Signal Image Detail Contrast

8. 5 % 10 % 20 % 50 %

9. Objectives • Describe three acquisitiontechniques • Time of Flight (TOF) • Contrast-enhanced MRA (CE-MRA) • Phase Contrast (PC) • 4D MRA ( time-resolved 3D MRA) • Discuss post-processing methods throughout

10. Objectives • Describe three acquisition techniques • Time of Flight (TOF) • Contrast-enhanced MRA (CE-MRA) • Phase Contrast (PC) • 4D MRA ( time-resolved 3D MRA) • Discuss post-processing methods throughout

11. RF Saturation Initial Value 1.0 0.5 0 RF Pulses Longitudinal Magnetization (along B0) Regrowth Equilibrium magnetization TR Time

12. Flow Related Enhancement Imaging Slice Flow 0 1 2 Mz Static Tissue Time

13. Flow Related Enhancement Vessels Static Tissue Image from Anderson, Edelman and Turski Clinical MRA

14. 2D Sequential TOF Acquisition slice advance

15. MIP Images from 2D TOF Volume Data coronal axial source image F. Korosec PhD Thesis U. of Wisconsin 1991 sagittal right carotid sagittal left carotid

16. Saturation Pulses Saturation Volume Imaging Slice

17. Saturation Pulses Isolate Arteries and Veins superior sat (arteries) inferior sat (veins) no sat F. Korosec, PhD Thesis, U. of Wisconsin, 1991

18. 2D TOF Acquisition Scan Time(TOF) = TR • NEX • # PE • # slices Advantages • Good inflow contrast • Trailing sat used for venous suppression • Retrospective reprojection Disadvantages • Voxel size greater in slice direction • Greater intravoxel dephasing due to large voxel and longer TE due to large slice select gradient amplitude • In-plane vessels can saturate • Pulsatility artifacts possible in ungated scans • Susceptible to motion

19. Pulsatility Artifact Unsaturated spins Saturated spins Systole Diastole Reduce flip angle to reduce saturation

20. Pulsatility Artifact tip angle = 90 tip angle = 30 Anderson. Edelman, and Turski, Clinical MRA

21. = SNR Voxel size * ADC time 3D TOF Acquisition

22. 3D Acquisition Scan time(TOF) = TR • NEX• # PE • # SE Advantages • More isotropic spatial resolution • Reduced intravoxel dephasing • Shorter TE than 2D • High SNR • Retrospective reprojection Disadvantages • Potential saturation in distal portions of volume • Saturation of slow flow

23. 3D TOF Multi-slab Acquisition

24. Summary

25. Objectives • Describe three acquisition techniques • Time of Flight (TOF) • Contrast-enhanced MRA (CE-MRA) • Phase Contrast (PC) • 4D MRA ( time-resolved 3D MRA) • Discuss post-processing methods throughout

26. Theory: T1 Shortening of Gadolinium • During rapid IV infusion, Gadolinium concentrated in arteries for 1 min. • Gadolinium is a potent T1 relaxation agent in blood • T1blood 1200 ms <100 ms • Arterial MR signal enhancement is proportional to T1 shortening

27. 0 60 Effect of Gadolinium on MRA Signal Through Plane 0 Gado 60 T1 = 50 Signal In-Plane 0 15 T1 = 1000 Number of RF Pulses

28. Pre-contrast Slice Post-contrast Slice

29. MIP

30. Volume Rendering

31. 2D TOF vs. 3D Contrast MRA 2D TOF 3D CE-MRA

33. Why use CE-MRA? • Fast: scans less than 1 minute • TR drops from 40 ms to 5ms • Free to orient slab along vessels, instead of perpendicular • easy to perform • eliminates respiratory motion artifact • Short TE • less sensitive to signal loss at a stenosis • less sensitive to susceptibility artifact • Uniform blood signal • independent of blood flow velocity • no “in-plane flow” saturation artifact

34. Renal CE-MRA Renal vein Renalartery Signal Renal parenchyma 0 10 20 30 40 Time (s)

35. Contrast preparation • MR compatible power injector • Gadolinium not nephrotoxic • Excreted by glomerular filtration • Load 0.3 mmol/kg Gd-DTPA (up to 40 cc) • Load 50 cc saline flush

36. Scan time for CE-MRA Scan time = TR x (Phase Enc) x (Slice Enc) • TR = repetition time • Phase Enc = # phase encoding steps • Slice Enc = number of slices Select parameters for scan <35 sec.

37. Methods to reduce image acquisition time • Shorter TR • Faster gradients • More efficient pulse sequences • Higher bandwidth • Partial Sampling of k-space • Partial Fourier (Echo, Readout, Phase encoding, Slice encoding), Projection Undersampling

38. TR=4.7 TR=8.5 RBW=64kHz RBW=32kHz SNR=22.9 SNR=44.0

39. Methods to reduce image acquisition time • Shorter TR • Faster gradients • More efficient pulse sequences • Higher bandwidth • Partial Sampling of k-space • Partial Fourier (Echo, Readout, Phase encoding, Slice encoding), Projection Undersampling

40. ky kx k-Space Signal Image

41. ky ky kx kx k-Space Signal kx • Partial Fourier • Fractional Echo

42. Zero Interpolation Processing ZIP x2 & ZIP 512 No ZIP

43. ZIP 512 No ZIP

44. Prescribing Volume Image Volume Monitor Volume

45. Timing • Injection begins prior to 3D CE-MRA • Variable scan delay • Patient instructed to hold breath • 3D CE-MRA acquisition begins

46. Arterial Venous Tissue Timing of MRA scan relative to bolus

47. Timing Artifacts Poorly timed Properly timed Maki, Prince et al JMRI 1996, 6:642-651

48. Timing Artifacts

49. Arteries & Veins Arteries

50. Coordination of Contrast Arrival Timing Scan • 2 cc test bolus • One sagittal slice per second Smart Prep • Large monitor voxel • Automatic trigger Fluoro Triggering • Real-time system • Thick 2D image @ 1 frame/s • Custom hardware Hany, JMRI 1997 Foo, Radiology 1997 Riederer, MRM 1997