Introduction to Magnetic Resonance Angiography

1. Introduction to Magnetic Resonance Angiography Geoffrey D. Clarke, Ph.D. Division of Radiological Sciences University of Texas Health Science Center at San Antonio

2. Overview Flow-Related Artifacts in MRI Time-of-Flight MR Angiography Contrast-Enhanced MR Angiography Phase-Contrast MR Angiography Quantitative Flow Imaging

3. Flow Voids & Enhancements In spin echo imaging vessels appear as signal voids same volume of blood does not experience both 90o and 180o pulses In flow effect may cause unsaturated blood to appear bright in slice that is most proximal to heart Saturation effects cause diminished signals in blood flowing parallel to image plane

4. Vessel Signal Voids

6. Field Echoes & Bright Blood Partial Flip Angle/Field Echo Images Short TR, Short TE Only one TX RF pulse (?o) Blood has Greater Proton Density than Stationary Tissues

7. Bright Blood Images

8. Motion Artifacts in read-out direction data acquired in time short compared to motion blurring of edges in phase-encode direction ghosting presenting as lines & smudges in slice-select direction variable partial volume, difficult to detect

11. Pulsatile Motion Artifact

13. Flow Artifact Correction

14. Magnetic Resonance Angiography (MRA)

15. MRA Properties Utilizes artifactual signal changes caused by flowing blood to depict vessel lumen May include spin preparation to suppress signal from stationary tissues or discriminate venous from arterial flow Does not require exogenous contrast administration, but contrast agents may be used to enhance MRA for fast imaging

18. Time of Flight Effect T1 of flowing water is effectively shorter than the T1 of stationary water Two contrast mechanisms are responsible: T1 saturation of the stationary tissue In-flow signal enhancement from moving spins

25. Gradient Echo with MTC Pulse

27. 2D TOF Application

29. Three Dimensional Gradient Refocused Echo Imaging

32. 3D-TOF Application:Cerebral Arteries – Circle of WIllis TR /TE = 40 / 4.7 ms 64 partitions, 48 mm slab, 0.75 mm per partition Flip angle = 25o 256 x 256, 18 cm FOV, 0.78 x 1.56 mm pixel MTC contrast Venous Presaturation

33. Circle of Willis Time of Flight MRA

35. Multi-Slab 3D TOF MRA Hybrid of 2D and 3D methods: Thin 3D slabs used Good inflow enhancement Multiples slabs to cover volume of interest High resolution Short TE Relatively time inefficient

36. Gd Contrast Enhanced MRA Gd contrast agents decrease T1 and increase CNR of blood and soft tissue Along with ultra-fast 3D sequences, allow coverage of larger VOI’s Shorter acquisition times allow breath-holding for visualization of central and pulmonary vasculature

37. MRI Compatible Power Injectors

38. 3D CE-MRA of Aortic Aneurysm

39. Bolus Chase 3D MRA

40. Normal Runoff MRA Image of tissue surrounding vessel can be manually striped off

44. Magnetic Field Gradients in MRI(Two More Functions) Slice Selection Phase Encoding Frequency Encoding Sequence Timing (Dephase/Rephase) Motion Compensation Motion Encoding

49. 3D Phase-Contrast MRA Renal Circulation

52. Flow Measurement with PC-MRI Typically uses 2DFT phase contrast method Slice positioned perpindicular to axis of vessel ROI drawn to delineate vessel lumen Average value in ROI is mean velocity Area of ROI is vessel cross-sectional area Flow = mean velocity * Area For pulsatile flow, multi-phase cine required

53.  Phase Contrast Velocity Images

55. 3D Cerebrovascular Flow