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
Early multi-slice spin echo images depicted vessels in the neck as signal voids

5. Spins do not get refocused by 180o pulse
Multi-slice Spin Echo
Long TR
90o-180o
Fast flow
MRI
Slices
Flowing
Blood
Stationary
Tissue
Spins do not get refocused by 180o pulse
Slice #1
Slice #2
Slice #3

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
Using gradient (field) echo images with partial flip angles allowed blood which flowed through the 2D image plane to be depicted as being brighter than stationary tissue.

8. Motion Artifacts in read-out direction in phase-encode 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

9. The MRI Signal: Amplitude & PhaseBo
rf = B1
Net Magnetization
Real
Imaginary
Real
Imaginary

10. Dephasing Due to Motion
Gslice
time
+180o
BLOOD: phase not zero
TISSUE: phase equals zero
PHASE
time
Phase Shift
Due to Motion
in a
Gradient Field
-180o
t = 0

11. Pulsatile Motion Artifact
Aorta
Artifact
Artifact
Artifact

12. Motion Compensation Gradients
Gslice
time
Phase Shift
Due to Motion
in a
Gradient Field
+180o
PHASE
time
-180o
BLOOD: phase equals zero
t = 0
TISSUE: phase equals zero
*Only applies for constant flow.
More gradient lobes needed for acceleration.

13. Flow Artifact Correction
Spatial pre-saturation pulses prior to entry of the vessel into the slices
Surface coil localization
Shortened pulse sequences
Cardiac & respiratory gating
Motion Compensation Gradients

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

16. Methods of Magnetic Resonance Angiography
Signal Amplitude Methods
2D Time-of-Flight
3D Time-of-Flight
Signal Phase Methods
2D Phase Contrast
(Velocity Imaging = Q-flow)
3D Phase Contrast

17. Time-of-Flight MRA MethodBo M
Imaginary
Real

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

19. 2D Time-of-Flight MRA Conditions Field Echo Imaging Short TE
Partial Flip Angle
generally large
keeps stationary tissues saturated
TR and flip angle
adjusted to minimize stationary tissue
adjusted to maximize blood

20. 2D Time-of-Flight MRA Advantages Good stationary tissue to blood
flow contrast
Sensitive to flow
Minimal saturation effects
Short scan times
Can be used with low flow rate

21. 2D Time-of-Flight MRA Limitations Relatively poor SNR
Poor in-plane flow sensitivity
Relatively thick slices
Long echo times (TE)
Sensitive to short T1 species

22. Improving Contrast in Time-of-Flight MRA 1. Venous Pre-saturation
(spatial suppression)
2. Magnetization Transfer Contrast
(frequency selective irradiation)
3. Fat Saturation
4. Cardiac Gated MRA
5. Spatial variation of flip angle

23. Spatial Pre-saturation in Time-of-Flight MRA
Saturates and dephases spins before they enter imaging slice
Can be used to isolate arteries or veins
Can be used to identify vessels feeding
a given territory
Can be used to establish the direction of flow in a particular vessel

24. Magnetization Transfer Contrast
PROTON SPECTRUM
Frequency (Hertz)
“Free” Water
Lipids
“Bound” Water
217 Hz
1500 Hz
Frequency (Hertz)

25. Gradient Echo with MTC Pulse
Off-resonance rf pulse
RF excitation TX
Digitizer On
Field Echo RX
Slice Select
Gsl
Rephasing
Spoilers
Read Out
Crushers or Spoilers
Dephasing
Gro
Phase Encode
Gpe

26. MIP #1
Maximum Intensity
Projections
MIP #2
OBJECT

27. 2D TOF Application
Abdominal
Aneurysm

28. 3D Time-of-Flight MRA Conditions
Uses two phase encode gradients and volume excitation
Maximum volume thickness limited by flow velocity
Use minimum TR, adjust flip angle for best contrast

29. Three Dimensional Gradient Refocused Echo Imaging
RF pulse (short time) TX
Field Echo RX
Slab Select
Secondary
Phase Encoding
Digitizer On
Gsl
Rephasing
Crusher
Read Out
Gro
Dephasing
Primary Phase Encoding
Phase Rewinder
Gpe

30. 3D Time-of-Flight MRA Advantages
Higher resolution (thinner slices) available allowing for delineation of smoother edges
Higher signal-to-noise than 2D methods
Lower slice select gradient amplitudes results in fewer phase effect artifacts than 2D method
Short duration RF pulses can be used to excite slab – TE can be reduced

31. 3D Time-of-Flight MRA Limitations
Blood signal is easily saturated with slow flow
Relatively poor background suppression
Short T1 tissues may be mistaken for vessels

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
90o
Time of Flight MRA

34. Cerebral Venous Angiogram
TOP
Use of arterial presaturation allows visualization of cerebral venous vessels
Saggital
Sinus
FRONT
Straight
Sinus
Transverse
Sinus
Confluence
Of Sinuses
Cerebven.mpeg

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
Programmable
Automatic Injection
MRI Compatible
Allows rapid arterial injection of Gd-DTPA

38. 3D CE-MRA of Aortic Aneurysm
44 slices
32 sec scan
TR/TE
= 2.3/1.1 ms
1.5 x 1.8 x 1.8 mm pixel
Phase Phase Phase 1
Digital
Subtraction
X-ray
Angiography
Phase Phase 1
Schoenberg SO, et al. JMRI 1999;
10:

39. Bolus Chase 3D MRA
Station Station Station 3
Earlier venous enhancement noted with fast injection
Ho VB et al. JMRI 1999; 10:

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

41. Phase-Contrast MRA MethodBo 
Imaginary
Real

42. Dephasing Due to Motion
Gslice
time
+180o
BLOOD: phase not zero
TISSUE: phase equals zero
PHASE
time
Phase Shift
Due to Motion
in a
Gradient Field
-180o
t = 0

43. Phase Contrast Imaging
Velocity Encoded Image
+180o
Phase Difference
PHASE
time
Velocity Compensated Image
-180o
Motion Compensation
Gradient (Bipolar) Applied
+180o
PHASE
time
Velocity Encoded Image
-180o
TISSUE: phase equals zero
in BOTH images
BLOOD: phase is DIFFERENT in each image

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

45. 2D Phase Contrast MRA Features Can use minimum TR Good for slow flow
doesn’t rely on T1 effects
Good for slow flow
Motion is imaged in only one direction
usually slice select
Requires 2 images
Velocity compensated / velocity encoded

46. 2D Phase Contrast MRA Advantages Short acquisition times
Variable velocity sensitivity
Good background suppression
Minimal saturation effects
Short T1 tissues do not show up on images

47. 2D Phase Contrast MRA Limitations Single thick section projection
Vessel overlap artifact
Sensitive to flow in only one direction
Unstructured flow may cause problems

48. 3D Phase Contrast MRA Features Images obtained at higher spatial
resolution than 2D PC
3D PC requires at least four images:
flow compensated
x-encoded
y-encoded
z-encoded
Low velocity imaging in tortuous vessels
Takes the most time

49. 3D Phase-Contrast MRA Renal CirculationFP
Coronal, 3D PC
TR/TE = 33/6 ms
20o flip
Coronal, Gd enhanced
TR/TE = 7/1.4 ms
40o flip, false renal stenosis (FP)

50. 3D Phase Contrast MRA Advantages Thin slices
Quantitative flow velocity and direction
Excellent background suppression
Variable velocity sensitivity
Short T1 tissues do not appear on images

51. 3D Phase Contrast MRA Limitations Long acquisition times
Long TE values

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
Magnitude
Phase Contrast
No Flow
Stationary In
Out
Flow
Velocity
29 cm/s

54. Velocity Encoding Range (Venc)
MRI Velocity (cm/s)
Phase Difference
(degrees)
-180o
-Venc
True Flow Velocity (cm/s)

55. 3D Cerebrovascular Flow
Flow Encoding Right to Left
Magnitude
Saggital Sinus
Flow Encoding Anterior to Posterior
Flow Encoding Cranial to Caudal
Straight
Sinus
Ant. Cerebral aa.
Basilar a.

56. Summary
Two different approaches to MRA are commonly used: Time-of-Flight (TOF-MRA) & Phase Contrast (PC-MRA)
TOF-MRA is easy to implement and is robust but has difficulty with slow flow
3D TOF can be combined with fast imaging methods and Gd contrast agents to obtain improved depiction of vascular structures

57. Summary
PC-MRA requires more time to acquire more images but can result in high resolution, fewer flow related artifacts, and quantitative measurement of flow
Phase-contrast MRI may provide the most accurate, noninvasive method for measuring blood flow in vivo