1. Artifacts and Pitfalls in MR Angiography (MRA) for Pulmonary Embolism Meyer CA, Schiebler ML, Reeder SB, Francois CJ, Nagle SK

2. Disclosures Financial: UW has a research agreement with GE Healthcare
Only gadofosveset (Ablavar®) is FDA approved for MRA
Use of other gadolinium based contrast agents is not FDA approved for MRA

3. Outline MRA Technique MRI Reconstruction Patient Contrast Related
Bolus Timing
Respiratory Motion
Gibbs Ringing
Bolus Duration
Wrap and Parallel Imaging
Transient Contrast Interruption
Amplifier Over-ranging
Dual Injection Error

4. MRA Technique Multiphasic acquisition Full chest coverage
fluoro triggered arterial phase
immediate post-injection phase
2nd post-injection “steady state” phase using lower flip angle
each phase done during single breath hold
Full chest coverage
High spatial resolution
2D parallel imaging
must use multichannel phased array coil
recommend ≥ 8 channels
Freq
encoding
Phase encoding
Slice encoding

5. MRA Technique k-space “corner cutting” Elliptical centric acquisition
Freq
encoding
Phase encoding
Slice encoding
k-space “corner cutting”
eliminates 22% of readouts
Elliptical centric acquisition
enables fluoro-triggering
0.1 mmol/kg of gadobenate dimeglumine
typically 15-20mL
diluted with saline to a total volume of 30 mL

6. MRA Technique

7. Slab Orientation Frequency encode long axis of slab (S/I)
Sagittal slab excitation to minimize aliasing
Freq
encode
arm
torso

8. Pulmonary emboli Findings:
Filling defects in pulmonary arteries (arrows)
Perfusion defects (arrowheads)

9. Outline MRA Technique MRI Reconstruction Patient Contrast Related
Bolus Timing
Respiratory Motion
Gibbs Ringing
Bolus Duration
Wrap and Parallel Imaging
Transient Contrast Interruption
Amplifier Over-ranging
Dual Injection Error

10. Bolus Timing ~15 sec elliptical centric acquisition
0.1 mmol/kg dose injected at 1.5 mL/s
200 lb patient: ~20 mL dose  13 s bolus
100 lb patient: ~10 mL dose  7 s bolus
The Problem:
bolus duration < acquisition time
The Solution:
extend bolus duration by diluting contrast in saline to 30mL injected at 1.5 mL/sec (20 s bolus)
Is this bolus timing or bolus duration?
Maki, et al., JMRI 6(4):642-51, 1996.

11. Bolus Timing Acquisition Enhancement Time inject
Contrast plateau leads to
sharper vessels due to
higher signal at edge of k-space
Enhancement
Dilute Bolus
Standard Bolus
Time
inject

12. Scan late or bolus short
Scan early
Ideal timing k0
kmax k0
kmax k0
kmax
Artifacts will not persist on repeat acquisitions or repeat injection PA PA
Blurred PA
Aorta
Aorta
Aorta
Edge
enhanced

13. Outline MRA Technique MRI Reconstruction Patient Contrast Related
Bolus Timing
Respiratory Motion
Gibbs Ringing
Bolus Duration
Wrap and Parallel Imaging
Transient Contrast Interruption
Amplifier Over-ranging
Dual Injection Error

14. Contrast Injection Pitfalls
Transient Interruption of Bolus*
Flow-related phenomenon
Related to valsalva during breath-hold
Increased unopacified blood return from the IVC
Dual Injector Error
Inadvertent reversal of the saline and contrast syringe
Fluoro-triggering detects the small amount of contrast mistakenly used to “flush” the IV line during set up.
Results in scanning when primarily saline is intravascular
* Wittram C, Yoo AJ. J Thorac Imaging 2007; 22: 125-9

15. Dual Injector Error Small amount of contrast in aorta
Dense contrast in subclavian vein
Small amount of contrast in aorta
No contrast in pulmonary artery
Pre-Injection
During Injection
1st Post Injection
2nd Post Injection
Need description

16. Outline MRA Technique MRI Reconstruction Patient Contrast Related
Bolus Timing
Respiratory Motion
Gibbs Ringing
Bolus Duration
Wrap and Parallel Imaging
Transient Contrast Interruption
Amplifier Over-ranging
Dual Injection Error

17. Gibbs Ringing Occurs near high-contrast edges
Occurs centrally in vessels 3-5 pixels wide based on true resolution
May be mistaken for pulmonary embolism
Measure signal drop within vessel
If < 50% signal drop, suspect artifact
If > 50% signal drop, suspect embolus
Do not aggressively window vessels
Ensure that background noise is visible in image

18. Gibb’s Ringing Pulmonary Embolism Simulated true vessel cross-section
Simulated Gibb’s ringing
Actual cross section

19. Corner Cutting With Corner Cutting Without Corner Cutting
2 pixels
4.5 pixels
7 pixels
Sagittal excitation
SI frequency-encoding
AP & RL phase-encoding
With and without corner-cutting
1 x 1 x 1 mm3 resolution
2 NEX
No parallel imaging
With corner cutting, the point-spread function is circularly symmetric and
Gibb’s ringing looks more like true embolism.
Without corner cutting, there is less risk of misinterpreting Gibb’s ringing as true embolism.

20. Outline MRA Technique MRI Reconstruction Patient Contrast Related
Bolus Timing
Respiratory Motion
Gibbs Ringing
Bolus Duration
Wrap and Parallel Imaging
Transient Contrast Interruption
Amplifier Over-ranging
Dual Injection Error

21. Wrap and Parallel Imaging
Wrap (aliasing)
occurs if excited tissue extends outside FOV
sagittal slab excitation avoids wrap from arms
Parallel imaging
propagates wrap artifacts into the center of the image
must completely include AP dimension of patient
Don’t rely only on mid-sagittal and mid-axial scout images!
Largest AP dimension usually at breasts or belly
image noise worse in center of image (increased G-factor)
Solution:
Increase number of AP slices while maintaining reasonable breath-hold time, even at the cost of lower AP resolution

22. Residual Aliasing & G-factor
Wrap
Parallel imaging
Wrap

23. Outline MRA Technique MRI Reconstruction Patient Contrast Related
Bolus Timing
Respiratory Motion
Gibbs Ringing
Bolus Duration
Wrap and Parallel Imaging
Transient Contrast Interruption
Amplifier Over-ranging
Dual Injection Error

24. Amplifier Over-ranging
Problem:
Prescan adjusts amplifier gain to use the entire dynamic range of the analog to digital converter
Actual signal intensity of the acquisition may exceed this range due to IV contrast
Solution:
Decrease amplifier gain and reinject
If this is a regular problem, then routinely decrease amplifier gain during manual prescan prior to injection.
True k-space
Over-range
portion
True image
Over-range
portion
Observed
Image

25. Amplifier Over-ranging

26. Outline MRA Technique MRI Reconstruction Patient Contrast Related
Bolus Timing
Respiratory Motion
Gibbs Ringing
Bolus Duration
Wrap and Parallel Imaging
Transient Contrast Interruption
Amplifier Over-ranging
Dual Injection Error

27. Respiratory Motion Artifact
Patient’s often dyspneic if PE suspected
If motion occurs in the middle of K space smearing occurs
Injection  startle  motion  quiescent
Solution:
Multiphasic injection – motion common on arterial phase (1st) acquisition
Repeat injection with fewer, thicker slices to shorten acquisition time

28. Respiratory Motion Artifact
1st Injection has respiratory motion
2nd Injection

29. Conclusion Contrast-enhanced MRA is a mature and robust technology
High quality scans require careful attention to k-space sampling strategies, injection protocols, and technologist training
Accurate interpretation requires under-standing common pitfalls and artifacts