Presentation is loading. Please wait.

Presentation is loading. Please wait.

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

Similar presentations


Presentation on theme: "Meyer CA, Schiebler ML, Reeder SB, Francois CJ, Nagle SK Artifacts and Pitfalls in MR Angiography (MRA) for Pulmonary Embolism Meyer CA, Schiebler ML,"— Presentation transcript:

1 Meyer CA, Schiebler ML, Reeder SB, Francois CJ, Nagle SK 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 Contrast Related MRI Reconstruction Related Patient Related Bolus Timing Bolus Duration Transient Contrast Interruption Dual Injection Error Gibbs Ringing Wrap and Parallel Imaging Amplifier Over- ranging Respiratory Motion

4 MRA Technique Multiphasic acquisition fluoro triggered arterial phase immediate post-injection phase 2 nd 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

5 MRA Technique 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 Findings: Filling defects in pulmonary arteries (arrows) Perfusion defects (arrowheads) Pulmonary emboli

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

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) Maki, et al., JMRI 6(4):642-51, 1996.

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

12 PA Aorta k0k0 k max PA Aorta Ideal timing Scan late or bolus short PA Aorta Scan early k0k0 k max k0k0 Edge enhanced Blurred

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

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 Pre-Injection During Injection1 st Post Injection2 nd Post Injection Small amount of contrast in aorta No contrast in pulmonary artery Dense contrast in subclavian vein

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

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 Gibbs RingingPulmonary Embolism Simulated true vessel cross-section Simulated Gibbs ringing Actual cross section

19 With Corner Cutting Without Corner Cutting 2 pixels 4.5 pixels 7 pixels Corner Cutting With corner cutting, the point-spread function is circularly symmetric and Gibbs ringing looks more like true embolism. Without corner cutting, there is less risk of misinterpreting Gibbs ringing as true embolism.

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

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 Dont 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

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

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

27 Respiratory Motion Artifact Patients often dyspneic if PE suspected If motion occurs in the middle of K space smearing occurs Injection startle motion quiescent Solution: 1.Multiphasic injection – motion common on arterial phase (1 st ) acquisition 2.Repeat injection with fewer, thicker slices to shorten acquisition time

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

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


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

Similar presentations


Ads by Google