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

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

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

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

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

MRA Technique

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

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

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

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.

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

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

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

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

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

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

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

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

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.

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

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

Residual Aliasing & G-factor Wrap Parallel imaging Wrap

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

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

Amplifier Over-ranging

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

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

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

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