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

OutlineMRA Technique

ContrastRelated

MRI Reconstruction

Related

PatientRelated

Bolus Timing

Bolus Duration

Transient Contrast Interruption

Dual Injection Error

Gibbs Ringing

Wrap and Parallel Imaging

Amplifier Over-ranging

Respiratory Motion

MRA Technique

Multiphasic acquisition 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 encodingS

lice

en

cod

ing

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

Freq

encoding

Phase encoding

Slic

e e

nco

din

g

MRA Technique

Slab Orientation

Frequency encode long axis of slab (S/I) Sagittal slab excitation to minimize aliasing

Freqencode armarm

torso

Findings:

Filling defects in pulmonary arteries (arrows)

Perfusion defects (arrowheads)

Pulmonary emboli

OutlineMRA Technique

ContrastRelated

MRI Reconstruction

Related

PatientRelated

Bolus Timing

Bolus Duration

Transient Contrast Interruption

Dual Injection Error

Gibbs Ringing

Wrap and Parallel Imaging

Amplifier Over-ranging

Respiratory Motion

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.

Time

inject

Enh

ance

men

t

Acquisition

Contrast plateau leads tosharper vessels due to

higher signal at edge of k-space

Dilute Bolus

Standard Bolus

Bolus Timing

PA

Aorta

k0 kmax

PA

Aorta

Ideal timingScan late or bolus short

PA

Aorta

Scan early

k0 kmax kmaxk0

Edgeenhanced

Blurred

OutlineMRA Technique

ContrastRelated

MRI Reconstruction

Related

PatientRelated

Bolus Timing

Bolus Duration

Transient Contrast Interruption

Dual Injection Error

Gibbs Ringing

Wrap and Parallel Imaging

Amplifier Over-ranging

Respiratory Motion

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

Pre-Injection During Injection 1st Post Injection 2nd Post Injection

Small amount of contrast in aorta

No contrast in pulmonary artery

Dense contrast in subclavian vein

OutlineMRA Technique

ContrastRelated

MRI Reconstruction

Related

PatientRelated

Bolus Timing

Bolus Duration

Transient Contrast Interruption

Dual Injection Error

Gibbs Ringing

Wrap and Parallel Imaging

Amplifier Over-ranging

Respiratory Motion

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-sectionSimulated Gibb’s ringingActual cross section

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 andGibb’s ringing looks more like true embolism.

Without corner cutting, there is less risk of misinterpreting Gibb’s ringing as true embolism.

OutlineMRA Technique

ContrastRelated

MRI Reconstruction

Related

PatientRelated

Bolus Timing

Bolus Duration

Transient Contrast Interruption

Dual Injection Error

Gibbs Ringing

Wrap and Parallel Imaging

Amplifier Over-ranging

Respiratory Motion

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

Wrap

Parallel imaging

OutlineMRA Technique

ContrastRelated

MRI Reconstruction

Related

PatientRelated

Bolus Timing

Bolus Duration

Transient Contrast Interruption

Dual Injection Error

Gibbs Ringing

Wrap and Parallel Imaging

Amplifier Over-ranging

Respiratory Motion

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-spaceOver-range

portion

True image Over-rangeportion

ObservedImage

Amplifier Over-ranging

OutlineMRA Technique

ContrastRelated

MRI Reconstruction

Related

PatientRelated

Bolus Timing

Bolus Duration

Transient Contrast Interruption

Dual Injection Error

Gibbs Ringing

Wrap and Parallel Imaging

Amplifier Over-ranging

Respiratory Motion

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:

1. Multiphasic injection – motion common on arterial phase (1st) acquisition

2. Repeat injection with fewer, thicker slices to shorten acquisition time

1st Injection has respiratory motion

2nd Injection

Respiratory Motion Artifact

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