4MRA Technique Multiphasic acquisition Full chest coverage fluoro triggered arterial phaseimmediate post-injection phase2nd post-injection “steady state” phase using lower flip angleeach phase done during single breath holdFull chest coverageHigh spatial resolution2D parallel imagingmust use multichannel phased array coilrecommend ≥ 8 channelsFreqencodingPhase encodingSlice encoding
5MRA Technique k-space “corner cutting” Elliptical centric acquisition FreqencodingPhase encodingSlice encodingk-space “corner cutting”eliminates 22% of readoutsElliptical centric acquisitionenables fluoro-triggering0.1 mmol/kg of gadobenate dimegluminetypically 15-20mLdiluted with saline to a total volume of 30 mL
10Bolus Timing ~15 sec elliptical centric acquisition 0.1 mmol/kg dose injected at 1.5 mL/s200 lb patient: ~20 mL dose 13 s bolus100 lb patient: ~10 mL dose 7 s bolusThe Problem:bolus duration < acquisition timeThe 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.
11Bolus Timing Acquisition Enhancement Time inject Contrast plateau leads tosharper vessels due tohigher signal at edge of k-spaceEnhancementDilute BolusStandard BolusTimeinject
12Scan late or bolus short Scan earlyIdeal timingk0kmaxk0kmaxk0kmaxArtifacts will not persist on repeat acquisitions or repeat injectionPAPABlurredPAAortaAortaAortaEdgeenhanced
14Contrast Injection Pitfalls Transient Interruption of Bolus*Flow-related phenomenonRelated to valsalva during breath-holdIncreased unopacified blood return from the IVCDual Injector ErrorInadvertent reversal of the saline and contrast syringeFluoro-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
15Dual Injector Error Small amount of contrast in aorta Dense contrast in subclavian veinSmall amount of contrast in aortaNo contrast in pulmonary arteryPre-InjectionDuring Injection1st Post Injection2nd Post InjectionNeed description
17Gibbs Ringing Occurs near high-contrast edges Occurs centrally in vessels 3-5 pixels wide based on true resolutionMay be mistaken for pulmonary embolismMeasure signal drop within vesselIf < 50% signal drop, suspect artifactIf > 50% signal drop, suspect embolusDo not aggressively window vesselsEnsure that background noise is visible in image
19Corner Cutting With Corner Cutting Without Corner Cutting 2 pixels4.5 pixels7 pixelsSagittal excitationSI frequency-encodingAP & RL phase-encodingWith and without corner-cutting1 x 1 x 1 mm3 resolution2 NEXNo parallel imagingWith 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.
21Wrap and Parallel Imaging Wrap (aliasing)occurs if excited tissue extends outside FOVsagittal slab excitation avoids wrap from armsParallel imagingpropagates wrap artifacts into the center of the imagemust completely include AP dimension of patientDon’t rely only on mid-sagittal and mid-axial scout images!Largest AP dimension usually at breasts or bellyimage 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
24Amplifier Over-ranging Problem:Prescan adjusts amplifier gain to use the entire dynamic range of the analog to digital converterActual signal intensity of the acquisition may exceed this range due to IV contrastSolution:Decrease amplifier gain and reinjectIf this is a regular problem, then routinely decrease amplifier gain during manual prescan prior to injection.True k-spaceOver-rangeportionTrue imageOver-rangeportionObservedImage
27Respiratory Motion Artifact Patient’s often dyspneic if PE suspectedIf motion occurs in the middle of K space smearing occursInjection startle motion quiescentSolution:Multiphasic injection – motion common on arterial phase (1st) acquisitionRepeat injection with fewer, thicker slices to shorten acquisition time
28Respiratory Motion Artifact 1st Injection has respiratory motion2nd Injection
29Conclusion Contrast-enhanced MRA is a mature and robust technology High quality scans require careful attention to k-space sampling strategies, injection protocols, and technologist trainingAccurate interpretation requires under-standing common pitfalls and artifacts