artifacts in cardiac mri andreas sigfridsson, imv/imt/cmiv andreas sigfridsson, imv/imt/cmiv...
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Artifacts in Cardiac MRIArtifacts in Cardiac MRI
Andreas Sigfridsson, IMV/IMT/CMIV Andreas Sigfridsson, IMV/IMT/CMIV <[email protected]><[email protected]>
2007-04-272007-04-27
Dynamic imagingDynamic imaging
Images are acquired line-by-line in Images are acquired line-by-line in k-k-spacespace Motion during acquisition causes Motion during acquisition causes artifactsartifacts, not , not
blurringblurring Special care must be takenSpecial care must be taken
TriggeringTriggering
Acquisition orderingAcquisition ordering
ExampleExample
Simulation exampleSimulation example 256x256, TR=20 ms, total scan time ~5 s256x256, TR=20 ms, total scan time ~5 s
Image domain k-space
Signal saturationSignal saturation
Signal recovery - relaxationSignal recovery - relaxation
11 T
t
z eM
0t
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1TTR
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Signal saturationSignal saturation
Progress to steady-stateProgress to steady-state
901 TTR
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Varying signal during inflowVarying signal during inflow
The MR signal is increased during inflowThe MR signal is increased during inflow
Signal increase related to flow velocitySignal increase related to flow velocity More magnetization because of T1 More magnetization because of T1
decay/recoverydecay/recovery
Static spins, saturated signal
Flowing spins, fresh signal
Varying signal intensityVarying signal intensity
Simulated six sinusoidal heart beatsSimulated six sinusoidal heart beats
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Intensity Image (animation)
Varying signal intensity, resultsVarying signal intensity, results
Reconstructed imageReconstructed image
High contrastNormal contrast
Varying signal intensity, k-spaceVarying signal intensity, k-space
Image artifacts because of k-space Image artifacts because of k-space samplingsampling Different lines are acquired with different Different lines are acquired with different
signal intensitysignal intensity
Varying signal intensity, analysisVarying signal intensity, analysis
Analyze signalAnalyze signal
dynamicstatic ImagetIntensityImagetImage *)()(
)(*)()()( ydynamicyystaticy kKkIntensitykKkK
dynamicstatictedreconstruc ImagetIntensityImageImage ))(FT(
)*FT()FT()FT( dynamicstatic KIntensityKK
)FT()FT()FT()FT( dynamicstatic KIntensityKK
Slow sine variationSlow sine variation
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Intensity FT(Intensity)
Varying signal intensity, resultsVarying signal intensity, results
Reconstructed imageReconstructed image
High contrastNormal contrast
Fast sine variationFast sine variation
Intensity FT(Intensity)
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Fast sine variationFast sine variation
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Intensity FT(Intensity)
““Physiological” variationPhysiological” variation
Aortic flow (handcrafted, not actual data)Aortic flow (handcrafted, not actual data)
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Intensity (one beat)
Intensity (whole scan)
““Physiological” variationPhysiological” variation
Intensity variation (animation)
““Physiological” variationPhysiological” variation
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Intensity
FT(Intensity)
““Physiological” variationPhysiological” variation
Reconstructed imagesReconstructed images
High contrastNormal contrast
RemediesRemedies
TriggeringTriggering Wait for ECG R-tagWait for ECG R-tag Wait additional trigger delay timeWait additional trigger delay time Acquire one (or several) k-space linesAcquire one (or several) k-space lines
Acquisition reorderingAcquisition reordering Measure current cardiac (or respiratory phase)Measure current cardiac (or respiratory phase) Choose k-space line to acquireChoose k-space line to acquire Creates a slow varying intensity curve along k-spaceCreates a slow varying intensity curve along k-space
Triggering - random phaseTriggering - random phase
If triggering is not accurate, random If triggering is not accurate, random variations will still be leftvariations will still be left
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Intensity FT(Intensity)
Random phase, reconstructedRandom phase, reconstructed
Normal contrast High contrast
Acquisition reorderingAcquisition reordering
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Intensity
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FT(Intensity)
Acquisition reorderingAcquisition reordering
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OrderedRandom
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Acquisition reorderingAcquisition reordering
Reconstructed imagesReconstructed images
High contrastNormal contrast
Dynamic imagingDynamic imaging
Images are acquired line-by-line in Images are acquired line-by-line in k-k-spacespace Motion during acquisition causes Motion during acquisition causes artifactsartifacts, not , not
blurringblurring Special care must be takenSpecial care must be taken
TriggeringTriggering
Acquisition orderingAcquisition ordering Artifacts might be predictable through use of Artifacts might be predictable through use of
Fourier transformFourier transform