Geometric Distortion Artifact Remedy in DWI: a novel comprehensive

approach

GGamal amal FFathalla athalla MM.. MMahdalyahdaly

gamal_mahdaly@hotmail.comgamal_mahdaly@hotmail.com

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Image Artifacts in EPIImage Artifacts in EPI

Chemical shift in Phase directionChemical shift in Phase direction N/2 Phase ghostingN/2 Phase ghosting Geometric distortionsGeometric distortions

Main causeMain cause : continuous phase information : continuous phase informationacross the whole K space (no return to across the whole K space (no return to center of K space between lines). Phase center of K space between lines). Phase errors may occur at any time.errors may occur at any time.

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Chemical Shift in Phase Chemical Shift in Phase DirectionDirection

Etiology:Etiology:Low frequency sampling in Ky (Phase Low frequency sampling in Ky (Phase

direction).direction).

Example : With ESP = 1ms, sampling rate = Example : With ESP = 1ms, sampling rate = 1 kHz,1 kHz,corresponds to 56 pixels of chemical shift @corresponds to 56 pixels of chemical shift @(1.5T, 256 matrix)(1.5T, 256 matrix)

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Chemical Shift in Phase Chemical Shift in Phase DirectionDirection

(256x256 matrix / 220 Hz Chemical Shift @ 1.5T)Over all Chem. Shift in pixels = [C.Shift. in Hz] x [Echo Spacing] x [Phase Matrix]

# Shots 1 2 4 8 16

Echo Spacing(ms)

1 0.5 0.25 0.125 0.062

Chem.Shift

(pixels)

56 28 14 7 3.5

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Chemical Shift in Phase Chemical Shift in Phase DirectionDirection

Remedy Tips :Remedy Tips :

– Decrease ESP (use multi-shot rather Decrease ESP (use multi-shot rather than single shot)than single shot)

– Use Fat SaturationUse Fat Saturation– Use Water-selective RF pulsesUse Water-selective RF pulses

e.g. Spectral -Spatiale.g. Spectral -Spatial

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Chemical Shift in Phase Chemical Shift in Phase DirectionDirection

Remedy Tips:Remedy Tips:Specific RF pulse for EPI: Specific RF pulse for EPI: Spectral - SpatialSpectral - Spatial

– Standard RF pulse = wide transmit. BWStandard RF pulse = wide transmit. BW(more than 1000 Hz), in order to excite both(more than 1000 Hz), in order to excite bothWater and FatWater and Fat

– Spectral - Spatial = narrow transmit. BWSpectral - Spatial = narrow transmit. BW(less than 200 Hz), in order to excite only(less than 200 Hz), in order to excite onlyone componentone component

– Higher efficiency than Fat SATHigher efficiency than Fat SAT(only one RF instead of two)(only one RF instead of two)

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N/2 Phase GhostingN/2 Phase Ghosting Etiology :Etiology :

Incorrect alignment (time shift) between the odd Incorrect alignment (time shift) between the odd and even lines of K space (due to patient periodic and even lines of K space (due to patient periodic brain motions, system fluctuations...)brain motions, system fluctuations...)

Remedy Tips:Remedy Tips:"Reference scan" (REF) = additional Prescan step "Reference scan" (REF) = additional Prescan step = data acquisition with no phase blips for time / = data acquisition with no phase blips for time / phase correction in the scan dataphase correction in the scan data

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Reference ScanReference Scan

Kx

Ky

FFT

raw data imageNo REF scan prior to scan

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Reference ScanReference Scan

FFT

image

Kx

Ky

raw dataREF scan prior to scan

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N/2Ghosting

Note theSignal drop

Due to Different

Susceptibility

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Geometric DistortionsGeometric Distortions

Etiology:Etiology:

Low sampling rate and continuous Low sampling rate and continuous Phase information in Ky (Phase dir.) Phase information in Ky (Phase dir.) make EPI very sensitive to patient-make EPI very sensitive to patient-induced Bo in-homogeneities or induced Bo in-homogeneities or system instabilities.system instabilities.

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Geometric Distortions Geometric Distortions Kx

Ky

Kx

Ky

perturbation

Non-EPI EPIEffect of single Excitation

corrupted raw data

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Geometric Distortion Geometric Distortion ArtifactsArtifacts

Etiology :Etiology :

Due to High B values used , MPG pulse duration Due to High B values used , MPG pulse duration increases & minimum TE increases. With the increases & minimum TE increases. With the following consequences:following consequences:

Increase of gradient amplitude per voxel increasing Increase of gradient amplitude per voxel increasing Microscopic Gradients Microscopic Gradients & thereby increasing the & thereby increasing the magnetic susceptibility artifacts.magnetic susceptibility artifacts.

Enhancing of Enhancing of ghostingghosting resulting from resulting from Periodic Brain Periodic Brain Motion.Motion.

Exaggerating the Exaggerating the Phase ErrorPhase Error Buildup due to Off- Buildup due to Off-Resonance Spins Resonance Spins

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Geometric Distortion Geometric Distortion ArtifactsArtifacts

Etiology :Etiology :

These effects are further complicated by:These effects are further complicated by:Partial Volume AveragingPartial Volume Averaging due to occurrence of multiple due to occurrence of multiple

signals in the same Voxel.signals in the same Voxel.Eddy CurrentsEddy Currents due to high amplitude gradients with due to high amplitude gradients with

resultant Feed through artifacts.resultant Feed through artifacts.Obligatory A/P phase directionObligatory A/P phase direction to prevent Peripheral to prevent Peripheral

Nerve stimulation thus accumulating the phase error in Nerve stimulation thus accumulating the phase error in the frontal region leading to distortion.the frontal region leading to distortion.

Narrow Receive BandwidthNarrow Receive Bandwidth slows down the sampling rate slows down the sampling rate thereby enhancing Ghosting & increasing Chemical Shift.thereby enhancing Ghosting & increasing Chemical Shift.

Occurrence of tissues of Occurrence of tissues of different susceptibilitiesdifferent susceptibilities in the in the Frontal & Temporal regions.Frontal & Temporal regions. Experta Medica

Geometric Distortion Geometric Distortion ArtifactsArtifacts

Manifested as: Manifested as: Bright and dark areas with huge spatial distortion Bright and dark areas with huge spatial distortion

of the anatomy especially in the frontal area of the anatomy especially in the frontal area obscuring parts of the frontal & temporal obscuring parts of the frontal & temporal regions.regions.

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Geometric Distortion Geometric Distortion ArtifactsArtifacts

Remedy TipsRemedy Tips

Reduce the Echo Sampling Time by:Reduce the Echo Sampling Time by:

1-Decreasing B value with trade off of Poor Diff 1-Decreasing B value with trade off of Poor Diff weighting & appearance of T2 Shine through artifact.weighting & appearance of T2 Shine through artifact.

2- Widening of RBW with trade off of low SNR.2- Widening of RBW with trade off of low SNR.3-Optimizing TE to allow maximum gradient amplitude 3-Optimizing TE to allow maximum gradient amplitude

at minimum possible TE.at minimum possible TE.

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ESP = 832s96 x 64BW = 100kHz

ESP = 960s128 x 96BW = 100kHz

ESP = 1216s192 x 128BW = 125kHz

ESP = 1472s256 x 192BW = 125kHz

Effect of Echo Spacing/Matrix/RBW combinations on the image distortion

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Effect of number of shots on distortionEffect of number of shots on distortion

Two ShotTwo Shot Four ShotFour Shot Eight ShotEight Shot Sixteen ShotSixteen Shot

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Geometric Distortion Geometric Distortion ArtifactsArtifacts

Remedy TipsRemedy Tips

Reduce the Echo Sampling Time by:Reduce the Echo Sampling Time by:

4-Ramp Sampling allowing echo sampling during ramp 4-Ramp Sampling allowing echo sampling during ramp ups & downs of the frequency encoding gradient, ups & downs of the frequency encoding gradient, thereby reducing echospacing & allowing faster thereby reducing echospacing & allowing faster sampling with tradeoff of wide RBW & hence reduced sampling with tradeoff of wide RBW & hence reduced SNR.SNR.

Read out Read out

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SE-EPI Single Shot :

Top : w/o Ramp SamplingBW = 100 kHzEcho Spacing = 960s

Bottom :with Ramp SamplingBW = 100 kHzEcho Spacing = 704s

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Other approaches to reduce Geometric distortion Other approaches to reduce Geometric distortion artifacts:artifacts:

1.1. Using Using an additional refocusing pulsean additional refocusing pulse to reduce the to reduce the eddy currents with a tradeoff of increasing the eddy currents with a tradeoff of increasing the minimum TE.minimum TE.

2.2. Using Using Parallel ImagingParallel Imaging techniques making use of techniques making use of spatial information related to the spatially varying spatial information related to the spatially varying sensitivity of different receive channels of the receive sensitivity of different receive channels of the receive coil. Using these pre-measured data, sensitivity coil. Using these pre-measured data, sensitivity profiles of the coil helps reconstruct the K-Space profiles of the coil helps reconstruct the K-Space trajectory missing lines which is under sampled during trajectory missing lines which is under sampled during the actual scan .This Tech is called the actual scan .This Tech is called ASSETASSET or or SMASHSMASH. .

3.3. SENSESENSE is another parallel imaging technique where K- is another parallel imaging technique where K-Space is under sampled by widening the K-Space Space is under sampled by widening the K-Space lines . The overflowing data / reduced number of lines lines . The overflowing data / reduced number of lines leads to aliasing. Each pixel of an aliased receiver leads to aliasing. Each pixel of an aliased receiver channel image reflects the signal from different spatial channel image reflects the signal from different spatial origins. The premeasured sensitivity profile helps origins. The premeasured sensitivity profile helps allocate each signal to its spatial origin. The trade off allocate each signal to its spatial origin. The trade off is low SNR due to lossy K-Space.is low SNR due to lossy K-Space. Experta Medica

4- Using 4- Using PROPELLERPROPELLER where whereK-Space trajectory is rotatedK-Space trajectory is rotatedacross the time domain toacross the time domain to define the areas of K-Spacedefine the areas of K-Space where distortion occurred & where distortion occurred & subtract them from the data subtract them from the data set with a tradeoff of limited set with a tradeoff of limited SNR because of lossy K-Space.SNR because of lossy K-Space.

Propeller filling

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Geometric Distortion Geometric Distortion ArtifactsArtifacts

Remedy Tips;Remedy Tips;

Use S/I Spatial saturation outside the FOV or at the Use S/I Spatial saturation outside the FOV or at the different susceptibility area. This increases the dielectric different susceptibility area. This increases the dielectric effect but reduces Susceptibility effects. ( Tradeoff: SAR effect but reduces Susceptibility effects. ( Tradeoff: SAR increases).increases).

Our new approachOur new approachIntroduces a spatial sat Introduces a spatial sat At the different At the different Magnetic susceptibilityMagnetic susceptibilityAdded to reducing the Added to reducing the Echosampling time by Echosampling time by Ramp sampling,Ramp sampling, optimizing TE & optimizing TE & Multishot excitation.Multishot excitation. Experta Medica

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Any Question???.

Again Any Question???

Thank you!

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