Geometric distortion of functional MR images especially of DWI hinders the visualization of the frontal & temporal regions. This is a simple & reliable approach to get rid of. This needs no special software or hardware & applies to old systems.

1. Geometric Distortion
Artifact Remedy in DWI:
a novel comprehensive
approach
GGamalamal FFathallaathalla MM..
MMahdalyahdaly
gamal_mahdaly@hotmail.comgamal_mahdaly@hotmail.com
Experta Medica

2. 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 information
across the whole K space (no return toacross the whole K space (no return to
center of K space between lines). Phasecenter of K space between lines). Phase
errors may occur at any time.errors may occur at any time.
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3. Chemical Shift in PhaseChemical Shift in Phase
DirectionDirection
 Etiology:Etiology:
Low frequency sampling in Ky (Phase direction).Low frequency sampling in Ky (Phase direction).
Example : With ESP = 1ms, sampling rate = 1Example : With ESP = 1ms, sampling rate = 1
kHz,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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4. Chemical Shift in Phase DirectionChemical Shift in Phase Direction
(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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5. Chemical Shift in PhaseChemical Shift in Phase
DirectionDirection
 Remedy Tips :Remedy Tips :
– Decrease ESP (use multi-shot rather thanDecrease ESP (use multi-shot rather than
single shot)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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6. Chemical Shift in Phase DirectionChemical Shift in Phase Direction
 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 both
Water 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 only
one 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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7. N/2 Phase GhostingN/2 Phase Ghosting
 Etiology :Etiology :
Incorrect alignment (time shift) between theIncorrect alignment (time shift) between the
odd and even lines of K space (due to patientodd and even lines of K space (due to patient
periodic brain motions, system fluctuations...)periodic brain motions, system fluctuations...)
 Remedy Tips:Remedy Tips:
"Reference scan" (REF) = additional Prescan"Reference scan" (REF) = additional Prescan
step = data acquisition with no phase blipsstep = data acquisition with no phase blips
for time / phase correction in the scan datafor time / phase correction in the scan data
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8. Reference ScanReference Scan
Kx
Ky
FFT
raw data image
No REF scan prior to scan
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9. Reference ScanReference Scan
FFT
image
Kx
Ky
raw data
REF scan prior to scan
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10. N/2
Ghosting
Note the
Signal
drop
Due to
Different
Susceptibility
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11. Geometric DistortionsGeometric Distortions
 Etiology:Etiology:
Low sampling rate and continuous PhaseLow sampling rate and continuous Phase
information in Ky (Phase dir.) make EPIinformation in Ky (Phase dir.) make EPI
very sensitive to patient-induced Bo in-very sensitive to patient-induced Bo in-
homogeneities or system instabilities.homogeneities or system instabilities.
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12. Geometric DistortionsGeometric Distortions
Kx
Ky
Kx
Ky
perturbation
Non-EPI EPI
Effect of single Excitation
corrupted
raw data
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13. Geometric Distortion ArtifactsGeometric Distortion Artifacts
Etiology :Etiology :
Due to High B values used , MPG pulse durationDue to High B values used , MPG pulse duration
increases & minimum TE increases. With theincreases & minimum TE increases. With the
following consequences:following consequences:
 Increase of gradient amplitude per voxel increasingIncrease of gradient amplitude per voxel increasing
Microscopic GradientsMicroscopic Gradients & thereby increasing the magnetic& thereby increasing the magnetic
susceptibility artifacts.susceptibility artifacts.
 Enhancing ofEnhancing of ghostingghosting resulting fromresulting from Periodic Brain Motion.Periodic Brain Motion.
 Exaggerating theExaggerating the Phase ErrorPhase Error Buildup due to Off-ResonanceBuildup due to Off-Resonance
SpinsSpins
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14. Geometric Distortion ArtifactsGeometric Distortion Artifacts
Etiology :Etiology :
These effects are further complicated by:These effects are further complicated by:
 Partial Volume AveragingPartial Volume Averaging due to occurrence of multiple signalsdue to occurrence of multiple signals
in the same Voxel.in the same Voxel.
 Eddy CurrentsEddy Currents due to high amplitude gradients with resultantdue to high amplitude gradients with resultant
Feed through artifacts.Feed through artifacts.
 Obligatory A/P phase directionObligatory A/P phase direction to prevent Peripheral Nerveto prevent Peripheral Nerve
stimulation thus accumulating the phase error in the frontalstimulation thus accumulating the phase error in the frontal
region leading to distortion.region leading to distortion.
 Narrow Receive BandwidthNarrow Receive Bandwidth slows down the sampling rateslows down the sampling rate
thereby enhancing Ghosting & increasing Chemical Shift.thereby enhancing Ghosting & increasing Chemical Shift.
 Occurrence of tissues ofOccurrence of tissues of different susceptibilitiesdifferent susceptibilities in the Frontalin the Frontal
& Temporal regions.& Temporal regions. Experta Medica

15. Geometric Distortion ArtifactsGeometric Distortion Artifacts
Manifested as:Manifested as:
Bright and dark areas with huge spatial distortion ofBright and dark areas with huge spatial distortion of
the anatomy especially in the frontal area obscuringthe anatomy especially in the frontal area obscuring
parts of the frontal & temporal regions.parts of the frontal & temporal regions.
Experta Medica

16. Geometric Distortion ArtifactsGeometric Distortion Artifacts
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 weighting &1-Decreasing B value with trade off of Poor Diff weighting &
appearance of T2 Shine through artifact.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 at3-Optimizing TE to allow maximum gradient amplitude at
minimum possible TE.minimum possible TE.
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17. ESP = 832µs
96 x 64
BW = 100kHz
ESP = 960µs
128 x 96
BW = 100kHz
ESP = 1216µs
192 x 128
BW = 125kHz
ESP = 1472µs
256 x 192
BW = 125kHz
Effect of Echo Spacing/Matrix/RBW
combinations on the image distortion
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18. 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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19. Geometric Distortion ArtifactsGeometric Distortion Artifacts
Remedy TipsRemedy Tips
 Reduce the Echo Sampling Time by:Reduce the Echo Sampling Time by:
4-Ramp Sampling allowing echo sampling during ramp ups &4-Ramp Sampling allowing echo sampling during ramp ups &
downs of the frequency encoding gradient, thereby reducingdowns of the frequency encoding gradient, thereby reducing
echospacing & allowing faster sampling with tradeoff of wideechospacing & allowing faster sampling with tradeoff of wide
RBW & hence reduced SNR.RBW & hence reduced SNR.
Read out Read out
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20. SE-EPI Single Shot :
Top :
w/o Ramp Sampling
BW = 100 kHz
Echo Spacing = 960µs
Bottom :
with Ramp Sampling
BW = 100 kHz
Echo Spacing = 704µs
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21. Other approaches to reduce Geometric distortionOther approaches to reduce Geometric distortion
artifacts:artifacts:
1.1. UsingUsing an additional refocusing pulsean additional refocusing pulse to reduce the eddyto reduce the eddy
currents with a tradeoff of increasing the minimum TE.currents with a tradeoff of increasing the minimum TE.
2.2. UsingUsing Parallel ImagingParallel Imaging techniques making use of spatialtechniques making use of spatial
information related to the spatially varying sensitivity ofinformation related to the spatially varying sensitivity of
different receive channels of the receive coil. Using these pre-different receive channels of the receive coil. Using these pre-
measured data, sensitivity profiles of the coil helps reconstructmeasured data, sensitivity profiles of the coil helps reconstruct
the K-Space trajectory missing lines which is under sampledthe K-Space trajectory missing lines which is under sampled
during the actual scan .This Tech is calledduring the actual scan .This Tech is called ASSETASSET oror SMASHSMASH..
3.3. SENSESENSE is another parallel imaging technique where K-Spaceis another parallel imaging technique where K-Space
is under sampled by widening the K-Space lines . Theis under sampled by widening the K-Space lines . The
overflowing data / reduced number of lines leads to aliasing.overflowing data / reduced number of lines leads to aliasing.
Each pixel of an aliased receiver channel image reflects theEach pixel of an aliased receiver channel image reflects the
signal from different spatial origins. The premeasuredsignal from different spatial origins. The premeasured
sensitivity profile helps allocate each signal to its spatial origin.sensitivity profile helps allocate each signal to its spatial origin.
The trade off is low SNR due to lossy K-Space.The trade off is low SNR due to lossy K-Space.
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22. 4- Using4- Using PROPELLERPROPELLER wherewhere
K-Space trajectory is rotatedK-Space trajectory is rotated
across 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 datasubtract them from the data
set with a tradeoff of limitedset with a tradeoff of limited
SNR because of lossy K-Space.SNR because of lossy K-Space.
Propeller filling
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23. Geometric Distortion ArtifactsGeometric Distortion Artifacts
Remedy Tips;Remedy Tips;
 Use S/I Spatial saturation outside the FOV or at the differentUse S/I Spatial saturation outside the FOV or at the different
susceptibility area. This increases the dielectric effect butsusceptibility area. This increases the dielectric effect but
reduces Susceptibility effects. ( Tradeoff: SAR increases).reduces Susceptibility effects. ( Tradeoff: SAR increases).
Our new approachOur new approach
Introduces a spatial satIntroduces a spatial sat
At the differentAt the different
Magnetic susceptibilityMagnetic susceptibility
Added to reducing theAdded to reducing the
Echosampling time byEchosampling time by
Ramp sampling,Ramp sampling,
optimizing TE &optimizing TE &
Multishot excitation.Multishot excitation.
Experta Medica

24. Experta Medica

25. Any Question???.
Again Any Question???
Thank you!

26. Have a nice dayHave a nice day