Noll Spin-Warp Imaging Spin-Warp Imaging • For each RF pulse: For each RF pulse: – Frequency encoding is performed in one Frequency encoding is performed in one direction direction – A single phase encoding value is A single phase encoding value is obtained obtained • With each additional RF pulse: With each additional RF pulse: – The phase encoding value is The phase encoding value is incremented incremented – The phase encoding steps still has the The phase encoding steps still has the appearance of “stop-action” motion appearance of “stop-action” motion
Transcript
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Spin-Warp ImagingSpin-Warp Imaging
• For each RF pulse: For each RF pulse: – Frequency encoding is performed in one direction Frequency encoding is performed in one direction – A single phase encoding value is obtainedA single phase encoding value is obtained
• With each additional RF pulse:With each additional RF pulse:– The phase encoding value is incrementedThe phase encoding value is incremented– The phase encoding steps still has the The phase encoding steps still has the
appearance of “stop-action” motionappearance of “stop-action” motion
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Spin-Warp Pulse SequenceSpin-Warp Pulse Sequence
Gx
Gy
Data Acq.
RF
PhaseEnc.
Freq.Enc.
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Spin-Warp Data AcquisitionSpin-Warp Data Acquisition
• In 1D, the Fourier transform produced a In 1D, the Fourier transform produced a 1D image.1D image.
• In 2D, the Fourier transform is applied in both In 2D, the Fourier transform is applied in both the frequency and phase encoding directions.the frequency and phase encoding directions.– This is called the 2D Fourier transform.This is called the 2D Fourier transform.
• Commonly we structure the samples in a 2D Commonly we structure the samples in a 2D grid that we call “k-space.”grid that we call “k-space.”– One line of k-space is acquired at a time.One line of k-space is acquired at a time.
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Spin-Warp Data AcquisitionSpin-Warp Data Acquisition
kxEach linehas a differentphase encode
Frequencyencodingalong each line
ky 2D Fourier2D FourierTransformTransform
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Echo-Planar ImagingEcho-Planar Imaging
• As with spin-warp imaging, echo-planar As with spin-warp imaging, echo-planar imaging (EPI) is just the combination of two imaging (EPI) is just the combination of two 1D localization methods1D localization methods
• EPI is also a combination of :EPI is also a combination of :– Frequency encoding in one direction Frequency encoding in one direction
(e.g. Left-Right)(e.g. Left-Right)– Phase encoding in the other direction Phase encoding in the other direction
• EPI uses a different phase encoding method.EPI uses a different phase encoding method.
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Echo-Planar ImagingEcho-Planar ImagingRF
Gx
Data Acq.
Frequency EncodingFrequency Encoding(in x direction)(in x direction)
Phase EncodingPhase EncodingMethod #1Method #1(in y direction)(in y direction)
RF
Data Acq.
Gy
Gx
PhaseEnc.
Freq.Enc.
RF
Gx
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Echo-Planar ImagingEcho-Planar Imaging
• For each RF pulse: For each RF pulse: – Frequency encoding is performed many timesFrequency encoding is performed many times– All phase encoding steps are obtainedAll phase encoding steps are obtained– The entire image is acquiredThe entire image is acquired
• With each additional frequency encoding With each additional frequency encoding (each additional line in the k-space grid):(each additional line in the k-space grid):– The phase encoding value is incrementedThe phase encoding value is incremented– The phase encoding steps still has the The phase encoding steps still has the
appearance of “stop-action” motionappearance of “stop-action” motion
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EPI Pulse SequenceEPI Pulse Sequence
RF
Data Acq.
Gy
Gx
PhaseEnc.
Freq.Enc.
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EPI Data AcquisitionEPI Data Acquisition
• As with Spin-Warp imaging, we put the As with Spin-Warp imaging, we put the acquired data for the frequency and phase acquired data for the frequency and phase encoding into the 2D grid called “k-space.encoding into the 2D grid called “k-space.
• Also, the 2D Fourier transform is used to Also, the 2D Fourier transform is used to create the image.create the image.
• In EPI, the data is filled into k-space in a In EPI, the data is filled into k-space in a rectangular “zig-zag”-like pattern.rectangular “zig-zag”-like pattern.
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EPI Data AcquisitionEPI Data Acquisition
kx
Each linehas a differentphase encode
Frequencyencodingalong each line
ky
Changing the signof the frequency enc.changes the directionthat the data is placedinto this 2D grid.
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EPI ImagingEPI Imaging
• In summary, EPI data is in many ways like In summary, EPI data is in many ways like Spin-Warp imaging:Spin-Warp imaging:– They are combinations of two kinds of 1D They are combinations of two kinds of 1D
localization.localization.– They have both frequency and phase encoding.They have both frequency and phase encoding.– Data are acquired on a 2D grid called k-space.Data are acquired on a 2D grid called k-space.– Images are reconstructed by a 2D Fourier Images are reconstructed by a 2D Fourier
transform.transform.
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EPI ImagingEPI Imaging
• It is also different from Spin-Warp Imaging:It is also different from Spin-Warp Imaging:– The image can be acquired with a single RF pulse.The image can be acquired with a single RF pulse.– The phase encoding steps all happen in rapid The phase encoding steps all happen in rapid
succession.succession.– The frequency direction alternates in sign.The frequency direction alternates in sign.– The time needed to acquire data after each RF The time needed to acquire data after each RF
pulse is very long.pulse is very long.– Special hardware is required.Special hardware is required.
• These differences are the focus of the rest of These differences are the focus of the rest of this presentation. this presentation.
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Variants on EPIVariants on EPI
• There are many variations on EPI.There are many variations on EPI.
• One technique that is useful for Spin-Warp One technique that is useful for Spin-Warp imaging that also works for EPI is “Partial k-imaging that also works for EPI is “Partial k-space” or “Half k-space” acquisitions.space” or “Half k-space” acquisitions.
• Like Spin-Warp imaging, this can be used to:Like Spin-Warp imaging, this can be used to:– Reduce echo-time. (phase)Reduce echo-time. (phase)– Improve spatial resolution. (frequency)Improve spatial resolution. (frequency)
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FullFullk-spacek-space
PartialPartialPhase DataPhase Data
PartialPartialFrequency DataFrequency Data
Partial k-space EPIPartial k-space EPI
Data notacquired
Data not acquired
kx
ky
kx
ky
kx
ky
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Multi-shot EPIMulti-shot EPI
• While possible to acquire an entire image with While possible to acquire an entire image with a single RF pulse (single-shot), it is a single RF pulse (single-shot), it is sometimes necessary to use multiple shots.sometimes necessary to use multiple shots.
• There are two common ways of doing this:There are two common ways of doing this:– InterleavingInterleaving– MosaicMosaic
• Multi-shot EPI is useful to:Multi-shot EPI is useful to:– Improve spatial resolutionImprove spatial resolution– Reduce artifactsReduce artifacts
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Multi-shot EPIMulti-shot EPI
InterleavedInterleavedEPIEPI
MosaicMosaicEPIEPI
kx
ky
kx
ky
#1#2
#1 #2
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Methods Similar to EPIMethods Similar to EPI
• One method that has very similar properties One method that has very similar properties to EPI is Spiral Imaging.to EPI is Spiral Imaging.
• Like EPI:Like EPI:– All image data can be acquired in a single-shot.All image data can be acquired in a single-shot.– Multi-shot variants also exist.Multi-shot variants also exist.– Many of the artifacts are similar.Many of the artifacts are similar.
• But:But:– Image reconstruction is complicated.Image reconstruction is complicated.– Some artifacts are different.Some artifacts are different.
• Many parameters are the same as in spin-Many parameters are the same as in spin-warp imaging:warp imaging:– SE vs. GRE or IRSE vs. GRE or IR– TR, TE, Flip Angle, TITR, TE, Flip Angle, TI– FOV, matrix size, spatial resolutionFOV, matrix size, spatial resolution
• Some parameters require extra thought, Some parameters require extra thought, however:however:– If only a single image is acquired using single-shot If only a single image is acquired using single-shot
EPI, the TR might be meaningless. (TR is infinite)EPI, the TR might be meaningless. (TR is infinite)
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Scan Time in EPIScan Time in EPI
• The scan time is most closely related to the The scan time is most closely related to the “number of shots” and not matrix size.“number of shots” and not matrix size.– Scan Time = (number of shots)*(TR)Scan Time = (number of shots)*(TR)– NotNot (number of phase encodes)*(TR) (number of phase encodes)*(TR)
• Consider 64x64 single-shot EPI and 128x128 Consider 64x64 single-shot EPI and 128x128 single-shot EPI - both are single-shot and single-shot EPI - both are single-shot and take a single RF pulse to acquire an image.take a single RF pulse to acquire an image.
• If 128x128 has artifacts that are too severe, If 128x128 has artifacts that are too severe, however, multi-shot EPI may be required.however, multi-shot EPI may be required.
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Echo Time in EPIEcho Time in EPI
• In EPI, it is often hard to achieve a short echo In EPI, it is often hard to achieve a short echo time.time.– The TE is defined as the time between the RF The TE is defined as the time between the RF
pulse and the acquisition of the center of k-space.pulse and the acquisition of the center of k-space.– In single-shot EPI, this could be a long time (often In single-shot EPI, this could be a long time (often
a minimum TE of 15-20 ms). a minimum TE of 15-20 ms).
• This can be addressed by doing a partial k-This can be addressed by doing a partial k-space acquisition in the phase encoding space acquisition in the phase encoding direction.direction.– This will allow much shorter TE’s (5-10 ms).This will allow much shorter TE’s (5-10 ms).
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Echo Time in EPIEcho Time in EPI
Full k-spaceFull k-space PartialPartialk-spacek-space
Data not acquired
kx
ky
kx
kyTE defined bythe time thecenter of k-spaceis acquired
Minimum TE -Time from firstsample to centerof k-space
Reduced TE bypartial k-space inthe phase enc.direction
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Pulse Sequence Options in EPIPulse Sequence Options in EPI
• Flow Compensation (Gradient Moment Nulling):Flow Compensation (Gradient Moment Nulling):– Flow Comp (GNM) is often not as effective with EPI Flow Comp (GNM) is often not as effective with EPI
due to the long echo times.due to the long echo times.– Partial k-space (phase) acquisitions reduce echo Partial k-space (phase) acquisitions reduce echo
time and make this technique more effective.time and make this technique more effective.
• Spatial and chemical presaturation can also be Spatial and chemical presaturation can also be used (fat saturation is nearly always used).used (fat saturation is nearly always used).
• There are also a 3D (volume) versions of EPI.There are also a 3D (volume) versions of EPI.
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T1 Weighting in EPIT1 Weighting in EPI
• In EPI, short TE’s are difficult to obtain and In EPI, short TE’s are difficult to obtain and the TR is often very long.the TR is often very long.– EPI is not well suited to T1-weighted imaging with EPI is not well suited to T1-weighted imaging with
the usual short TR pulse sequences.the usual short TR pulse sequences.
• On the other hand, one shot (or a small On the other hand, one shot (or a small number of shots) is required for an image.number of shots) is required for an image.– EPI is well-suited to inversion recovery T1-EPI is well-suited to inversion recovery T1-
weighted imaging.weighted imaging.
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Artifacts in EPIArtifacts in EPI
• The ability to acquire images very rapidly is The ability to acquire images very rapidly is the strength of the EPI method.the strength of the EPI method.
• As a result, artifacts resulting from subject As a result, artifacts resulting from subject motion are nearly non-existent when imaging motion are nearly non-existent when imaging with single-shot EPI.with single-shot EPI.
• Ghosting artifacts resulting from pulsatile Ghosting artifacts resulting from pulsatile blood flow are also extremely rare with single-blood flow are also extremely rare with single-shot EPI.shot EPI.
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Artifacts in EPIArtifacts in EPI
• There are however, several kinds of image There are however, several kinds of image artifacts that are very different from those artifacts that are very different from those seen in spin-warp imaging:seen in spin-warp imaging:
– ““N/2” or Nyquist ghostingN/2” or Nyquist ghosting
– Distortions from magnetic field inhomogeneityDistortions from magnetic field inhomogeneity
– Chemical shift and susceptibility artifactsChemical shift and susceptibility artifacts
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N/2 GhostingN/2 Ghosting
• N/2 (“N over 2”) or Nyquist ghosting artifacts N/2 (“N over 2”) or Nyquist ghosting artifacts are unique to EPI.are unique to EPI.– Caused by imperfections in the image acquisition.Caused by imperfections in the image acquisition.
• There are two distinct kinds:There are two distinct kinds:– Even and Odd GhostsEven and Odd Ghosts
• ““Ghost tuning” procedures can reduce or Ghost tuning” procedures can reduce or eliminate these ghosts.eliminate these ghosts.– Tuning can be done for each day, subject, or scan.Tuning can be done for each day, subject, or scan.– Might also be done automatically (with prescan).Might also be done automatically (with prescan).
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N/2 GhostingN/2 Ghosting
Even GhostEven Ghost Odd GhostOdd GhostOriginal ImageOriginal Image
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Distortions from InhomogeneityDistortions from Inhomogeneity
• EPI is very sensitive to center frequency EPI is very sensitive to center frequency adjustments and inhomogeneities.adjustments and inhomogeneities.
• For a misadjusted center frequency, the For a misadjusted center frequency, the image is shifted in the “phase” direction.image is shifted in the “phase” direction.– Careful prescan tuning is necessary.Careful prescan tuning is necessary.
• For misadjusted shims, the image can be For misadjusted shims, the image can be twisted, stretched or squeezed.twisted, stretched or squeezed.– Shimming is often necessary (esp. at high fields).Shimming is often necessary (esp. at high fields).
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Distortions from InhomogeneityDistortions from Inhomogeneity
CenterCenterFrequencyFrequency
MisadjustmentMisadjustment
OriginalOriginalImageImage
““X” shimX” shim(L/R shim)(L/R shim)
““Y” shimY” shim(A/P shim)(A/P shim)
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Fat and Susceptibility ArtifactsFat and Susceptibility Artifacts
• In EPI, unsuppressed fat is often shifted 2 cm In EPI, unsuppressed fat is often shifted 2 cm or more.or more.– Fat suppression (Fat Sat) is always required.Fat suppression (Fat Sat) is always required.
• At areas of high magnetic susceptibility, a At areas of high magnetic susceptibility, a “piling-up” artifacts is often seen.“piling-up” artifacts is often seen.– Prevalent near frontal sinuses, above ears, etc.Prevalent near frontal sinuses, above ears, etc.– Pulse sequence parameters can affect this:Pulse sequence parameters can affect this:
• Interleaving usually reduces this artifact.Interleaving usually reduces this artifact.• Increasing resolution in the frequency direction often Increasing resolution in the frequency direction often
worsens the artifact.worsens the artifact.
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Fat and Susceptibility ArtifactsFat and Susceptibility Artifacts
• EPI is an extremely rapid and useful imaging EPI is an extremely rapid and useful imaging method.method.
• It does, however, require some special, high It does, however, require some special, high performance hardware. Why?performance hardware. Why?– In spin-warp, we acquire a small piece of data for In spin-warp, we acquire a small piece of data for
an image with each RF pulse.an image with each RF pulse.
– However in EPI, we try to acquire all of the data However in EPI, we try to acquire all of the data for an image with a single RF pulse.for an image with a single RF pulse.
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Spin-Warp vs. EPI Pulse SequencesSpin-Warp vs. EPI Pulse Sequences
RF
Data Acq.
Gy
GxGx
Gy
Data Acq.
RF
EPIEPISpin-WarpSpin-Warp
Many acquisitionsMany acquisitionsto make a one image.to make a one image.
One acquisitionOne acquisitionto make one image.to make one image.
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T2 Decay and Acquisition TimeT2 Decay and Acquisition Time
• In spin-warp imaging, only a single phase In spin-warp imaging, only a single phase encode need to be acquired.encode need to be acquired.– Only takes a short time.Only takes a short time.
• In EPI, all phase encode lines need to be In EPI, all phase encode lines need to be acquired.acquired.– Takes longer.Takes longer.– Without special hardware - 100 ms to 1 second.Without special hardware - 100 ms to 1 second.
• T2 decay reduces signal throughout data T2 decay reduces signal throughout data acquisition.acquisition.
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T2 Decay and Acquisition TimeT2 Decay and Acquisition Time
RF
Data Acq.
Gy
Gx
SignalStrength
Signal decays away during acquisition.Signal decays away during acquisition.
