Spin-Warp Imaging

1
Spin-Warp Imaging

• For each RF pulse
• Frequency encoding is performed in one direction
• A single phase encoding value is obtained
• With each additional RF pulse
• The phase encoding value is incremented
• The phase encoding steps still has the appearance
  of stop-action motion

2
Spin-Warp Pulse Sequence
3
Spin-Warp Data Acquisition

• In 1D, the Fourier transform produced a 1D
  image.
• In 2D, the Fourier transform is applied in both
  the frequency and phase encoding directions.
• This is called the 2D Fourier transform.
• Commonly we structure the samples in a 2D grid
  that we call k-space.
• One line of k-space is acquired at a time.

4
Spin-Warp Data Acquisition
2D FourierTransform
5
Echo-Planar Imaging

• As with spin-warp imaging, echo-planar imaging
  (EPI) is just the combination of two 1D
  localization methods
• EPI is also a combination of
• Frequency encoding in one direction (e.g.
  Left-Right)
• Phase encoding in the other direction (e.g.
  Anterior-Posterior)
• EPI uses a different phase encoding method.

6
Echo-Planar Imaging
Frequency Encoding(in x direction)
Phase EncodingMethod 1(in y direction)
7
Echo-Planar Imaging

• For each RF pulse
• Frequency encoding is performed many times
• All phase encoding steps are obtained
• The entire image is acquired
• With each additional frequency encoding (each
  additional line in the k-space grid)
• The phase encoding value is incremented
• The phase encoding steps still has the appearance
  of stop-action motion

8
EPI Pulse Sequence
9
EPI Data Acquisition

• As with Spin-Warp imaging, we put the acquired
  data for the frequency and phase encoding into
  the 2D grid called k-space.
• Also, the 2D Fourier transform is used to create
  the image.
• In EPI, the data is filled into k-space in a
  rectangular zig-zag-like pattern.

10
EPI Data Acquisition
11
EPI Imaging

• In summary, EPI data is in many ways like
  Spin-Warp imaging
• They are combinations of two kinds of 1D
  localization.
• They have both frequency and phase encoding.
• Data are acquired on a 2D grid called k-space.
• Images are reconstructed by a 2D Fourier
  transform.

12
EPI Imaging

• It is also different from Spin-Warp Imaging
• The image can be acquired with a single RF pulse.
• The phase encoding steps all happen in rapid
  succession.
• The frequency direction alternates in sign.
• The time needed to acquire data after each RF
  pulse is very long.
• Special hardware is required.
• These differences are the focus of the rest of
  this presentation.

13
Variants on EPI

• There are many variations on EPI.
• One technique that is useful for Spin-Warp
  imaging that also works for EPI is Partial
  k-space or Half k-space acquisitions.
• Like Spin-Warp imaging, this can be used to
• Reduce echo-time. (phase)
• Improve spatial resolution. (frequency)

14
Partial k-space EPI
Fullk-space
PartialPhase Data
PartialFrequency Data
15
Multi-shot EPI

• While possible to acquire an entire image with a
  single RF pulse (single-shot), it is sometimes
  necessary to use multiple shots.
• There are two common ways of doing this
• Interleaving
• Mosaic
• Multi-shot EPI is useful to
• Improve spatial resolution
• Reduce artifacts

16
Multi-shot EPI
InterleavedEPI
MosaicEPI
17
Methods Similar to EPI

• One method that has very similar properties to
  EPI is Spiral Imaging.
• Like EPI
• All image data can be acquired in a single-shot.
• Multi-shot variants also exist.
• Many of the artifacts are similar.
• But
• Image reconstruction is complicated.
• Some artifacts are different.

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Spiral Imaging
k-SpaceData
Pulse Sequence
19
EPI Parameters

• Many parameters are the same as in spin-warp
  imaging
• SE vs. GRE or IR
• TR, TE, Flip Angle, TI
• FOV, matrix size, spatial resolution
• Some parameters require extra thought, however
• If only a single image is acquired using
  single-shot EPI, the TR might be meaningless. (TR
  is infinite)

20
Scan Time in EPI

• The scan time is most closely related to the
  number of shots and not matrix size.
• Scan Time (number of shots)(TR)
• Not (number of phase encodes)(TR)
• Consider 64x64 single-shot EPI and 128x128
  single-shot EPI - both are single-shot and take a
  single RF pulse to acquire an image.
• If 128x128 has artifacts that are too severe,
  however, multi-shot EPI may be required.

21
Echo Time in EPI

• In EPI, it is often hard to achieve a short echo
  time.
• The TE is defined as the time between the RF
  pulse and the acquisition of the center of
  k-space.
• In single-shot EPI, this could be a long time
  (often a minimum TE of 15-20 ms).
• This can be addressed by doing a partial k-space
  acquisition in the phase encoding direction.
• This will allow much shorter TEs (5-10 ms).

22
Echo Time in EPI
Partialk-space
Full k-space
23
Pulse Sequence Options in EPI

• Flow Compensation (Gradient Moment Nulling)
• Flow Comp (GNM) is often not as effective with
  EPI due to the long echo times.
• Partial k-space (phase) acquisitions reduce echo
  time and make this technique more effective.
• Spatial and chemical presaturation can also be
  used (fat saturation is nearly always used).
• There are also a 3D (volume) versions of EPI.

24
T1 Weighting in EPI

• In EPI, short TEs are difficult to obtain and
  the TR is often very long.
• EPI is not well suited to T1-weighted imaging
  with the usual short TR pulse sequences.
• On the other hand, one shot (or a small number of
  shots) is required for an image.
• EPI is well-suited to inversion recovery
  T1-weighted imaging.

25
Artifacts in EPI

• The ability to acquire images very rapidly is the
  strength of the EPI method.
• As a result, artifacts resulting from subject
  motion are nearly non-existent when imaging with
  single-shot EPI.
• Ghosting artifacts resulting from pulsatile blood
  flow are also extremely rare with single-shot EPI.

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Artifacts in EPI

• There are however, several kinds of image
  artifacts that are very different from those seen
  in spin-warp imaging
• N/2 or Nyquist ghosting
• Distortions from magnetic field inhomogeneity
• Chemical shift and susceptibility artifacts

27
N/2 Ghosting

• N/2 (N over 2) or Nyquist ghosting artifacts
  are unique to EPI.
• Caused by imperfections in the image
  acquisition.
• There are two distinct kinds
• Even and Odd Ghosts
• Ghost tuning procedures can reduce or eliminate
  these ghosts.
• Tuning can be done for each day, subject, or
  scan.
• Might also be done automatically (with prescan).

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N/2 Ghosting
Even Ghost
Odd Ghost
Original Image
29
Distortions from Inhomogeneity

• EPI is very sensitive to center frequency
  adjustments and inhomogeneities.
• For a misadjusted center frequency, the image is
  shifted in the phase direction.
• Careful prescan tuning is necessary.
• For misadjusted shims, the image can be twisted,
  stretched or squeezed.
• Shimming is often necessary (esp. at high
  fields).

30
Distortions from Inhomogeneity
Center FrequencyMisadjustment
OriginalImage
X shim(L/R shim)
Y shim(A/P shim)
31
Fat and Susceptibility Artifacts

• In EPI, unsuppressed fat is often shifted 2 cm or
  more.
• Fat suppression (Fat Sat) is always required.
• At areas of high magnetic susceptibility, a
  piling-up artifacts is often seen.
• Prevalent near frontal sinuses, above ears, etc.
• Pulse sequence parameters can affect this
• Interleaving usually reduces this artifact.
• Increasing resolution in the frequency direction
  often worsens the artifact.

32
Fat and Susceptibility Artifacts
Susceptibility (piling-up) Artifact
Fat Artifact
Original Image
33
EPI Hardware

• EPI is an extremely rapid and useful imaging
  method.
• It does, however, require some special, high
  performance hardware. Why?
• In spin-warp, we acquire a small piece of data
  for an image with each RF pulse.
• However in EPI, we try to acquire all of the data
  for an image with a single RF pulse.

34
Spin-Warp vs. EPI Pulse Sequences
EPI
Spin-Warp
Many acquisitionsto make a one image.
One acquisitionto make one image.
35
T2 Decay and Acquisition Time

• In spin-warp imaging, only a single phase encode
  need to be acquired.
• Only takes a short time.
• In EPI, all phase encode lines need to be
  acquired.
• Takes longer.
• Without special hardware - 100 ms to 1 second.
• T2 decay reduces signal throughout data
  acquisition.

36
T2 Decay and Acquisition Time
DataAcq. takes longer.
Signal decays away during acquisition.