An organizational post I'd been meaning to get to for a while. There are some posts to come in this series, in parentheses below. I'll update this page with links as these posts get published.
An introduction to the post series, defining what we mean by "good" data, and general discussion on viewing and interpreting EPI artifacts in a time series.
Good data
Includes cine loops through time series EPI and statistical images to evaluate the data.
Includes cine loops through time series EPI and statistical images to evaluate the data. (The notes include a description of the slice-dependent gradient switching limits that can prohibit certain slice orientations.)
Common persistent EPI artifacts
Aliasing effects in the frequency and phase encoding dimensions.
Common persistent EPI artifacts: Gibbs artifact, or ringing
Common persistent EPI artifacts: Gibbs artifact, or ringing
The origin of the ringing problem and demonstrations in phantom and brain data.
Subject-dependent conditions:
- Asymmetric orientation of the subject's head leading to a poor shim
- Poor shim as a result of subject motion during/immediately after shimming
- Presence of FOD or an implant causing a poor shim
Scanner-dependent conditions:
- Rotated read/phase encode axes
- No fat suppression
- Mechanical resonances
- Excessive ramp sampling
A brief overview of these two plagues of EPI.
RF screening, adding devices to the scanner environment, modifying the magnet room, and standard operating procedures for fMRI labs.
Receive fields for phased-array RF coils, and removing receive field heterogeneity with prescan normalization.
Common intermittent EPI artifacts
- Eye movements
- Head nodding
- Talking
- Coughing, swallowing, yawning and sneezing
- Body movements
(Common intermittent EPI artifacts: Signal drift due to gradient heating)
Rare intermittent EPI artifacts
Most common causes:
- Movement of locking nuts or some other metal component of the gradient electrical cables
- Movement of locking nuts on shim trays or other components inside the bore tube
- Conductive debris in the RF coil sockets on the patient bed
- Other sources of metal-on-metal friction inside the magnet room
- Items of clothing on subjects
Detecting spikes outside of QA tests.
(Rare intermittent EPI artifacts: Fluctuating ghosts with a 32-ch Rx coil)
(Rare intermittent EPI artifacts: Spurious signals due to imperfect crusher gradients)
Rare persistent EPI artifacts
(Rare persistent EPI artifacts: Poor shim, even on a phantom)
Movement/loosening of passive shim trays. (May also manifest as an inability to achieve good fat suppression for EPI of head.)
Loss of echoes that refocus away from theoretical k-space center.
(Rare persistent EPI artifacts: Increased signal dropout with partial Fourier)
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