What is artifact reduction?
What is artifact reduction?
Artifact reduction (removal) is a process of identifying artifact components in the EEG signal and separating them from the neuronal sources.
What is phase encoding and frequency encoding in MRI?
Spatial encoding in MRI The second step of spatial localization is called phase encoding. A magnetic gradient field is applied briefly in one direction. As the change in frequency is very brief, when the gradient is switched off, it causes a change in phase that is proportional to the distance.
How do you choose phase encoding?
Three factors are considered in selecting phase- and frequency-encode directions: 1) reducing artifacts, 2) minimizing scanning time, and 3) accommodating restrictions imposed by coil design or parallel imaging. The phase-encoding direction is associated with two major artifacts: wrap-around and flow/motion.
What is CT artifact?
In computed tomography (CT), the term artifact is applied to any systematic discrepancy between the CT numbers in the reconstructed image and the true attenuation coefficients of the object.
What is the difference between a frequency encoding and a phase encoding?
The frequency-encoding direction is along the x-axis in K-space (may or may not be that axis in the image, if it is rotated); this represents the time samples of the signal. The y-axis is the phase-encoding direction: each phase-encoding step yields a separate horizontal line.
What is the purpose of the phase encoding gradient?
The phase encoding gradient is used to position the spin system at a specific line in k-space. Application of the frequency encoding gradient and recording signal as a function of time moves the spin system across a line in k-space.
How can we reduce artifacts?
Several methods of reducing motion artifacts are then suggested. These include: randomization of views, averaging views, matching repeat times to the respiratory period, hybrid imaging, ROPE and COPE. The latter two methods reorder the data acquisition to destroy the coherence of the motion.
What is metal artifact reduction?
Projection-based metal artifact reduction (MAR) algorithms act in projection space and replace corrupted projections caused by metal with interpolation from neighboring uncorrupted projections. MAR algorithms primarily suppress artifacts that are due to photon starvation.
How does randomly ordered phase encoding affect motion artifact appearance?
The resulting effect of the randomly ordered phase encoding is a noise-like appearance of the motion artifact in contrast to the more conspicuous appearance of image ghosts in the case of a sequential (standard) order of phase encoding ,
How to choose the correct phase encoding direction?
The phase encoding direction can be chosen, e.g. whenever oblique MR images are acquired or when exchanging frequency and phase encoding directions to control wrap around artifacts. The temporal order in which the phase encoding gradient pulses are applied.
What is ‘phase encoding’ in EPI?
‘Phase Encoding’. In the blipped phase encoding variant of EPI, the k-space position in the phase encoded direction is incremented by gradient ‘blips’ of the appropriate area. These, when timed to occur during the reversals of the read-out gradient, produce a rectilinear path in k-space. The artifacts in an EPI image can arise from both hardware…
How does phase encoding level affect spatial resolution?
Spatial resolution is directly related to the number of phase encoding levels (gradients) used. The phase encoding direction can be chosen, e.g. whenever oblique MR images are acquired or when exchanging frequency and phase encoding directions to control wrap around artifacts.