MRI Artifacts Flashcards
(14 cards)
Motion artifact
Phase encoding direction
Correction: swap PE and FE direction, sat band over large blood vessels, deepen anesthesia/neuromuscular blockade
Correction of flow: gradient moment rephrasing (nulling) aka flow compensation- applies additional gradients so there is no phase shift with the moving protons (works best on slow laminar flow I.e venous), this will increase the TE that is used, which may reduce the number of slices possible
Correction of TOF/entry slice phenomenon: using saturation bands adjacent to FOV or in slice direction
Pulsatility artifact
Occurs in PE direction
Flow compensation and radial encoding of K space may reduce pulsatility artifact
Zipper
Frequency encoding direction
Leakage of external RF signals
Break in room shielding or due to electrical equipment in room (monitoring equipment/lights etc)
Spike or herringbone
Multiple parallel dark stripes
Loose electrical connection
Build up of static electricity
Need engineer
Three interdependent factors in MRI
SNR
Image resolution
Acquisition time
Cross talk
Overlapping slices
Results in horizontal black bands where there was overlap
Cross excitation
Loss of signal during slice selection if there is excitation of a slice from adjacent slices resulting in partial saturation of spins
Occurs due to shape of frequency profiles of RF pulse
Occurs if no gap between slices
Worst on inversion recovery sequences
Solution: have an interslice gap of at least 1/3 of slice thickness
Phase wrap
Wrap around
Aliasing
When FOV in PE direction too small
Correction: FOV in PE direction needs to be larger than area of anatomy being imaged, or phase over sampling needs to be used
Gibbs
Truncation
Ringing
Data undersampling, cannot completely eliminate
PE direction
Correction: sample more datapoints by increasing size of image matrix and voxel size
Chemical shift
Misregistration of signal or phase cancellations
FE direction
Use fat sat or increase bandwidth
Phase cancellation/ black boundary artifact
Occurs on GE but not SE
PE direction
Choose a TE when fat and water are in phase
Susceptibility artifacts
Decrease TE- shorter time for dephasing of spins
Thin slices
Avoid gradient echo pulse sequences
Swap SE and FE
Magic angle artifact
Tendons/ligaments appear hyperintense when at 55 degree + 10 on short TE sequences (esp T1, but also proton density, T2*, and STIR).
Not seen on long TE sequences such as T2 W
Partial volume averaging
Occurs in all 3d imaging
Image in 3d voxel, displayed in 2d pixel
Pixel displays intermediate of tissue intensities, particularly at boundary of tissues
Correction: decrease slice thickness
