Artifacts Flashcards
How to reduce Gibbs truncation artifact?
Increase phase matrix or reduce field of view
What causes zipper artifact?
Leak in the RF shielding
How to reduce magnetic susceptibility artifact?
Use smaller voxels, shorter TE, increased receiver bandwidth, high TSE factors Thin slices to reduce partial volume averaging Use STIR instead of fat suppression Increase NSA/NEX to add signal
Avoid gradient echo sequences and parallel imaging techniques.
How to reduce chemical shift?
Higher bandwidth
How to correct parallel imaging artifact?
Increase FOV or reduce parallel imaging factor. Consider reducing slides and increasing slice thickness to recapture a atomic coverage without dramatically increasing scan time
How to address aliasing?
Implementation of “no phase wrap” or “anti-alisasing” options or increase phase dimension sampling.
What is the cost of using no phase wrap or anti aliasing options?
NSA/NEX is usually halved. Decrease SNR or increased time
What causes partial volume averaging?
Multiple tissue types contained within a single voxel.
How to correct partial volume average of?
Smaller voxels are needed, usually thinner slices
What causes dielectric effect?
Increase in inhomogeneity. Local eddy currents due to increased conductivity of body tissues. Manifests as areas of shading or signal loss
How correct for dielectric effect?
No real correction outside of placing patient in lower field strength systems, employing dielectric pads or using smaller FOV
What causes cross talk?
When using groups of angled slices. Slices overlap or are too close together. Creates band of signal loss where they overlap.
How to correct for cross talk artifact?
Increase gaps between slices, reduce slice overlap, utilize “stacks as packages” function on scanner.
What causes cross excitation?
Signal loss due to partial excitation and subsequent saturation in adjacent slices in multi-slice imaging.
How to correct for cross excitation?
Selection of interleaved method of slice acquisition
What causes Moire fringes/ Zebra artifact?
Interference of aliased signals at different phases. Most often in 3D gradient echo images at higher field strengths
What causes FID artifacts?
Incompletely crushed FID signal from an RF refocusing pulse. Reduced refusing pulses, increases in FOV, decreases in slice thickness all contribute.
How to correct for FID artifact?
Increase NSA/NEX or enable flow compensation
What causes magic angle artifact?
Tendon or ligament is positioned approx 55degrees to the direction of the main magnetic field and a short TE value pulse sequence is used. Can appear similar to tendonopathy
How to correct magic angle artifact?
Use long TE sequence
Cause of Annefact artifact?
Feather like bright signals. Usually in spine imaging. Active RF coil channels or elements outside the scanning FOV.
