Artefacts Flashcards
(22 cards)
What could be cause of MRI spatial distortion?
B0 inhomogeneity
nonlinear gradients
coil heating
eddy currents
unbalanced gradients
What could cause MRI non-uniformity artefact?
Rx coil issues
Cause of MRI RF spike artefact?
Transient discharge of static
What does MRI RF spike artefact look like?
point in k-space => regular spatial pattern
What does MRI ADC saturation look like?
centre of k space has highest signal, so this saturates first
signal where there should be none
Cause of MRI ADC saturation?
Rx gain too high
What does MRI zipper artefact look like?
alternating bright and dark pixels in PE dir
What causes MRI zipper artefact?
continuous RF interference
What does Tx RF breakthrough look like in MRI?
zipper artefact in centre of image (at Larmor frequency)
How to prevent aliasing in MRI?
use surface coil / phase oversampling
What causes T2 blurring in MRI?
Use of multiple TE => different contrast for different k space points
What is truncation / Gibbs ringing in MRI?
modulation at boundaries due to sampling of sharp features with too low a sampling freq
What is an artefact?
A structure in the image that does not provide a true representation of the scanned object
What is phase abberration in US?
varying speed of sound in inhomogeneous tissue leads to wavefront distorting and hence poor resolution
What causes boundary distortion in US?
variations in speed of sound
what causes edge shadowing , ghosts and size errors in US?
refraction
explain how edge shadowing occurs in US
refraction in cyst towards centre leaves signal voids at edges
what causes post-cystic enhancement in US?
low attenuation in cyst
How can specular reflection affect image (three types of artefact)?
dark appearance
mirror image multipath artefact
reverb and comet tail artefact
Name three beam dimension artefacts in US
beam width
slice thickness
side-lobe
What causes ring-down artefact in US?
particular configuation of insonated resonating gas bubbles
What does ring-down artefact look like in US?
like a comet-tail artefact
