artifacts Flashcards
Sources of artifacts
hardware
software
physiological phenomena
physics limitations
types of artifacts
chemical shift aliasing black boundry gibbs or truncation zipper motion entry slice field inhomogeneity slice overlap (crosstalk)
typesof artifacts
magic angle moire fringe RF overflow central point quadrature ghost susceptibility zero fill eddy current diastolic pseudogating gadolinium pseudogating
frequency encoding direction different resonant F between water and fat vertebral bodies, orbits, solid organs worse at higher field strength less with stronger gradients
chemical shift artifact
occurs when FOV is smaller than part being imaged
caused by undersampling in the phase direction
end slices of 3D
Aliasing or wrap around
increase FOV
oversample data in F direction and increase phase steps in phase direction
swap phase and Freq direction
use surface coil so no signal out of FOV
correct aliasing or wrap around
black line at fat-water interfaces
occurs at TE when fat/water spins located in the same pixel are out of phase
GE sequences
1.5T occur at 4.5ms multiples starting at 2.3 ms
black boundry artifact
use in phase TE’s
fat suppression
increase bacndwidth or matrix size
Min (full) TE
correct black boundry artifact
bright or dark lines parallel to borders of abrupt intensity change
may simujlate a syinx on sagittal spine
related to finite number of steps used by fourier transform
Gibbs
truncation artifact
more encoding steps lessen the intesity and narrow the artifact
correct Gibbs /truncation artifact
related to hardware/software
occur from RF energy entering room perpendicular to the F direction
zipper artifacts
noise or repeating densities in phase direction
extend across entire FOV
arterial, CSF, swallowing etc…
motion artifacts
flow comp and cardiac gating
spatial presaturation
surface coil localization
correcting motion
loss of signal in image from multi angle multi slice acquisition
same mechanism as spatial presat for motion
slice overlap
crosstalk
avoid steep change in angle between slice groups
use separate acquisitions
use small flip angle
correct slice overlap
crosstalk
seen mostly in tendons and ligaments at 55 angle to mag field
magic angle
unsaturated spins in blood of CSF entering initial slices result in greater signal then reduces on subsequent slices
confused with thrombus
entry slice artifact
use spatial saturation pulses
correct entry slice artifact
main magnetic RF coil inhomogeneity dielectric effects (worse at 3.)T) variation in intensity across image non-uniform fat suppresion
field inhomogeneity
shimming area of interest
use STIR instead of fatsat
use volume coil vs surface coil
use phased array coil
correct field inhomogeneity
cause nonuniform washed out image
occurs when signal received from amp exceeds the dynamic range of the A-D converter
RF overflow (clipping)
autoprescanning usually adjusts rcvr gain to prevent
correct RF overflow
interference pattern most commonly seen on gradient images
aliasing from one side of body to the other results in superimposition of signals that add or cancel
similar to looking through two window screens
Moire fringes
focal dot of inc reased/decreased signal in cent of image
caused by constant offset of DC voltage
central point artifact
recalibrate by engineer
constant temp in equipment room
central point corrections
unbalanced gain between two channels of a quadrature coil
combining two signals of different intensities causes some Freq to become less than zero causeing a 180 deg ghost
quadrature ghost
repair by engineer
correct quadrature ghost
variations in mag strength that occurs near interfaces of substance of different magnetic susceptibillity
causes dephasing of spins and freq shifts
worse with long echo times and GE sequences
less with FSE
susceptibility artifacts
band like usually oblique stripes
data in K-space array will be missing or set to zero
abrupt change from signal to no signal or normal to high
zebra artifacts
varying mag field strength induces electrical currents in conductors causing distortion
problem with echo-planar imaging using strong gradients
eddy current artifact
precompensation
shielded gradients
eddy current corrections
change in intensity of blood flow in large vessel
diastolic pseudogating
three density layers in bladder after Gd
T2 shortening overshadows normal T1 effects at high concentrations
gadolinium pseudolayering
