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Z MRI Physics > artifacts > Flashcards

artifacts Flashcards

1
Q

Sources of artifacts

A

hardware
software
physiological phenomena
physics limitations

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2
Q

types of artifacts

A 
chemical shift
aliasing
black boundry
gibbs or truncation
zipper
motion
entry slice
field inhomogeneity
slice overlap (crosstalk)
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3
Q

typesof artifacts

A 
magic angle
moire fringe
RF overflow
central point
quadrature ghost
susceptibility
zero fill
eddy current
diastolic pseudogating
gadolinium pseudogating
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4
Q 
frequency encoding direction
different resonant F between water and fat
vertebral bodies, orbits, solid organs
worse at higher field strength
less with stronger gradients
A

chemical shift artifact

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5
Q

occurs when FOV is smaller than part being imaged
caused by undersampling in the phase direction
end slices of 3D

A

Aliasing or wrap around

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6
Q

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

A

correct aliasing or wrap around

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7
Q

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

A

black boundry artifact

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8
Q

use in phase TE’s
fat suppression
increase bacndwidth or matrix size
Min (full) TE

A

correct black boundry artifact

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9
Q

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

A

Gibbs

truncation artifact

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10
Q

more encoding steps lessen the intesity and narrow the artifact

A

correct Gibbs /truncation artifact

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11
Q

related to hardware/software

occur from RF energy entering room perpendicular to the F direction

A

zipper artifacts

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12
Q

noise or repeating densities in phase direction
extend across entire FOV
arterial, CSF, swallowing etc…

A

motion artifacts

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13
Q

flow comp and cardiac gating
spatial presaturation
surface coil localization

A

correcting motion

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14
Q

loss of signal in image from multi angle multi slice acquisition
same mechanism as spatial presat for motion

A

slice overlap

crosstalk

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15
Q

avoid steep change in angle between slice groups
use separate acquisitions
use small flip angle

A

correct slice overlap

crosstalk

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16
Q

seen mostly in tendons and ligaments at 55 angle to mag field

A

magic angle

17
Q

unsaturated spins in blood of CSF entering initial slices result in greater signal then reduces on subsequent slices
confused with thrombus

A

entry slice artifact

18
Q

use spatial saturation pulses

A

correct entry slice artifact

19
Q 
main magnetic
RF coil inhomogeneity
dielectric effects (worse at 3.)T)
variation in intensity across image
non-uniform fat suppresion
A

field inhomogeneity

20
Q

shimming area of interest
use STIR instead of fatsat
use volume coil vs surface coil
use phased array coil

A

correct field inhomogeneity

21
Q

cause nonuniform washed out image

occurs when signal received from amp exceeds the dynamic range of the A-D converter

A

RF overflow (clipping)

22
Q

autoprescanning usually adjusts rcvr gain to prevent

A

correct RF overflow

23
Q

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

A

Moire fringes

24
Q

focal dot of inc reased/decreased signal in cent of image

caused by constant offset of DC voltage

A

central point artifact

25
Q

recalibrate by engineer

constant temp in equipment room

A

central point corrections

26
Q

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

A

quadrature ghost

27
Q

repair by engineer

A

correct quadrature ghost

28
Q

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

A

susceptibility artifacts

29
Q

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

A

zebra artifacts

30
Q

varying mag field strength induces electrical currents in conductors causing distortion
problem with echo-planar imaging using strong gradients

A

eddy current artifact

31
Q

precompensation

shielded gradients

A

eddy current corrections

32
Q

change in intensity of blood flow in large vessel

A

diastolic pseudogating

33
Q

three density layers in bladder after Gd

T2 shortening overshadows normal T1 effects at high concentrations

A

gadolinium pseudolayering

Z MRI Physics (12 decks)

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