Chapter 8 Artifacts Flashcards
In TOF MRA, a 2D or 3D imaging volume is pulsed rapidly so that full recovery of longitudinal magnetization is not possible between excitations and causes a phenomenon
saturation
what is predictable distribution of flow velocities in layers that parallel the vessel wall called
laminar flow
TOF effects in MR arise from the movement of what?
longitudinal magnetization
what is the term used when blood velocities exceed a critical threshold and disrupt the laminar flow state?
turbulent flow
artifacts from motion will appear in which direction
phase encoding direction
what is the term for the artifact that is caused when two different frequencies with the same peak are placed in the same voxel
chemical shift
the artifact that appears as multiple rings or bands of regular periodicity or duplication at transitions between high and low intensity is termed
truncation
aliasing occurs because tissue outside the selected FOV is
undersampled
a line of low signal in the cervical cord due to truncation is called
Gibbs artifact
what can affect the TOF phenomena
TE
slice thickness
velocity of flow
(ALL OF THE ABOVE)
name the type of flow when the velocity is greater in the center of the lumen of the vessel and slower at the walls
Laminar flow
this flow phenomenon is when a nucleus may have only received a 90 degree excitation or a rephasing 180 degree because it is moving
TOF
flow related enhancement results in
bright, high signal from moving nuclei
in entry slice phenomena, nuclei that are flowing and have not received repeated RF pulses are said to be
fresh
the magnitude of intra-voxel dephasing depends on the degree of
turbulent flow
spatial pre sat is effective in reducing flow artifact provided that the saturation pulse is placed _______ the flow and the imaging stack
between
what is the appearance of truncation artifact
interfaces of bright and dark ringing
what is the appearance of a zipper artifact
bright and dark lines across the image
what type of sequence does a black blood acquisition produce where blood flow is dark
spin echo equence
in bright blood imaging, flowing spins in blood vessels are hyper intense to stationary tissue and are based on what type of sequence
GRE
what type of sequence is a TOF MRA
imaging technique that relies on flow related enhancement
chemical pre sat saturates a particualr tissue by targeting its specific
frequency
the tissue most often saturated with chemical pre sat is
fat
the cause of Moire pattern
wrap and field inhomogeneities on GRE
black blood acquisitions produce images where blood flow is dark are based on
spin echo sequence
in bright blood imaging, flowing spins in blood vessels are hyper intense to stationary tissue and are based on
GRE sequences
what can be used on SE and GRE sequences to enhance the signal from flowing blood and suppress the signal from stationary tissue
- pre sat pulses
- flow comp
- magnetization transfer
TOR MRA is most sensitive to flow that is ________ to the FOV, the slices, and/or slab
perpendicular
what imaging technique can be used to overcome artifacts and the problem with loss of signal due to in plane flow
contrast enhanced MRA
