Image Artifacts Module 8 Image Artifacts Module 8

16 Pages·2016·2.65 MB·English

IImmaaggee AArrttiiffaaccttss
MMoodduullee 88
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IImmaaggee AArrttiiffaaccttss
(cid:1)Definition
(cid:1)False information appearing on images
that can mimic pathology and often
obliterates the tissue’s true representation.
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MMoottiioonn
(cid:1)Appearance
(cid:1)Produces faint to prominent ghost images of the
anatomy in the phase encoding direction.
(cid:1)Can cause blurring, fuzziness
(cid:1)More apparent in higher field strengths due to
the number of spins responding during the
acquisition.
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MMoottiioonn
(cid:1) Origin or Cause
(cid:2)Gross patient motion
(cid:2)patient uncooperative
(cid:2)Physiological motion
(cid:2)pulsations from heart
(cid:2)fast arterial flow and slow venous flow
(cid:2)peristalsis in the bowel
(cid:2)respiratory
(cid:2)cerebrospinal fluid
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MMoottiioonn
(cid:1)Origin or Cause
(cid:1)Failure of gradients to completely eliminate the
phase contribution of either the slice-select or
frequency encoding gradients.
(cid:1)inaccurate gradient digitization
(cid:1)quadrature imbalance
(cid:1)stimulated echo ghosting
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MMoottiioonn
(cid:1) Solutions
(cid:2)Sponges
(cid:2)Supports the patient’s body.
(cid:2)Provides reference point for patient.
(cid:2)Sedation
(cid:2)When patient is unable to cooperate.
(cid:2)Monitor patient heart and respiration.
(cid:2)Cardiac gating
(cid:2)Synchronizes the collection of MR data to
patient’s heartbeat.
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MMoottiioonn
(cid:1)Solutions
(cid:1)Respiratory gating
(cid:1)Synchronize collection of MR data to patient’s
respiration.
(cid:1)Breath hold imaging
(cid:1)Collection of MR data during suspension of
respiration.
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MMoottiioonn
(cid:1)Solutions
(cid:1)Swapping phase and frequency encodes
(cid:1)Minimizes motion in the phase direction.
(cid:1)Fast imaging
(cid:1)Rapid scanning to minimize scan time.
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MMoottiioonn && GGhhoossttiinngg
GGhhoossttiinngg oonnllyyaappppeeaarrss iinn tthhee PPEE ddiirreeccttiioonn..
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EEYYEE
MMOOTTIIOONN
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CCAARRDDIIAACC MMOOTTIIOONN
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The image on the left was acquired without any form of
motion compensation technique for cardiac motion although
respiratory motion was compensated for. Notice the blurring
of the cardiac structures.
The image on the right was obtained using cardiac gating. The
imaging sequence is triggered by the R-wave and data
collection occurs during the same portion of the cardiac cycle
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2 for each TR period. This effectively eliminates cardiac motion.
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FFllooww
(cid:1)Flow
(cid:1)Fast Flow
(cid:1)arterial
(cid:1)Pulsatile Flow
(cid:1)cerebral spinal fluid
(cid:1)Slow Flow
(cid:1)venous, lymphatic
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FFllooww
(cid:1)Cause
(cid:1)Flow-Related Enhancement
(cid:1)Flow Void
(cid:1)Flow-Induced Phase artifacts
(cid:1)Mechanisms
(cid:1)Transport of spins into and out of volume of
interest
(cid:1)Flow-induced phase changes
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FFllooww RReellaatteedd EEnnhhaanncceemmeenntt
(cid:1)Appearance - Bright signal
(cid:1)Flow can affect the NMR signal either by introduction of
unsaturated spins into the volume or through the excited spins
from the volume.
(cid:1)It occurs in sequences where the TR is too short to allow full
longitudinal relaxation.
(cid:1)Partial saturation remains for long T1 tissues.
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FFllooww VVooiidd
(cid:1)Appearance - Dark signal
(cid:1)Spins excited by the RF pulse are swept out of
the selected slice prior to the application of the
next RF pulse.
(cid:1)No intensity is induced.
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FFllooww--iinndduucceedd pphhaassee aarrttiiffaaccttss
(cid:1)Appearance - Ghosts of flowing fluid
(cid:1)Flow is pulsatile and irregular.
(cid:1)Changes in flow causes phase shifts.
(cid:1)Flow across a gradient causes phase shifts.
(cid:1)Inconsistent phase encoding result in
incorrect spatial assignment.
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FFllooww
(cid:1) Solutions
(cid:2)Presaturation
(cid:2)Pre-saturates inflowing spins to void signal
flowing into region of interest.
(cid:2)Flow compensation
(cid:2)Gradient reversals to reduce phase shift.
(cid:2)Even-echo rephasing
(cid:2)Refocus of phase dispersions by using
symmetrical echo collection.
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FFllooww
AArrttiiffaacctt
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Note the appearance of two spinal cords in the first T2-
weighted FSE axial image.
The artifactual extra spinal cord is due to flow artifact
from the pulsatile CSF flow.
Flow compensation (of the first order of flow) is applied
2 in the second image, eliminating the artifact.
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PPUULLSSAATTIILLEE
BBLLOOOODD
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Wraparound
Or Aliasing
Whenever there is anatomy outside of
the field of view which has been excited
by the RF pulses there is the potential
for aliasing artifacts to occur.
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WWrraappaarroouunndd oorr AAlliiaassiinngg
(cid:1)Appearance
(cid:1)Signals beyond ROI superimpose on
opposite image boundaries.
(cid:1)Directions
(cid:1)Frequency encode
(cid:1)Phase encode
(cid:1)Slice select encode
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WWrraappaarroouunndd oorr AAlliiaassiinngg
(cid:1)Cause
(cid:1)Diameter of the excited anatomy imaged is
larger than the prescribed field of view (FOV).
(cid:1)Finite number of frequencies used to represent
the FOV is inappropriate causing sampled data
points to be indistinguishable.
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WWrraappaarroouunndd oorr AAlliiaassiinngg
(cid:1)Cause
(cid:1)Two different frequencies can have the same
peaks, placing them in the same voxel.
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Tissues outside the field of view do not get properly phase
encoded relative to their actual position.
This excited tissue gives up signal during readout but as
it has not been correctly phase encoded appears in a
position other than its actual location.
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EElliimmiinnaattiinngg AAlliiaassiinngg
(cid:1) Select a no frequency-
wrap and/or a no phase-
wrap option prior to
running the sequence.
(cid:1) Select a larger field-of-
view
(cid:1) Use a saturation pulse on
areas outside the field-of-
view.
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No phase-wrap doubles
the field-of-view by
doubling the number of
phase encodes;
then halves the number
of averages (to keep scan
time constant);
then discards the
additional data encoded
but not needed for the
desired field-of-view.
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AAlliiaassiinngg
FFrreeqquueennccyy EEnnccooddee DDiirreeccttiioonn
(cid:1)Solutions
(cid:1)Oversample in readout direction at twice the
frequency required for FOV.
(cid:1)Reduce the amplitude of the time domain signal
using a steeper band pass filter before sampling
the data.
(cid:1)Surface coils
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AAlliiaassiinngg
PPhhaassee EEnnccooddee DDiirreeccttiioonn
(cid:1)Solutions
(cid:1)Use more phase encode steps.
(cid:1)Swap phase and frequency directions.
(cid:1)Selective excitation
(cid:1)Surface coils
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