ARTIFACTS IN COMPUTED
TOMOGRAPHY
Image artifact
• Artifact is a distortion or error in an
image that is unrelated to the subject
being studied.
• Artifacts can degrade image quality.
• It can affect the perceptibility of detail.
• It can lead to misdiagnosis.
Classification of artifacts
Based on appearance
bands Streaks shading rings
Artifacts arise from range of sources:
• Physics based artifacts
• Patient based artifacts
• Scanner based artifacts
• Helical & multisectional technique artifacts
PATIENT MOTION ARTIFACT
VOLUNTARY INVOLUNTARY
E.g. : swallowing, E.g. : peristalsis,
respiratory Cardiac motion
motion
•Appears as streaks that are usually tangential to high contrast
edges of the moving parts.
•It produces “ghosting effect” object in ct image appears as if
composed of superimposed images.
•Blurred images.
REMEDY
• Voluntary motion:
Explanation of the procedure and good
communication with a patient.
Immobilizing pads
• Involuntary :
Short scan time.
Software correction.
CT images of the body created with conventional
reconstruction (a) and with motion artifact correction .
METAL ARTIFACT
• Metal objects : prosthetic devices, dental
fillings, surgical clips, electrodes.
• Appears as streaks.
• Because of highly attenuating metal objects ,
the error occurs as a result of combination of
beam hardening, partial volume and limited
dynamic range of acquisition and
reconstruction resulting in star shaped streak
artifact
REMEDY
Removal of external metal objects from
patients.
Software such as Metal Artifact Reduction
program that can be used to complete the
incomplete profile through interpolation.
Gantry angulations.
CT images of a patient with metal spine implants,
reconstructed without any correction (a) and with metal
artifact reduction
BEAM HARDENING ARTIFACT
• Refers to increase in the mean energy of x-ray
beam as it passes through the patient.
• Beam is “hardened” high energy photons are
attenuated less by the tissue .As a result ,they
pass through the patient and reach the detectors.
• This artifact is also called “Cupping” artifact
because the hardening is most pronounced in the
center and less at the periphery. It resembles as a
cup.
REMEDY
• Filtration( bow-tie filters).
• Iterative reconstruction
PARTIAL VOLUME ARTIFACT
• Partial volume averaging is a technique where
the ct numbers are based on the average of
tissue voxels.
• When the voxel contains multiple materials that
are significantly different, partial volume
averaging results in partial volume artifacts.
• Appears as dark thin spot
REMEDY
• Thin slice selection
• Thin slice incremation
TUBE ARCHING
• Tungsten vapor from anode and cathode
intercepts the projectile electrons intended for
collisions with the target. Crackling sound.
• REMEDY : tube replacement
gas burn off
OUT OF FIELD ARTIFACT
• Patient is not entirely enclosed in the scanning
field of view. Patients body can obstruct the
detectors.
• In addition, patient tissue outside the SFOV will
further Harden the X ray beam.
• Appears as shading or streaks
REMEDY
• Selection of larger SFOV.
• Raising patients arms above the head on the scan
of chest and abdomen.
ALIASING ARTIFACT
• Too coarse sampling interval leads to Misregistration by
the computer information relating to sharp edges and
small objects.
• Appears as streaks after reconstruction.
CT image of a Teflon block in a
water phantom shows aliasing (arrow) due to
under sampling of the edge of the block.
REMEDY
• Increase scan time.
• Use complete arc scan.
• High resolution techniques like Quarter detector
shift, Flying focal spot.
NOISE INDUCED ARTIFACT
Noise is influenced partially by photons that
strike the detector.
• Poor selection of exposure techniques(kVp,
mA) scan speed, poor patient positioning in
SFOV.
• Detector electronic noise
• More photons less noise.
REMEDY
• Increase mAs.
• Adaptive filtration.
• Automatic tube current modulation.
RING ARTIFACT
• Detector malfunction usually appears in
3rd generation CT scanner.
REMEDY
Detector calibration.
Detector replacement.
LINE IN TOPOGRAM
• Bad detector causes continuous line on the
topogram.
• REMEDY: replacing detectors
AIR CONTRAST INTERFACE
ARTIFACT
• Significant difference in density between contrast
and air; motion may contribute
• Most frequent in gastric air fluid level.
• Appears as streaks
• REMEDY: change the patient position
HELICAL CT ARTIFACT
STAIR STEP
ARTIFACT
CONE
ZEBRA
BEAM
ARTIFACT
ARTIFACT
STAIR STEP ARTIFACT
• Occurs when the slice thickness is larger than the
pixel length.
• Stair step appearance
REMEDY
• Stair-step artifacts are virtually eliminated in
multiplanar and 3D reconstructions of thin slice
data from today’s multi-slice scanners.
CONE BEAM ARTIFACT
• Caused by incomplete or insufficient
projection samples as a result of cone
beam geometry of multi- slice ct.
• Appears as streaks & altered density in
image periphery.
CT images from data collected by an outer detector row (a)
and an inner detector row (b) show cone beam artifacts around a Teflon rod,
REMEDY
• Cone beam reconstruction algorithm.
ZEBRA ARTIFACT
• Faint stripes may be apparent in multiplanar and
the three-dimensional reformatted images from
helical data.
• This Zebra effect becomes more pronounced
away from the axis noise inhomogenity is worse
off-axis
REFERENCE
• Computed Tomography
Euclid Seeram
