CT SCAN
ARTIFACTS
CAUSES AND SOLUTIONS
By, Mr. Chetan Shende.
Clinical Application and
Product Specialist.
CT ARTIFACTS
In computed tomography (CT), the term artifact is
applied to any systematic discrepancy between the
CT numbers in the reconstructed image and the true
attenuation co-efficients of the object.
CT images are inherently more prone to artifacts than
conventional radiographs because the image is
reconstructed from something on the order of a million
independent detector measurements.
Artifacts can seriously degrade the quality of
computed tomographic (CT) images, sometimes to the
point of making them diagnostically unusable.
To optimize image quality, it is necessary to
understand why artifacts occur and how they can be
prevented or suppressed
CAUSES OF IMAGE ARTEFACTS CAN BE
GROUPED INTO A FEW CATEGORIES:
Physics based
Patient properties
Scanner based
Helical and multislice artefacts
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PHYSICS-BASED ARTIFACTS
Result from the physical processes involved in the
acquisition of CT data .
Beam Hardening
I. Cupping Artifacts.
II Streaks and Dark
Bands.
Partial Volume
Photon Starvation
BEAM HARDENING
An x-ray beam is composed of individual photons
with a range of energies (polychromatic).
As the beam passes through an object, it becomes
"harder" that is to say its mean energy increases,
because the lower energy photons are absorbed
more rapidly than the higher- energy photons
Beam hardeningartefact
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BEAM HARDENING
This is particularly common in the posterior fossa on a
CT head scan due to the dense petrous bones.
The energy distribution of X-rays shifts toward the higher
side as X-rays go through an object.
This phenomenon is called beam hardening.
High-energy X-rays are also known as “hard” X-rays,
which can penetrate denser, thicker materials better
than low-energy X-rays or “soft” X-rays. In other words,
X-rays harden as they penetrate an object. Hence the
name, beam hardening.
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CUPPING ARTEFACT
This beam hardening artefact also produces another
type of artefact called the cupping artefact
The centre of an object is usually the thickest and,
therefore, the beam will become harder in the
centre than at the periphery and is assigned lower
Hounsfield units. This can be corrected with a
‘beam hardening correction’ algorithm.
SOLUTIONS TO BEAM HARDENING
Pre-patient filter: This absorbs the soft x-rays and
minimises the beam hardening artefact.
Bow-tie filter: Pre-harden the x-ray beam
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STREAKS AND DARK BANDS ARTEFACT
In very heterogeneous cross sections, dark bands
or streaks can appear between two dense objects
in an image.
Occur because the portion of the beam that
passes through one of the objects at certain tube
positions is hardened less than when it passes
through both objects at other tube positions.
Can occur both in bony regions of the body and in
scans where a contrast medium has been used
Streak artifacts at interface between high-density
material surrounded by low-density material
System unable to process high spatial frequency
signal required for sharp edges Common sites: -
Dense bone and surrounding tissue - Bowel loops
with contrast Blood vessels with IV contrast Areas
around biopsy needles
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PARTIAL VOLUME ARTEFACT
occurs when tissues of widely different absorption are
encompassed on the same CT voxelproducing a beam
attenuation proportional to the average value of these
tissues.
The latest generation of CT scanners with an associated
reduction in the volume of a voxel has substantially
reduced the occurrence of this artifact.
Partial volume averaging is particularly problematic in CT
angiography (e.g. misdiagnosis of an apparent contrast
filling defect caused by the artifact as PE). Therefore the
use of thin section reconstructions (1-1.5 mm) are
recommended where the impact of this artifact is
negligible
If a dense object only partially protrudes into a
detector stream the attenuation is averaged
with its surroundings and it will be assigned a
lower Hounsfield unit. In the image above, the
dense circle lies on a less dense background.
The object fills detector stream 2 resulting in a
very high attenuation (white). In detector
stream 3 none of the dense object is imaged
and so the attenuation is low (black). In detector
stream 1 the object is only partially imaged and
so the attenuation is an average between the
dense object and the less dense background.
N.B. partial voluming will only ever reduce the
apparent attenuation of an object, it will never
increase the apparent attenuation.
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INCOMPLETE PROJECTION
Streak artefact
An object may protrude into the slice in one
projection but not in the opposing projection,
especially at the periphery of the image where the
beam is more divergent. If this happens, a variant of
partial voluming artefact occurs in which the
object appears streaked due to the
inconsistencies producedduring imaging.
These streak artefacts can be caused, for example,
when a patient’s arms are by their side and are
imaged in some projections but not others.
SOLUTION
Smaller slice thickness
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PHOTON STARVATION ARTIFACT
is one source of streak artifact which may occur in CT.
It is seen in high attenuation areas, particularly behind metal
implants. Because of high attenuation, insufficient photons
reach the detector.
During the reconstruction process, the noise is greatly
magnified in these areas leading to characteristic streaks in
the image .
An object may protrude into the slice in one projection but
not in the opposing projection, especially at the periphery of
the image where the beam is more divergent. If this
happens, a variant of partial voluming artefact occurs in
which the object appears streaked due to the
inconsistencies produced during imaging.
These streak artefacts can be caused, for example, when a
patient’s arms are by their side and are imaged in some
projections but not others.
SOLUTION
Smaller slice thickness
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PHOTON STARVATION ARTIFACT
Photon starvation.
This is another cause of streak artefacts. In projections that
have to travel through more material,
e.g. across the shoulders, as the x-ray beam travels through
more x-ray photons are absorbed and removed from the
beam.
This results in a smaller proportion of signal reaching the
detector and, therefore, a larger proportion of noise. The
streaks are due to the increased noise which is why they
occur in the direction of the widest part of the object being
scanned
SOLUTION
Adaptive filtering: the regions in which the attenuation
exceeds a specified level are smoothed before undergoing
backprojection
Photon starvation mA modulation The tube current (mA)
can be varied with the gantry rotation. Higher mA's (greater
signal) are used for the more attenuating projections to
reduce the effect of photon starvation. The mA required can
either be calculated in advance from the scout view or
during the scan from the feedback system of the detector
Avoidance using mA modulation
Allows an increase of photon flux through widest parts
without unnecessary dose through narrower parts
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PATIENT BASED ARTIFACTS:
Caused by patient movement, presence of metallic
materials in or on the patient :
Motion artifact
Metallic artifact
Transient interruption of contrast
MOTION ARTIFACT
Motion artefact can be caused by:
Patient swallowing
Breathing
Pulsatility of heart and vessels
Patient moving
If a patient or structure moves as the gantry rotates the
object will be detected as being in several positions and
represented in the image as such.
SOLUTIONS MOTION ARTIFACT
Scan parameters
Shorten scan time
Spiral scanning
ECG gating: this can be used prospectively to trigger
image acquisition during a specific point on the ECG
when heart motion is lowest, or retrospectively by
reconstructing acquired data from specific ECG phase
Patient parameters
Breath hold
Ensure comfortable patientposition
Tell patient to stay still and give clear instructions
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MOTION ARTIFACT
Motion artifacts ordinary are associated with involuntary
movements of patient’s body:
heart beating and respiratory chest movements, vascular
pulsation movements, seizures type movements. Voluntary
movements include cases scanning small age (pediatric)
patients, patients with alcohol or drugs intoxication, patients
with various degree consciousness disorders, and
sometimes incompletely sedated patients.
Patient motion commonly can cause shading or streaking
artifacts in the reconstructed images.
CT scans axial projection ,In both panels presented artifacts
arising from the movement of the patient during the study
CT scans axial
projection: motion
artifacts cover the
hemorrhage in the
right frontal part of the
brain
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TRANSIENT INTERRUPTION OF CONTRAST
ARTIFACT (TIC)
(TIC) is a common flow artifact seen in CT pulmonary
angiography (CTPA) studies.
The contrast opacificiation of the pulmonary arteries is
suboptimal due to an increase in the flow of unopacified
blood from the inferior vena cava (IVC) to the right side of
the heart, often during deep inspiration
Although CT pulmonary angiography protocols vary between
radiology departments, a common protocol involves injecting
a contrast bolus in the veins of the upper extremity, followed
by a timed or bolus-triggered breathing instruction to suspend
breathing.
The instruction is intended to occur during a normal tidal
volume. If the patient instead takes in a deep breath before
holding it (i.e., inspiratory breath-hold, which is often employed
in routine chest CTs), the intrathoracic pressure suddenly
decreases.
The decreased intrathoracic pressure increases systemic
venous return, including of unopacified blood from the inferior
vena cava, to the right heart and subsequently pulmonary
arteries.
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SOLUTION (TIC)
The solution to transient interruption of contrast flow of the
pulmonary arteries is to reduce the volume of unopacified
blood entering the right atrium from the inferior vena cava.
Prescanning hyperventilation is likely the cause; with the
implementation of faster scanners, prescanhyperventilation
should be dropped.
Transverse CT pulmonary angiogram at the level of the right
pulmonary artery showing grade 1 transient interruption of
contrast (TIC)
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METALLIC ARTEFACTS
Highly dense metal objects in and around patients cause
significant image streaking Artifacts occur from combination
of beam hardening and partial volume effect
Special software correction along with correction of beam
hardening must be used to overcome this artefact
Metal based artifacts are observed in cases, when artificial
(mainly metal) objects are presented on/in scanning body or
scanning field. Mostly it can be various types of earrings, metal
clips, metal plates, dental braces, artificial heart or joint, dental
and other body site implants. The same we can say about
permanent make-up and tattoos, what is popular recently.
These objects evoke streaking like artifacts which occur
because the density of the metal is beyond the normal range
that can be computed by the software, resulting in an
incomplete attenuation profiles.
The metal produces a beam-hardening and
photon starvation artefact. This can also happen
with other high attenuation materials such as IV
contrast.
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METALLIC ARTEFACTS
CT scans axial projection. Visible bright metal artifacts
arising from the right ear (marked by arrows)
SOLUTIONS POSSIBLE:
Remove metal object
Decrease section width
Increase KVp for better penetrability
Adjust W / L settings
Reorient data acquisition (e.g. gantry tilt)
Metallic artifact reduction (MAR) software
CT scans axial projection metal artifact on the abdominal
wall projection, CT topogram with clearly visible origin of
artifact – navel ring, surrounded by a circle
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SPIRAL AND MULTISLICE SCANNING ARTEFACTS
Ring artefact
Helical artefact
-Cone beam artefact
multiplanar reconstruction (MPR) artifact
-zebra artifact
-stair step artifact
CONE BEAM ARTEFACTS
This is a particular artefact caused by multislice scanners. As the
section scanned increases per rotation, a wider collimation is
used.
Because of this the x-ray beam becomes cone-shaped instead
of fan-shaped and the area imaged by each detector as it
rotates around the patient is a volume instead of a flat plane.
The resulting artefact is similar to the partial volume artefact for
off-centre objects.
This is particularly pronounced at the edges of the image. With
modern scanners cone beam reconstruction algorithms correct
this artefact.
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Solution
Reconstruction algorithm minimises cone beam
artefacts
RING ARTEFACTS
CT phenomenon that occurs due to the miscalibration or
failure of one or more detector elements in a CT scanner.
Less often, it can be caused by insufficient radiation dose or
contrast material contamination of the detector cover 2.
They occur close to the isocenter of the scan and are
usually visible on multiple slices at the same location. They
are a common problem in cranial CT
Recalibration of the scanner will usually rectify the artifact.
Occasionally detector elements need replacing, which can
be costly. The referring clinician should be notified that the
concerning ring shadows are artifactual.
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RING ARTEFACTS
Ring artefacts happen when at least one of the detectors is
out of calibration on a spiral MDCT scanner or when one of
the detectors is out of place, so that the detector will give a
consistently erroneous reading at each angular position,
resulting in a circularartefact
Solution
Ring artifact is caused by a miscalibrated or defective
detector element, which results in rings centered on the
center of rotation. This can often be fixed by recalibrating
the detector.
Usually, recalibrating the detector is sufficient to fix this
artifact, although occasionally the detector itself needs to
be replaced.
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ZEBRA AND STARIR -STEP ARTEFACTS
In CT a zebra artifact from 3D reconstructions and a zebra
sign from hemorrhage in the cerebellar sulci, and
potentially-confusingly a zebra stripe sign in the bones of
those treated with cyclical bisphosphonates for
osteogenesis imperfecta
It therefore seems prudent to use "zebra" with a term like
"stripes" rather than "artifacts"
In multidetector row CT, the projection planes (defined by
the x-ray source and the detector row) are not exactly
parallel to the axial plane (except for the center detector
row). In the simplest 2D FBP reconstruction, the projection
planes for each detector row are assigned to the closest
axial plane based on where they intersect the center of
rotation. If there is a high-contrast edge in the z-direction
between the axial plane and the projection plane, this
creates streaks, as well as stair-step artifacts
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THANK YOU.
shendechetan21@gmail.com
chetan.shende@pacificafrica.com
Chetan Shende
+91 9158993366
