Artefacts mr & ct

“A structure or substance not
normally present but produced by
an external agent or action”

Introduction
• There are numerous kinds of artifacts that can
occur in MRI.
• Some affect the quality of the MRI exam.
• Others may be confused with pathology.
• Some artifacts can be mitigated by the MR
Tech while others require an engineer.

Sources of Artifacts
• Hardware Issues e.g. calibration, power stability
• Software problems e.g. programming errors
• Physiological phenomena e.g. blood flow
• Physics limitations e.g. Gibbs and susceptibility

Classification
Patient Related Equipment Related

Patient Related
• Motion Artefacts
• Susceptibility Artefacts
• Metal Artefacts
• Chemical Shift Artefacts
• Black Line Artefacts
• Partial Volume Artefacts
• Zebra Stripes
• Moire Fringes
• Entry Slice Artefacts
• Magic Angle Artefacts

Motion Artefacts
• Motion results in two effects on MR images.
• View-to-view effects are caused by motion that occurs between the
acquisitions of successive phase-encoding steps.
• The reason for mainly affecting data sampling in the phase-encoding
direction is the significant difference in the time of acquisition in the
frequency and phase-encoding directions.
– Frequency-encoding sampling in all the rows of the matrix (128, 256 or 512)
takes place during a single echo (milliseconds).
– Phase-encoded sampling takes several seconds, or even minutes, owing to the
collection of all the k-space lines to enable Fourier analysis.
– Major physiological movements are of millisecond to seconds duration and
thus too slow to affect frequency-encoded sampling, but they have a
pronounced effect in the phase-encoding direction.
• Periodic physiologic motions that commonly result in ghosting artifacts
include cardiac motion, respiratory motion, vascular pulsation, and
cerebrospinal fluid (CSF) pulsation.

• Motion occurring between the time of radiofrequency (RF)
excitation and echo collection (i.e., within-view) results in a
lack of coherent phase among the population of moving
spins at the time of echo formation.
• This incoherence manifests as blurring and increased
image noise.
• Unlike phase errors encountered in view-to-view motion
effects, this within-view effect is expressed throughout the
image.
• It is most frequently associated with random motion, as can
occur with gastrointestinal peristalsis, swallowing,
coughing, eye motion, and gross patient movement.

Respiratory motion
• Respiratory motion
results in ghosting
artifacts and blurring
that can obscure or
simulate lesions.
Motion artefact (T1 coronal study of lumbar
vertebrae).

Respiratory motion - Remedy
1. Mechanical methods –
– Breath hold
– use of an abdominal or thoracic binder
– taking images with the patient in a prone position
– Disadv:
• patient discomfort counterproductive effects.

Motion-related artifacts.
(a) Abdominal image obtained without breath hold shows breathing-related motion artifacts.
(b) Abdominal image obtained with breath hold shows a minimal artifact due to respiration and
several motion artifacts due to cardiac pulsation.

2. Signal averaging
– use of multiple data acquisitions to improve the
signal-to-noise ratio (SNR) of the image.
– the prominence of ghosting artifacts is reduced by
approximately the square root of the number of
signal averages obtained.
– Disadv:
• imaging time increases linearly with the number of
signal averages;

The image on the left exhibits respiratory motion as blurring of the structures
as well as motion-induced ghosting. By using multiple averages motion can be
reduced in the same way that multiple averages increase the signal to noise ratio. The
image on the right was obtained with 16 averages.

3. Respiratory Triggering/Gating
– data are collected only during a limited portion of
the respiratory cycle, usually near end-expiration,
when respiratory movement is minimal.
– Drawback:
• marked prolongation of imaging time (because so much
of the time is not being used productively for data
acquisition)

4. Respiratory Ordered Phase Encoding (ROPE)
– involves monitoring the patient’s respiratory cycle
during imaging using a bellows device.
– Phase encoding steps are ordered on the basis of
the phase of the respiratory cycle.
– (Unlike respiratory triggering) data are collected in
a continuous fashion.
– does not restrict the operator’s selection of TR
– does not significantly extend imaging time;

Images acquired without and with compensation for respiratory motion.
(a, c) Images acquired without compensation show respiration-induced artifact.
(b, d) Images acquired with compensation are unaffected by respiratory motion.

5. Gradient moment nulling
– involves the application of additional gradient pulses
to correct for phase shifts among a population of
moving protons at the time of echo collection.
– This method corrects for constant-velocity motion and
helps reduce the signal loss and ghosting associated
with such movement.
– Disadv:
• Use of gradient moment nulling requires a nominal TE,
which may preclude its use on T1-weighted pulse sequences.
– Adv:
• This method does not prolong image acquisition time;

6. Real-time navigator echo gating
– an elegant non–breath-hold technique that can be
used to compensate for several different types of
motion.
– With this technique, an echo from the diaphragm is
obtained to determine the diaphragmatic position,
and the timing of the acquisition is adjusted so that
data are acquired only during a specific range of
diaphragmatic motion.
– The navigator echo part of the acquisition is
interleaved with the actual imaging sequence to
facilitate real-time monitoring.

Placement of the navigator section for respiratory motion compensation.
(a) Image with aqua overlay shows the navigator section from which the displacement
information is obtained to determine the diaphragmatic position.
(b) Graph shows diaphragmatic movement, indicated by the white wave and green line. The
yellow boxes represent the best time to image (“window of opportunity”).

7. Fat Suppression Methods
– With this method, not only subcutaneous fat but
also mediastinal, mesenteric, retroperitoneal, and
other stores of internal fat are suppressed and are
thus less capable of generating ghosting artifacts.

8. Choosing the shorter dimension of the matrix
as the phase-encoding direction, and
9. Swapping phase and frequency-encoding
directions
– Moves the artefact out of the field of interest.

Cardiac motion
• Cardiac motion produces a series of ghost
artifacts along the phase-encoding direction of
the image, in addition to blurring and signal
loss of cardiac and juxtacardiac structures.

Cardiac motion - Remedy
1. Electrocardiographic Triggering / Gating
– data collection is synchronized with cardiac phase.
– This synchronization enables cardiac tissue to be
located in a consistent position as each successive
phase-encoding step is acquired, resulting in
increased tissue signal intensity and decreased
phase errors.

The image on the left was acquired without any form of
motion compensation technique for cardiac motion.
The image on the right was obtained using cardiac gating.

Cardiac motion - Remedy
2. Fast Imaging Sequences
– reduce the opportunity for motion during data
acquisition
3. Gradient moment nulling, and
4. Spatial RF pre-saturation pulses.

Vascular Pulsation
• Vascular pulsation
artifacts are recognized
by their alignment with
the responsible vessel
along the phase-encoding
direction of
the image.

Vascular Pulsation
• These artifacts
reproduce the cross-sectional
size and shape
of the responsible
vessel, but not
necessarily its signal
intensity.

Vascular Pulsation - Remedy
1. Spatial RF presaturation pulses applied
outside the field of view help reduce the
signal intensity of inflowing blood and, hence
reduce the resultant pulsation artifact.
2. Positioning the section of interest in the
middle of a multisection acquisition, thus
reducing any potential entry phenomenon,
and maximizing the saturation of flowing
spins.

Control of flow-related artifacts.
(a) Image shows a cardiac pulsation artifact.
Images obtained with a saturation band superior to the imaging section (b) show less
severe flow-related artifacts.

Vascular Pulsation - Remedy
3. Flow Compensation or Gradient Moment Nulling.
• In this technique, flowing spins are rephased by using
motion-compensating gradient pulses.
• If no motion compensation is applied with the
gradients, the flowing spins are not in phase with the
static spins when the echo forms.
• With motion compensation, the flowing spins with
constant velocity (irrespective of what the velocity is)
are brought back into phase, with no effect on static
spins.
• The penalty for using motion compensation is
increased echo time.

Schematics show the MR signal effects of flow compensation with gradient moment nulling. Top: Typical
readout gradient waveform. Bottom: Phase of cumulative stationary spins (solid line) and constant-velocity
spins (dotted and dashed lines). (a) During imaging without flow compensation, the moving spins are not
refocused at the desired echo time (TE), and this leads to the loss of signal from flowing spins. (b) During
imaging with gradient moment nulling, all the spins are refocused, and flow velocity is compensated for by
the 1:2:1 ratio of the gradient lobe areas. GGMR gradient moment nulling gradient, GR readout gradient.

Control of flow-related artifacts.
(a) Image shows a cardiac pulsation artifact.
Images obtained with first-order motion compensation (c) show less severe flow-related
artifacts.

Susceptibility Artefacts
• Materials of different magnetic
susceptibility will concentrate or disperse
magnetic field
• At interface between tissue of different
susceptibility get:-
– signal dephasing (signal loss)
– Frequency (image) shift (signal gain or loss)
• Artefact occurs in both frequency and
phase encoding directions

Where susceptibility artefacts occur
• Large susceptibility artefact seen around
metal screws, clips and plates
• More subtle effect seen at natural interfaces:-
– iron content of haemoglobin (haemorrhage)
– trabecular bone,
– paranasal sinuses,
– skull base,
– sella.

Images of Susceptibility Artefacts

Reducing Susceptibility Artefacts
• Use spin echo sequences rather than gradient
echo (180o refocusing pulse cancels the
susceptibility gradient)

• Swap frequency and phase encoding direction
(changes artefact size and shape, perhaps
removing from area of interest): but this can
lead to aliasing

Echo-planar images show magnetic susceptibility artifacts.
(a) Left-right phase encoding causes severe distortion of the signal (arrows).
(b) Anterior-posterior phase encoding minimizes the distortion, since the phase axis is
symmetric around the susceptibility gradients.

• Minimise TE, so artefact has less time to build-up:
difficult if want T2 weighting

Metal artefacts
• More severe because most
metals have much higher
magnetic susceptibility than
does body tissue
• Complete signal loss
• Part of a section may
appear on images acquired
in a completely different
section because of the
frequency difference
between the spins in the
metal and those next to the
metal Sagittal MR image shows a magnetic
susceptibility artifact that resulted from the
presence of metallic dental fillings.

Reducing Metal Artefacts
• Orientating the long axis of an implant or device
parallel to the long axis of the external magnetic field
• Appropriate frequency encoding direction (since metal
artefacts are most pronounced in this direction)
• Using smaller voxel sizes
• Fast imaging sequences
• Increased readout bandwidth
• Avoiding gradient-echo imaging when metal is present.
• MARS (Metal Artefact Reduction Sequence)
– applies an additional gradient, along the slice select
gradient at the time the frequency encoding gradient is
applied.

Chemical shift artifacts
• At 1.5T the Larmor frequency of fat is approximately 220 Hz
lower than water.
• The shift in Larmor frequency between water protons and fat
protons is referred to as chemical shift.

Chemical Shift Artifacts
• Chemical shift artifacts produce high intensity and
low intensity bands near fat/water interfaces,
primarily in the frequency encode direction.
• Dark boundaries between regions are sometimes
called “contour artifacts”.

Williams, R. L. et al. Radiology 1999;212:151-154

Chemical shift at 7T: oil and water

CSA = . Nfreq/BWrec
– CSA is a chemical shift artifact
–  - frequency difference between fat and water,
– Nfreq - number of samples in the frequency
encoding direction
– BWrec - receiver bandwidth

How To Avoid Chemical Shift
• Increasing receiver bandwidth
• Low magnetic field strength,
• Decreasing voxel size
• More prominent on T2-weighted than on T1-
weighted images.
• Fat suppression methods often eliminate visible
artifacts, and
• Gradient reorientation can redirect chemical shift
artifacts to another portion of the image.

Black Line Artifact
• Since the difference in frequency between fat and water is about
220 Hz, the fat and water signals are in phase every 1/220 Hz.
• In other words, they are in phase at 4.45 msec, 8.9 msec, and so on
(these time points correspond to the first cycle, second cycle, and
later cycles).
• Within each cycle, the fat and water signals would be 180° out of
phase with one another at 2.23 msec, 6.69 msec, and so on.
• At the in-phase echo times, the water and fat signals are summed
within any pixel that contains a mixture of the two components,
whereas at opposed-phase echo times, the signal in these pixels is
cancelled out, leading to the appearance of a dark band at the fat-water
interface.

Black Line Artifact
• An artificially created
black line located at fat-water
interfaces such as
muscle-fat interfaces.

Black Line Artifact: Mitigation
• Use in-phase TE’s
• Fat suppression
• Increase bandwidth or matrix size.

In-phase versus
Opposed-phase Imaging
• Chemical shift can be used to advantage, as is
routinely done in liver imaging for the
detection of fatty infiltration.

(a) In-phase MR image acquired with an echo time of 2.2 msec.
(b) Opposed-phase MR image acquired with an echo time of 4.4
sec. Both images were acquired at 1.5 T.
In-phase and opposed-phase imaging sequences are sometimes also referred to as
CHEMICAL SHIFT SEQUENCES

Partial Volume Artifacts
• Partial volume artefacts arise from the size of the voxel
over which the signal is averaged.
• For example, if a small voxel contains only fat or water
signal, and a larger voxel might contain a combination of
the two, the large voxel possess a signal intensity equal to
the weighted average of the quantity of water and fat
present in the voxel.
• Volume averaging is most likely to occur in the slice-selection
direction of the image, which has the largest voxel
dimension.
• Volume averaging can simulate abnormalities, decrease the
visualization of low-contrast abnormalities, and blur or
distort affected structures

Partial Volume Artifacts - Remedy
• Smaller pixel size and/or
• Smaller slice thickness.
• Acquisition of additional two-dimensional images in
alternate imaging planes is very helpful for resolving
issues relating to partial volume averaging.
Retrospective reformatting of two-dimensional data
can also be performed using an interpolation algorithm
and can be of further assistance
• Three-dimensional Fourier transform imaging
– it provides thin sections with no intervening gaps and is
conductive to reformatting in alternate imaging planes

Zebra Stripes / Zero-fill
• Zebra stripes can be observed along the
periphery of gradient-echo images where
there is an abrupt transition in magnetization
at the air-tissue interface.
• They are accentuated by aliasing that results
from the use of a relatively small field of view.

Zebra Stripes - Remedy
• Expanding the field of view
• Using spin-echo pulse sequences
• Using oversampling techniques to reduce
aliasing.

Moire Fringes
• Moire fringes are an interference pattern most
commonly seen when doing gradient echo images.
• One cause is aliasing of one side of the body to the
other resulting in superimposition of signals of
different phases that add and cancel.
• Can also be caused by receiver picking up a
stimulated echo.
• Similar to the effect of looking though two window
screens.

Entry slice (Inflow) artifact
• Unsaturated spins in blood or CSF entering the
initial slices results in greater signal which
then reduces on subsequent slices.
• May be confused with thrombus.

How to avoid?
• The characteristic location
• If necessary, the use of gradient echo flow
techniques can be used to differentiate entry
slice artifacts from occlusions.
• Can use spatial saturation to reduce.
– Mechanism for TOF angiography.

Magic Angle Effects
• Seen most frequently in
tendons and ligaments
that are oriented at a
55o angle to the main
magnetic field.

Magic Angle Effects
• Normal dipolar
interactions between
the H+’s in water
molecule aligned in
tendons shortens T2,
causing loss of signal.
• The dipolar interactions
go to zero at about 55o
increasing the signal.

Equipment Related
Aliasing / Wrap Around Artefacts
Ringing Artefacts
Slice Overlap Artefacts
RF Overflow Artefacts
RF Noise
Zipper Artefacts
Cross Talk Artefacts
Shading Artefacts
Central Point Artefacts
Quadrature Ghost
Field Inhomogenity Artefacts
Surface Coil Artefacts
Eddy Current Artefacts
Gadolinium Pseudolayering

Aliasing or Wrap Around Artefact
• The selected field of view is smaller than the size of
the imaged object.
• when the anatomy exceeds the field of view

Wrap-around artifact
The anatomy is usually displaced to the opposite side of the image

Aliasing can occur in the frequency encode
direction or the phase encode direction.
Note, 3D techniques can also get aliasing artifact in slice direction, since
slice selection achieved using same technique as phase encoding

Wrap Around: Phase Encoding
Phase Encoding Phase Encoding
FOV

How to Eliminate Aliasing/Wrap
• Extend the FOV to include the whole anatomy: but
reduces resolution
Phase encoding
• Swap frequency and phase encoding direction if
anatomy thinner in original frequency encoding
direction : motion artefacts in phase encoding
direction
Phase encoding

Aliasing artifact caused by imaging with too small a field of view.
(a) Image shows aliasing artifact.
(b) Image obtained with the phase encoding and frequency
encoding axes exchanged shows no aliasing.

• Apply phase oversampling, increase phase encoding
outside field of view and ‘throw away’ the data
acquired.
• Penalty:
– scanning time increases
Phase oversampling
Phase encoding

• Apply spatial saturation pulse, (flip and completely dephase
with gradient) so no signal is produced from the area
which will wrap
• Penalities:
– SAR increased slightly
Spatial saturation pulse
Phase encoding

• Surface coils do NOT pick up signal beyond the edges
• So a smaller surface coil is a possible way of
eliminating aliasing/wrap
Surface Coil
Phase encoding

Gibbs Phenomenon (Ringing Artefact)
• Multiple parallel lines near high contrast interfaces
(ie narrow black or white line)
• Due to under-sampling of data (128/256 cf infinite
matrix size)
• Usually in phase encoding direction, but can occur in
frequency encoding
• Matrix size in phase encoding usually less than
frequency encoding, so artefact in phase encoding
direction

Solution to Truncation Artefact
• Reduce the voxel size (more sampling) by
– larger higher resolution matrix (disadv: time
penalty)
– smaller FOV (disadv: wrap around)
• filtering the K space data prior to Fourier
transform
• Gegenbauer reconstruction
• Bayesian approach

Slice-overlap (Cross-slice) Artifacts
• Loss of signal seen in an image from a multi-angle,
multi-slice acquisition.

Example: Two groups of non-parallel slices in the same sequence, e.g.,
L4-5 and L5-S1. The level acquired second will include spins that have
already been saturated.

Slice-overlap (Cross-slice) Artifacts
• Loss of signal seen in an image from a multi-angle,
multi-slice acquisition.
• Same mechanism as spatial presaturation for
reduction of motion and flow artifacts.

Slice-overlap Artifacts: Correction
• Overlap of sections within areas of diagnostic
interest should be carefully avoided.
• Use separate acquisitions.
• Avoid steep change in angle between slice
groups.
• Use small flip angle, i.e. GE sequence

RF Overflow Artifacts (Clipping)
• Causes a non-uniform, washed-out
appearance to an image.
• Occurs when the signal received from the
amplifier exceeds the dynamic range of the
analog-to-digital converter causing clipping.

RF Overflow Artifacts: Remedy
• Autoprescanning usually adjusts the receiver
gain to prevent this from occurring.
• If the artifact still occurs, the receiver gain can
be decreased manually.

RF noise
• RF pulses and precessional frequencies of MRI
instruments occupy the same frequency
bandwidth as common sources such as TV,
radio, fluorescent lights and computers.
• Stray RF signals can cause various artefacts.

RF noise - Remedy
• Eliminate stray RF interference
– Appropriate site planning
– Proper installation and
– RF shielding (faraday cage)

Zipper Artifacts
• The zipper artifacts that can be controlled
easily are those due to RF entering the
scanning room when the door is open during
acquisition of images.
• Broad-band noise degrades the entire image,
whereas narrow frequency noise produces
linear bands that transverse the phase-encoding
direction of the image.

Zipper Artifacts
Zipper artifacts from RF entering room are oriented perpendicular to the
frequency direction.

Solutions
• Identifying and removing external RF sources
• Ensuring that the door to the imaging room
remains closed, and
• Verifying the integrity of the magnet room
enclosure and associated seals.

Cross-talk Artifact
• Result of imperfect slice excitation, (i.e. non-rectangular),
of adjacent slices causing
reduction in signal over entire image.

Solution
• May be reduced by using gap, interleaving
slices and optimized (but longer) RF pulses.

Solution
• optimized RF pulses that have a more
rectangular slice profile can be implemented

Shading
• Manifest as foci of relatively reduced signal
intensity involving a portion of the image.
• Abnormalities contained in the shaded
portion of the MR image may be obscured.
• There are many potential causes for this
artifact, including partial volume averaging
malfunction of the RF transmitter, amplifier, or
receiver, excessive RF absorption, etc.

Shading: Remedy
• To minimize shading artifacts, the anatomy of
interest should be centered within the
magnet, within the coil, and within the group
of sections to be acquired.

Central Point Artifact
• A focal dot of increased or decreased signal in
the center of an image.
• Caused by a constant offset of the DC voltage
in the amplifiers.

Central Point Artifact: Correction
• Requires recalibration by engineer
• Maintain a constant temperature in
equipment room for amplifiers.

Quadrature ghost artifact
• Another amplifier artifact caused by
unbalanced gain in the two channels of a
quadrature coil.
• Combining two signals of different intensity
causes some frequencies to become less than
zero causing 180 degree “ghost.”

Quadrature ghost artifact
• This is the result of a hardware failure and
must be addressed by a service
representative.

Field inhomogeneity
• Types:
– Main magnetic field (B0)
– Gradient field artefacts (B1) inhomogeneity
– RF coil inhomogeneity
– Dielectric effects – worst at 3T+
• May cause variation in intensity across image
• May cause non-uniform fat suppression

External magnetic field (B0)
inhomogeneity
• Inhomogeneous
external magnetic field
causes distortions
– Intensity distortion
– Spatial distortion

Gradient field artefacts (B1)
inhomogeneity
• Any deviation in the gradient would be represented as
a distortion.
• Ideally the phase gradient should be assigned to the
smaller dimension of the object and the frequency
gradient to the larger dimension.
• Correction:
– reducing the field of view, by lowering the gradient field
strength or by decreasing the frequency bandwidth of
radio signal.
• If correction is not achieved, the cause might be either
a damaged gradient coil or an abnormal current
passing through the gradient coil.

RF inhomogeneity
• Due to the failure of the
RF coil
• non-uniform B1 field,
• non-uniform sensitivity
of the receive only coil
(spaces between wire in
the coil, uneven
distribution of wire)
Field inhomogeneity- RF coil

Dielectric effects
• Presence of non-ferromagnetic material in the
imaged object

Field inhomogeneity: Mitigation
• Shimming, area of interest in near isocenter
• Use STIR for Fat sat vs. Chess. Caution with
Gad.
• Coil – Use volume vs. surface coil, allow
space between coil and body.
• Dielectric – use phased array coils, software
compensation

Surface coil artefacts
(attenuation of signal)
• Close to the surface coil, the
signals are very strong
resulting in a very intense
image signal.
• Further from the coil the
signal strength drops rapidly
due to the attenuation with
a loss of image brightness
and significant shading to
the uniformity.
Surface coil artefact: high signal at anterior
thoracic wall adjacent to surface coil.

Eddy Current Artifacts
• Varying magnetic field
from gradients can induce
electrical currents in
conductors such as the
cryostat causing
distortion of the gradient
waveforms.
• Particularly a problem
with echo-planar imaging
that uses strong, rapidly
changing gradients.

Eddy Current Artifacts: Mitigation
• Precompensation- A “distorted” gradient
waveform is used which corrects to normal
with the eddy current effects.
• Shielded gradients – Active shielding coils
between gradient coils and main gradients.

Gadolinium “Pseudolayering”
• Three density layers in the bladder after Gd
– Low conc. Gd top layer = dark
– Med conc. Gd middle layer = bright
– High conc. Gd lower layer near ureters = dark
• T2 shortening overshadows normal T1 effects
at high concentrations.

Gadolinium “Pseudolayering”
Low conc. Gd top layer = dark
Med conc. Gd middle layer = bright
High conc. Gd lower layer near ureters = dark

artifact
• Any systematic discrepancy between the CT
numbers in the reconstructed image and the
true attenuation coefficients of the object.
• Inherently more prone to artifacts than
conventional radiographs because the image
is reconstructed from something on the order
of a million independent detector
measurements.

Types of Artifact
• Streaking: Due to an inconsistency in a single
measurement.
• Shadowing: Due to a group of channels or
views deviating gradually from the true
measurement.
• Rings: Due to errors in individual detector
calibration.
• Distortion: Due to helical reconstruction.

Classification of Artefacts
• Physics based artifacts: Results from the physical
processes involved in the acquisition of CT data.
• Patient based artifacts: Caused by patient
movement / presence of metallic materials in or
on the patient.
• Scanner based artifacts: Results from
imperfection in scanners function.
• Helical & multisection artifacts: Produced by the
image reconstruction process.

Classification of artefacts
• Physics based artifacts
– Beam hardening artifacts.
– Partial volume artifacts.
– Phantom starvation.
• Patient based artifacts
• Scanner based artifacts
• Helical & multi-section artifacts.

Beam hardening artifact:
• X-ray beam is composed of individual photons
with a range of energies.
• As the beam passes through the object, it
becomes “harder”.

Beam hardening
• The mean energy
increases.
• The lower energy
photons are absorbed
more rapidly than the
higher energy photons
and it becomes harder.

Two types of artifacts can result from this effect.
1. Cupping artifact.
2. Appearance of dark bands / streaks between dense
objects in the image.

Cupping artifact
• X-rays passing through the
middle portion of a uniform
cylinder phantom are
hardened more than those
passing through the edges
because they are passing
through more material.
• As the beam becomes
harder, the rate at which it
is attenuated decreases and
the resultant attenuation
profile differs from the ideal
profile.

Streaks and Dark Bands
• In very heterogeneous cross sections, dark bands or
streaks appear between two dense objects in an
image.
• Occurs because portion of the beam that passes
through one of the objects at certain tube portions is
hardened less than when it passes through both
objects at other tube positions.

Streaks and Dark Bands
• Can occur in
– bony regions of the body
– where contrast medium
has been used.

Built-in Features for Minimizing Beam
Hardening
• Filtration
• Calibration correction
• Beam hardening correction software

Hardening
Filtration
• A flat piece of attenuating
metallic material is used to
pre-harden the beam by
filtering out the low energy
components before it
passes through the thinner
parts of the patient.
• An additional bowtie filter
further hardens the edges
of the beam, which will pass
through the thinner parts of
the patient.

Hardening
Beam hardening
correction software
• An iterative correction
algorithm is applied
when images of bony
regions are being
reconstructed.

Hardening
Beam hardening
correction software
• minimize blurring of the
bone-soft tissue
interface in brain scans
• reduces the appearance
of dark bands in
nonhomogeneous
cross section.

Avoidance of Beam Hardening by the
Operator
• It is sometimes possible to avoid scanning
bony regions, either by means of patient
positioning or by tilting the gantry.
• It is important to select the appropriate scan
field of view to ensure that the scanner uses
the correct calibration and beam hardening
correction data and, on some systems, the
appropriate bowtie filter.

Physics based artifacts
• Beam hardening artifacts.
• Partial volume artifacts.
• Phantom starvation.

Partial volume artifact
• Arise when voxel contain
many types of tissues.
• It produces CT numbers
as an average of all types
of tissues.
• It will appear as bands or
streaks.
• Posterior cranial fossa is
the most critical region to
produce partial volume
artifact.

It can be best avoided by using a thin
acquisition section width.

Physics based artifacts
• Beam hardening artifacts.
• Partial volume artifacts
• Phantom starvation
• Undersampling

Photon Starvation
• Potential source of serious streaking artifacts.
• Can occur in highly attenuating areas – shoulders.
• When x-ray beam is traveling horizontally, the
attenuation is greatest and insufficient photons
reach the detector & very noisy projections are
produced at these tube angulations.
• The reconstruction has the effect of magnifying
the noise resulting in horizontal streaks in the
image.

Photon Starvation - Solution
• Increase the tube current.
– Penalty: Unnecessary dose exposure to patient

Built-in Features for Minimizing
Photon Starvation
Automatic Tube Current
Modulation
• On some scanner models,
the tube current is
automatically varied during
the course of each rotation,
a process known as
milliamperage modulation.
• This allows sufficient
photons to pass through the
widest parts of the patient
without unnecessary dose
to the narrower parts

Photon Starvation
Adaptive Filtration
• Some manufacturers use
a type of adaptive
filtration to reduce the
streaking in photon-starved
images.
• This software correction
smooths the attenuation
profile in areas of high
attenuation before the
image is reconstructed

Original axial CT images (top) and coronal reformatted images
(bottom) in their original form (a) and after reconstruction with
multidimensional adaptive filtration (b)

Undersampling
• The number of projections
used to reconstruct a CT image
is one of the determining
factors in image quality.
• Too large an interval between
projections (undersampling)
can result in misregistration by
the computer of information
relating to sharp edges and
small objects.
• This leads to an effect known
as view aliasing, where fine
stripes appear to be radiating
from the edge of, but at a
distance from, a dense
structure.

Undersampling
• Stripes appearing
close to the
structure are more
likely to be caused
by undersampling
within a projection,
which is known as
ray aliasing.

Undersampling - Remedy
• View aliasing can be
minimized by acquiring
the largest possible
number of projections
per rotation.
• Ray aliasing can be
reduced by using
specialized high-resolution
techniques,
– quarter- detector shift or
– flying focal spot
• increase the number of
samples within a
projection.

– Metal artefacts
– Motion artefacts
– Incomplete projections
• Helical & multisection artifacts.

Metallic materials
• Presence of metal object in the scan field can
lead to streaking artifacts because the density
of the metal is beyond the normal range that
can be handled by the computer, resulting in
incomplete attenuation profiles.

METAL ARTIFACT
• Manifest itself as “star
streaking” artifact.
• It’s caused by presence
of metallic objects
inside or outside the
patient.
• Metallic object absorbs
the photons causing an
incomplete attenuation
profile

Avoidance of Metal Artifacts by the
Operator
• Take off removable
metal objects before
scanning commences
• Gantry angulation to
exclude the metal
inserts from scans
• Increasing technique,
especially kilovoltage

Software Corrections for Metal
Artifacts
• Interpolation techniques
– substitute the over range values in attenuation profiles
– loss of detail around the metal-tissue interface

Patient based artifacts
• Metal artefacts
• Motion artefacts
• Incomplete projections

MOTION ARTIFACT CAUSED BY:
• VOLUNTARY MOTION
• INVOLUNTARY MOTION
IT PRODUCES “GHOSTING” EFFECT
CT IMAGE APPEARS– AS IF COMPOSED OF
SUPERIMPOSED IMAGES

Avoidance of Motion Artifacts by the
Operator
• Positioning aids - prevent voluntary
movement in most patients.
• Sedation - to immobilize the patient
(eg,pediatric patients)
• Short scan time
• Breath hold

Motion Artifacts
• Overscan and underscan modes
• Software correction
• Cardiac gating.

Motion Artifacts
Overscan and Underscan Modes
• The maximum discrepancy in detector readings occurs
between views obtained toward the beginning and end
of a 360° scan.
• Some scanner models use overscan mode for axial
body scans, whereby an extra 10% or so is added to the
standard 360° rotation. The repeated projections are
averaged, which helps reduce the severity of motion
artifacts.
• The use of partial scan mode can also reduce motion
artifacts, but this may be at the expense of poorer
resolution.

Motion Artifacts
Software correction
• Automatically apply reduced weighting to the beginning
and end views to suppress their contribution to the final
image.
• More noise in the vertical direction of the resultant
image

Motion Artifacts
Cardiac gating
• Using data from just a fraction of the cardiac
cycle, when there is least cardiac motion.
• This is achieved by combining
electrocardiographic gating techniques with
specialized methods of image reconstruction

Incomplete Projections
• If any portion of the
patient lies outside the
scan field of view, the
computer will have
incomplete information
relating to this portion
and streaking or shading
artifacts are likely to be
generated.
– Eg: arms down by the sides
– IV contrast tube outside
the scan field

Avoid artifacts due to incomplete
projections?
• Selection of larger SFOV
• Taping patient tissue
• Raising patients arms
above their head on the
scan of chest and
abdomen
• radiation therapy
planning
– wider bores and larger
scan fields of view
– permit greater versatility in
patient positioning

– Ring artifacts
– Air-contrast interface
– Tube arcing
– Line in topogram
• Helical & multisection artifacts.

Ring artifacts
Ring artifacts
• If one of the detector is
out of calibration on a
third generation
scanner (rotating x-ray
tube and detector
assembly), the detector
will record incorrect
data in each angular
position, resulting in
circular artifact.

Ring artifacts
Ring artifacts
• This misinformation is
reconstructed as a ring in
the image, with the
radius of the ring
determined by the
position of the faulty
detector in the detector
array.
• Scanners with solid state
detectors – more prone

Avoidance of Ring Artefacts
• Detector calibration
• Detector replacement
• Selecting the correct scan field of view
• Software corrections

Air-Contrast Interface
• STREAKING

Air-Contrast Interface - Remedy
• Change the patient position

Tube Arcing
• Tungsten vapor from
anode and cathode
intercepts the projectile
electrons intended for
collisions with the
target.
• Crackling sound!!!!

Tube Arcing -Remedy
• GAS BURNOFF
• TUBE REPLACEMENT

Line In Topogram
• Bad detector causes
continuous line on the
topogram

Line In Topogram - REMEDY
• Detector replacement

• Helical & multi-section artifacts.
– Helical Artefacts in Single-Section Scanning
– Helical Artefacts in Multi-section Scanning
– Cone Beam Effect
– Multiplanar and Three dimensional Reformation
•

Helical Artefacts in Single-Section
Scanning
• The additional artifacts that can occur in
helical CT are due to helical interpolation &
reconstruction process.
• The artifacts occur when anatomic structures
change rapidly in the z direction (at the top of
the skull).
• Worse for higher pitches.

Consecutive axial CT images from a helical scan of a
cone-shaped phantom lying along the scanner axis.

Series of CT images from a helical scan of the
abdomen shows helical artifacts (arrows).

Solutions
• Low pitch
• 180° rather than 360° helical interpolator
• Thin acquisition sections rather than thick.
• Use axial rather than helical imaging to avoid
helical artifacts (eg, in brain scanning).

Helical and Multisection CT Artifacts
• Helical Artefacts in Single-Section Scanning
• Helical Artefacts in Multi-section Scanning
• Cone Beam Effect
• Multiplanar and Three dimensional
Reformation

Helical Artefacts in Multi-section
Scanning
• More complicated form
of axial image distortion
• “Windmill-like”
appearance of artefacts

Solutions
• Z-filter helical interpolators
– Reduce the severity of windmill artefacts
• Non-integer pitch values relative to detector
acquisition width

Cone beam effect
• As the number of sections acquired per rotation increases,
a wider collimation is required and the x-ray beam
becomes cone shaped rather than fan shaped.

Cone beam effect
• As the tube and detectors
rotate around the patient,
the data collected by each
detector correspond to a
volume contained b/n
two cones, instead of the
ideal flat plane.
• This leads to artifacts
similar to those caused by
partial volume around
off-axis objects.

Cone beam effect
• The artifacts are more pronounced for the outer
detector rows than the inner ones.
• Cone beam effect get worse for increasing number
of detector rows.

Cone Beam Reconstruction
CT images of a phantom, obtained by using four-section acquisition and standard reconstruction
(a), 16-section acquisition and standard reconstruction (b), and 16- section acquisition and cone
beam reconstruction (c).

Multiplanar and Three dimensional
Reformation
Stair step artifacts
• Appear around the
edges of structures
• Wide collimations and
non-overlapping
reconstruction intervals
• Less severe with helical
scanning

Multiplanar and Three dimensional
Reformation
Stair step artifacts - Remedy
• Thin slice use
• 50% overlap on recon slice incrementation

Conclusion
• Artifacts originate from a range of sources and
can degrade the quality of an image to varying
degrees.
• Design features incorporated into modern
scanners minimize some types of artifact, and
some can be partially corrected by the scanner
software.
• However, there are many instances where careful
patient positioning and the optimum selection of
scan parameters are the most important factors
in avoiding image artifacts.

Artefacts mr & ct

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