CT artifact

1. CT ARTIFACTS
G.YOGANANTHEM
GUIDED BY PROF.DR.I.GURUBHARATH

2. ARTIFACTS

5. ‘beam hardening. X-ray photons coming from a tube are
made up of a full spectrum of X-ray energies, not just the
voltage that the tube is set at. The energy of a photon
determines its level of attenuation when passing through a
sample, with low-energy photons attenuating much faster
than high-energy ones. Therefore, when an X-ray beam
begins to penetrate a sample material, the lower energy X-
rays preferentially attenuate, resulting in an overall higher
energy of the X-ray beam - ‘beam hardening’.
The effect of beam hardening on a single material can
usually be managed by adjusting CT reconstruction
algorithms. However, if the sample contains multiple
materials within a single scan volume, imaging differing
densities simultaneously tends to result in artifacts.

6. Beam hardening artifact

10. Streak artifact from biopsy needle during CT-
guided abdominal biopsy

11. Shading from missing data in shadow of
biopsy needle.

18. 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.
Solutions
•Adaptive filtering: the regions in which the attenuation exceeds a
specified level are smoothed before undergoing backprojection.

19. 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

25. Ring artifacts resulting from defective detector element and
mis calibiration in 3rd generation single-slice scanner.

27. Ring artefact
Due to the failure of a particular detector, incorrect data in every projection will appear as a ring in the image.
Radius of the ring is determined by the position of the detector in the array and virtually disappeared in
contemporary CT units.

28. Cone beam artefact
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.

31. Tube arcing artifact is known to be caused by a temporary short circuit in the X-ray
tube causing momentary loss of X-ray output. It is seen as near-parallel and an
equidistant streak pattern on transaxial computed tomography (CT) images and as a
“horizontal” hypodense band on the coronal and sagittal CT images.

34. Movement artifact
The image on the left shows the result of movement during scanning.
The degraded image was repeated and no pathology was shown

39. Spiral and multislice scanning artefacts
Helical artefacts
In spiral scanning, as the gantry rotates it is also moving in the z-axis. This
means that a row of detectors is moving in a spiral path. This can cause
artefactual representation of structures that are changing in shape or position in
the z-axis as they will be in different positions for different projections used in the
reconstruction of the image. Nowadays this artefact is rare as scanners have a
large number of detectors and pitch <1.
Worsened by:
•Increasing pitch
•Increased contrast between object and surrounding structures