CT artifact

CT artifact

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

2. ARTIFACTS

3. ‘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 .

4. Beam hardening artifact

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

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

7. 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 .

8.   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  

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

10. 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.

11. 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.  

12. 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.

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

14. 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