1. CT ARTIFACTS

2. Artefact
Is defined as a feature appearing in an image that does not correspond to or represent an actual anatomical or pathological structure.
Classification based on:
Appearance: dot, streak like, ring like,
Cause of artefact

3. CLASSIFICATION OF CT ARTIFACTS :
Based on Origin :
Physics based artifacts : Result from physical processes involved in the acquisition of CT data: beam hardening, photon starvation, undersampling A.
Patient based artifacts : Caused factors related to patient-
motion A, metallic A, incomplete projection artefacts.
Scanner based artifacts : Result from imperfections in scanner function: undersampling A, ring A, Cone beam A,
Helical and Multisection artifacts : Produced by the image reconstruction process.
Based on Appearance : Streaking : Due to an inconsistency in a single measurement.
Shading : 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.

4. PHYSICS BASED ARTIFACTS :
Beam hardening and resultant artifacts.
Partial volume effect.
Photon starvation
Undersampling

5. BEAM HARDENING AND RESULTANT ARTIFACTS :
Cause : X ray beam is composed of individual photons with a wide range of energies.
Mechanism: As the beam passes through an object it becomes HARDER , because lower energy photons are absorbed more rapidly than higher energy photons. This means x-ray beam exiting the patient has lower no. of energy photons than beam when it entered the patient.
Types of artifacts :
Cupping artifacts
Appearance of dark bands / streaks between dense objects in the image.
Effect: CT values are less in center of patients.

6. BEAM HARDENING EFFECT : BUILT-IN MINIMIZING FEATURES :
Filtration :
A flat piece of attenuating material,usually metal, is used to pre-harden the beam by filtering out lower energy components before it passes through patient.
Bow-tie filter can also be used.
Calibration correction :
Scanners are calibrated using phantoms in a range of sizes.
Bem hrdening correction softwre.

7. BEAM HARDENING EFFECT : AVOIDANCE BY OPERATOR :
Using proper patient positioning and tilting of gantry.
Select appropriate FOV- if possible void dense object.
Using appropriate bow-tie filters.

8. VOLUME AVERAGING : Cause: variable density objects.
Not an artifact in itself but a Mechanism : limitation of image reconstruction.
Refers to the smallest unit of spatial resolution of a CT machine being a single voxel.
Thus all the components present within a voxel will influence its final attenuation coefficient or CT number.
Effect : Thus the appearance of an entire voxel may be adversely
affected due to the presence in it of a small part of a high or low
attenuation or object.

9. PARTIAL VOLUMING : AVOIDANCE :
Using a thin acquisition section width – especially in imaging a body part whose anatomy is rapidly changing such as the posterior fossa ,lung –diaphragm interface.

10. PHOTON STARVATION :
Occurs in highly attenuating areas such as the shoulders.
X ray beam travelling horizontally is greatly attenuated and insufficient no. of photons reach the detector.
Very noisy projections are produced at these tube angulations.
Reconstruction process has the effect of greatly magnifying the noise, resulting in horizontal streaks in the image.

11. PHOTON STARVATION : IN-BUILT MINIMIZING TECHNIQUES :
Automatic tube current modulation :
Tube current is automatically varied during the course of each rotation, this process is known as Milliamperage modulation.
This allows sufficient photons to pass through the widest parts of the pt. without unneccesary dose to narrower parts.

12. UNDERSAMPLING :
The number of projections used to reconstruct a CT image is one of the determining factors in image quality.
Too large interval between projections (ie undersampling ) results in misregisteration of information relating to sharp edges and small objects.
This leads to View ALIASING , where fine stripes appear to be radiating from the edge of, but at a distance from a dense structure.
Stripes appearing close to the structure are called RAY ALIASING and is caused by undersampling within a projection.
View aliasing
Ray aliasing

13. UNDERSAMPLING : AVOIDANCE :
Must be avoided where resolution of fine detail is important.
VIEW ALIASING – minimized by acquiring largest possible number of projections per rotation which may be done by a slow rotating speed.

14. PATIENT BASED ARTIFACTS :
Metal artifacts
Motion artifacts
Incomplete projection.

15. METAL ARTIFACTS :
Presence of metal objects in scan field leads to severe streaking artefacts.
They occur because the density of metal is beyond the normal range that can be handled by the computer, resulting in incomplete attenuation profiles.
Additional artefacts due to beam hardening, partial voluming and aliasing compound the problem.

16. METAL ARTIFACTS : AVOIDANCE :
Ask patients to remove all metal objects and jewellery before scanning.
For nonremovable items, it is sometimes possible using gantry angulation to exclude the metal inserts from scans of nearby anatomy.
When absolutely impossible to eliminate them,
Using high kVp may help penetrate some objects.
Using thin sections will reduce contribution due to partial volume artifacts.

17. METAL ARTIFACTS : IN-BUILT MINIMIZING TECHNIQUES :
Special software corrections apply interpolation techniques to substitute the over-range values in attenuation profiles.
Usefulness is limited because, although streaking distant from the metal implant is reduced, there still remains a loss of detail around the metal-tissue interface, which is often the main area of diagnostic interest.
Beam hardening correction software must also be applied when scanning metal objects to minimize additional artifacts.

18. MOTION ARTIFACTS :
Patient motion causes misregistration artifacts, which appear as shading/streaking in the image especially at high-low density interfaces.

19. MECHANISM OF MOTION ARTEFACT
Due to motion , structures at different positions when different views are taken ,so back projections are not added correctly.

20. MOTION ARTIFACTS : IN-BUILT MINIMIZING TECHNIQUES :
OVERSCAN AND UNDERSCAN MODES :
Maximum discrepancy in detector readings occurs between veiws obtained towards the begining and end of a 360 degree rotation.
In overscan mode an extra 10 degree is added to standard 360 degree rotation,which helps reduce artifacts.
Underscan mode can also reduce artifacts but at the expense of poorer resolution.

21. MOTION ARTIFACTS : IN-BUILT MINIMIZING TECHNIQUES :
Software correction :
Automatic applying of reduced weighting to the beginning and end views to suppress their contribution to the final image.
Additional specialized motion correction is also available.
Cardiac gating :
Images are produced by using data from just a fraction of the cardiac cycle when there is least cardiac motion.
Achieved by combining ECG gating techniques.
Types: prospective, retrospective.

22. MOTION ARTIFACTS : AVOIDANCE BY OPERATOR :
Use of positioning aids to prevent voluntary movement.
Sedation.
Shortest possible scan times.
Breath hold .

23. INCOMPLETE PROJECTION :
If any portion of the patient lies outside the scan FOV the computer will have incomplete information relating to this portion and streaking or shading artifacts will be generated.
This will also occur of object not to be scanned lies close to patient and interferes with x ray beam transmission.
Eg Pt scanned with arms down instead of being raised out of the way of scan for CT thorax.
Blocking of reference channels at the sides of detector array may also interfere with data normalization and cause streaking artefacts.

24. INCOMPLETE PROJECTION : AVOIDANCE :
Position the pt properly.
Some machines monitor the reference data channels for inconsistencies and avoid using reference data that appears suspicious.

25. SCANNER BASED ARTIFACT : RING ARTIFACT :
If one of the detectors is out of calibration on a third-generation (rotating x-ray tube and detector assembly) scanner, the detector will give a consistently erroneous reading at each angular position, resulting in a circular artifact .
AVOIDANCE : Detector recalibration
Selecting the correct FOV and using calibration data that fits more closely to pt anatomy.

26. 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 fanshaped.
As the number of rows of detectors increases in a multislice scanners , divergence of the cone beam along the z-axis increases
As the tube and detectors rotate around the patient, the data collected by each detector correspond to a volume contained between two cones, instead of the ideal flat image .
The artifacts are more pronounced for the outer detector rows than for the inner ones , where the data collected correspond more closely to a plane.

27. HELICAL ARTIFACTS IN SINGLE SECTION SCANNING :
In general, the same artifacts are seen in helical scanning as in sequential scanning. However, there are additional artifacts that can occur in helical scanning due to the helical interpolation and reconstruction process.
The artifacts occur when anatomic structures change rapidly in the z direction (eg, at the top of the skull) and are worse for higher pitches.
If a helical scan is performed of a cone-shaped phantom lying along the z axis of the scanner, the resultant axial images should appear circular.

28. HELICAL ARTIFACTS IN SINGLE SECTION SCANNING :
To keep helical artifacts to a minimum, steps must be taken to reduce the effects of variation along the z axis. This means using, where possible,
a low pitch.
a 180° rather than 360° helical interpolator if there is a choice
thin acquisition sections rather than thick.
Sometimes, it is still preferable to use axial rather than helical imaging to avoid helical artifacts (eg, in brain scanning)

29. HELICAL ARTIFACTS IN MULTISECTION SCANNING :
The helical interpolation process leads to a more complicated form of axial image distortion on multisection scanners than is seen on single-section scanners.
The typical windmill-like appearance of such artifacts is due to the fact that several rows of detectors intersect the plane of reconstruction during the course of each rotation. As helical pitch increases, the number of detector rows intersecting the image plane per rotation increases and the number of “vanes” in the windmill artifact increases.