Artifact In CT
Artifact In CT
Artifact Origins
Physics Based
Beam hardening
Partial volume effect.
Photon starvation.
Under sampling.
Patient Based
Metal.
Patient motion
Inadequate scan field.
Scanner Based
Detector sensitivity
Spiral Artifacts in MSCT
3D and MPR artifacts:
Stair step artifacts.
Zebra artifact
Windmill artifact.
Cone beam artifact
Physics Based
Beam Hardening
Causes
The average energy of the X-ray beam becomes greater (Harder) as it passes through tissue.
Because the attenuation of bone is greater than tissue, bone causes more beam Hardening than soft tissue.
The beam hardening occurs artifacts in CT
because rays from some projection angle are hardened to a differing extent than rays from other angles, confusing the reconstruction algorithm.
Beam hardening cause broad dark or streaks artifacts.
(1)
Physics Based
Beam Hardening
Correction
Prefilter the X-ray beam near the focus ( bowtie)
Avoid highly absorbing bony regions by patient
positioning or gantry tilt.
Algorithms.
Beam hardening correction soft ware.
(2
Physics Based
Partial Volume Effect
Causes
Occurs where object does not fill scan
plain.
CT number is based on μ for tissue voxels.
If voxel non - uniform ( contains several materials)
detection process will average.
Partial volume effect cause band and streak artifacts.
Correction
Use thinner slices.
ct artefact
Physics Based
Photon Starvation
Causes
Parts of individual projections can be very noise due to insufficient photons passing through widest part of patient.
Photon starvation caused streak artifacts.
Correction
Increase of photon through widest parts without unnecessary doe through narrower parts.
Patient Based
Metal Artifacts
Causes
Caused by metallic materials e.g. Dental Fillings, Surgical Clips , Electrodes and Jewellery.
Metal artifacts caused streak artifacts.
Correction
Avoid metal parts.
Gantry angulation e.g. to avoid dental fillings.
Algorithm.
Patient Based
Patient Motion
Causes
Patient motion, Organ motion, Heart beating, Breathing and Swallowing.
Motion artifacts caused streaks and blurring artifacts.
Correction
Fast scanning.
Algorithm
Patient Based
Inadequate Scan Field
Causes
Patient scanned with armsdown.
Arms outside scan field.
Computer has incomplete information relating to arms.
Inadequate scan field caused streaks artifacts.
Scanner Based
Detector sensitivity
In third generation scanners, detector gain differences can cause circular artifacts to be present in the image.
1 or more bad detector.
Presence of circular artifacts indicates that the detector gain needs re-calibration.
3D & MPR Artifacts
Stair Step Artifacts
Causes
Large reconstruction interval.
Asymmetric helical interpolation.
Correction
Reduced with thinner slices.
Adaptive interpolation.
Spiral Artifacts
Zebra Artifact
Helical interpolation process gives rise tovariations in noise along the z-axis.
Noise in homogeneity increases with distance from axis of rotation.
This can show up as faint stripes in MIPS, which get more pronounced away from the centre.
Spiral Artifacts in MSCT
Windmill Artifact
Windmill artifacts are propeller like dark / bright structures in the vicinity of their sources, that seem to rotate around their center when scrolling through the stack of axial images
Windmill artifacts in axial images may lead to streak artifacts in reconstructions, e.g. horizontal streaks Coronal or Sagital MPR$.
Windmill Artifact
The angle between the vanes of the artifact equals the projection angle between detector rows intersecting the plain of reconstruction.
Use thicker slice to avoid the vanes.
Cone Beam Artifact
Cone artifacts typically appear in the periphery.
Cone beam depending on the number of detector rows.
Cone Beam Effect
As number of slices increases, beam is more diverging, outer slices are distorted.
Effect gets worse the larger the cone beam angle and the thinner the slice.
Negligible up to 8 slices, significant for 16 slice scanners.
Cone Beam Effect
Cone beam reconstruction reduced cone beam artifact.
Double Z-sampling technology improve the resolution.
The CT stair-step artifact is found in straight structures which are oriented obliquely with respect to movement of the table and appear around the edges of sagittal and coronal reformatted images when wide collimations and non-overlapping reconstruction intervals are used.
It is also seen in coronary CT angiography when step-wise reconstructions are from different cardiac phases. This is associated with heart rate variability and irregular heart rates.
Solution
This can be minimized by, using smaller collimation and overlapping reconstruction in helical imaging.
In coronary CT angiography, 256 and 320-detector CT scanners typically avoid this artifact. Some authors recommend beta-blockers to reduce stair-step artifact, others report limited results in achieving target heart rates with their use.
Ring artifacts are a CT phenomenon that occur due to miscalibration or failure of one or more detector elements in a CT scanner. They occur close to the isocentre of the scan and are usually visible on multiple slices at the same location. They are a common problem in cranial CT.
The remedy is usually simple, recalibrate the scanner and let the referrer know that the worrying-looking ring shadows are artifactual. Occasionally detector elements need replacement which can be costl
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