Week 9 - Artifacts Flashcards
Any appearance on the image that is not in the object scanned
Image artifact
What are the classifications of image artifacts?
-physics based
-patient based
-equipment induced
T/F
Artifacts degrade diagnostic value of CT images
True
What is the best way to identify motion?
Lung window
T/F
Individual rays are hardened differently
The degree of hardening is dependent on the composition of part examined and degree of travel through various tissues
True
Beam hardening
How is the beam hardened?
Low energy photons are preferentially absorbed in the body creating a harder beam
Beam is hardened more by dense objects (metal, dense bone…)
What are the two types of beam hardening artifacts?
-cupping artifacts (periphery of image is lighter)
-appearance of dark bands or streaks between dense objects in the image
What are some common beam hardening artifact corrections?
-AI filtration: filter out low energies
-additional filtration
-calibration correction
-correction software (can be included in algorithms)
-raising kvp
What is the best method for correcting beam hardening artifacts?
Select correct SFOV
(Small, medium, large)
Ex. Head = small
-occurs when one or more type of tissue is within a voxel
-occurs when dense object lies to the edge of the FOV
-inconsistency between views causes shading artifacts
Partial volume artifact
How is a partial volume artifact reduced?
By selecting thinner slices
Can occur when dense objects lie to the edge of the SFOV and are only present in some views used to create the image
Partial volume artifact
-undersampling caused inaccuracies
-shows stripes radiating from fine structures
-should be avoided for protocols needing high detail
*not enough views taken to produce detail needed
Aliasing
*causes moire pattern
How is aliasing avoided?
By reducing helical pitch and slowing gantry speed
(Ensuring all anatomy is scanned)
-results from pronounced differences in density of anatomical structures
-streak artifact appearance
Ex. Barium and air in GI tract
Edge gradient effect
How can the edge gradient effect be reduced?
-thinner slices
-low HU value barium
-appearance of shading, ghosting, streaking or blurring
Motion artifact
How can motion artifacts be reduced?
-overscan (pitch)
-partial scan modes
-software correction
-cardiac gating
-communication
-positioning aids
-short scan times
-streak artifact
-density of metal beyond any “normal” HU
Metallic artifacts
How are metallic artifacts reduced?
-increased kvp
-removing metals
-thin slices
T/F
Newer systems have HU detection of up to 4000 which reduces metallic artifacts
*beam hardening still contributes
True
-anatomy out of SFOV will attenuate and harden the beam
*but is ignored in the image reconstruction phase
-streaks and shading on image
Out of field artifacts
How are out of field artifacts reduced?
-selecting a SFOV larger than the patient
-not always possible
-raise patients arms up
-ring or concentric rings centered on the rotational axis
-caused by detector fault
-appear from a malfunctioning or miscalibrated detector element in a third generation scanner
Ring artifact
What reduces ring artifacts?
Recalibration
-undesired surge of electric current
-occurs with large differences in electrical potential
-caused by residual gas molecules in tube (xray output is affected)
-no specific appearance
Tube arching
T/F
Interpolation can result in artifacts
(Especially when changes of an atomic structures occur in the x axis
True
*helical and cone beam effect
-higher pitches shows this effect more prominently
-becomes more prominent with MDCT
Helical and cone beam effect
-appear as streaks, or bright and dark sharding near large differences in densities
-affect structures closer to the periphery of FOV
-more pronounced for outer detector rows
Cone beam effect
T/F
Tube arching causes a momentary loss of xray output
True
