Computed Tomography- Exam 3 || 100% Correct Answers.
Factors controlling image appearance: correct answers -artifacts
-windowing
-noise
-resolution
Artifact correct answers - a distortion or error in an image that is unrelated to the subject being
studied
-CT image artifact is any discrepancy between the reconstructed CT numbers in the image and
true attenuation coefficients of the object
-artifacts degrade image quality and can cause misdiagnosis
-classified according to appearance & cause
Streak Artifact correct answers -appear as straight lines across the image
-caused by:
*improper sampling of data
*uneven x-ray beam
*noise
*patient motion
Ring or Band Artifacts correct answers -are produced when the projection readings of a
detector(s) is/are defective
-found in 3rd generation CT scanners
Shading Artifacts correct answers -appear near objects of high densities
-caused by beam hardening (same energy)
-differential absorption= tissue
*attenuates the x-ray beam differently
*atomic # of tissue (bone is higher)
*pathology
*thickness
Beam Hardening Artifact correct answers -creates an artifact because the beam is not uniform.
There is less beam hardening at the edges of an object than the center which is reflected by the
CT numbers assigned
-is material dependent. Dense bone and contrast agents result in dark shading artifacts
, -corrected by polynomial mapping
Patient Motion Artifact correct answers -voluntary or involuntary
-appears as streaks because the inability of the reconstruction algorithm to deal with the data
inconsistencies
-method to reduce motion
*immobilization
*proper breathing instructions
*short scan times
Metal Artifacts correct answers -are caused by the presence of the metal objects in the patient.
Ex. prosthetic devices, dental fillings, surgical clips
-appears as star shaped streaks due to the attenuation of the metal object and error in profile
information
-methods to reduce metal artifacts:
*removal of all external metal
*use of metal artifacts reduction (MAR) software program
Partial Volume Artifacts correct answers -appears as bands and streaks and is caused by spatial
volume averaging especially when the materials are significantly different. Ex. bone and soft
tissue
-partial volume averaging is when a voxel is given a CT number based on the average of the
different materials located within the voxel (3D pixel)
-corrected by thinner slices
Aliasing Artifacts correct answers -appear as streaks and are a result of insufficient projection or
view sampling
-to minimize aliasing, increase the number of views or ray samples per view
Noise Induced Artifacts correct answers -are a result of insufficient photons at the detector
caused by poor patient position & technique factors
-fewer photons result in more noise and weaker detector signal
-corrective action is to optimize patient position and technique factors and filtering algorithms
Scatter Shading Artifact correct answers -as the tissue volume exposed to radiation increases, the
scatter to primary signal increases
Factors controlling image appearance: correct answers -artifacts
-windowing
-noise
-resolution
Artifact correct answers - a distortion or error in an image that is unrelated to the subject being
studied
-CT image artifact is any discrepancy between the reconstructed CT numbers in the image and
true attenuation coefficients of the object
-artifacts degrade image quality and can cause misdiagnosis
-classified according to appearance & cause
Streak Artifact correct answers -appear as straight lines across the image
-caused by:
*improper sampling of data
*uneven x-ray beam
*noise
*patient motion
Ring or Band Artifacts correct answers -are produced when the projection readings of a
detector(s) is/are defective
-found in 3rd generation CT scanners
Shading Artifacts correct answers -appear near objects of high densities
-caused by beam hardening (same energy)
-differential absorption= tissue
*attenuates the x-ray beam differently
*atomic # of tissue (bone is higher)
*pathology
*thickness
Beam Hardening Artifact correct answers -creates an artifact because the beam is not uniform.
There is less beam hardening at the edges of an object than the center which is reflected by the
CT numbers assigned
-is material dependent. Dense bone and contrast agents result in dark shading artifacts
, -corrected by polynomial mapping
Patient Motion Artifact correct answers -voluntary or involuntary
-appears as streaks because the inability of the reconstruction algorithm to deal with the data
inconsistencies
-method to reduce motion
*immobilization
*proper breathing instructions
*short scan times
Metal Artifacts correct answers -are caused by the presence of the metal objects in the patient.
Ex. prosthetic devices, dental fillings, surgical clips
-appears as star shaped streaks due to the attenuation of the metal object and error in profile
information
-methods to reduce metal artifacts:
*removal of all external metal
*use of metal artifacts reduction (MAR) software program
Partial Volume Artifacts correct answers -appears as bands and streaks and is caused by spatial
volume averaging especially when the materials are significantly different. Ex. bone and soft
tissue
-partial volume averaging is when a voxel is given a CT number based on the average of the
different materials located within the voxel (3D pixel)
-corrected by thinner slices
Aliasing Artifacts correct answers -appear as streaks and are a result of insufficient projection or
view sampling
-to minimize aliasing, increase the number of views or ray samples per view
Noise Induced Artifacts correct answers -are a result of insufficient photons at the detector
caused by poor patient position & technique factors
-fewer photons result in more noise and weaker detector signal
-corrective action is to optimize patient position and technique factors and filtering algorithms
Scatter Shading Artifact correct answers -as the tissue volume exposed to radiation increases, the
scatter to primary signal increases
