CT Image Quality and Image Artifact Flashcards
what are some ways to look at CT image quality
CT number and accuracy, linearity, uniformity, high contrast spatial resolution, contrast, noise, CNR, low-contrast resolution, temporal resolution, artifacts
CT number is related to Kv
sometimes?
what is high contrast spatial resolution
scanner’s ability to resolve smallest features (high contrast), and measured in two orthogonal directions: in plane and cross plane
what are quantitative descriptors of spatial domain
point spread function and line spread function
what are quantitative descriptors of frequency domain
modulation transfer function
how do you determine in-plane spatial resolution in CT
focal spot size and shape, magnification factor, detector aperature size, reconstruction kernel, number of projections, and detector pitch
what is cross plane spatial resolution in the spacial domain
slice sensitivity profile and slice thickness
what is cross plane spatial resolution in the frequency domain
it is x modulation transfer function
what is the slice sensitivty profile
the line spreasd function along the z direction
how do you measure the SSP
a thin disk perpendicular to the z-axis, and small interval of image slices and it is in alignment with the x-y plane
how is CT number dtermined
it is based on the linerar attenuation coefficient relative to water. Cs= CTs-CTb and it is compared to water
how to find noise in the ct
the STD is the magintidie of the square root of the noise
what is the CNR in CT
a simple measurement of singal and noise and it is a useful metric to compare image quality when other factors are fixed
what is low contrast resolution
it is the smallest object that can be visulaized at a given contrast level
what is low contrast resolution dependent on
it is dependent on dose, contrast, and slice thickness, kernel and noise
what does a reconstruction kernel work on
reconstruction kernel involves the change of noise level, noise spatial correlation adn spatial resoliton of which affect the low contrast resolution
what is temporal resoltion
the time span of the raw data used for reconstructing each image
what does faster rotation time do to temporal resolution
it increases temporal resolitoon and this results in less motion artifacts overall
what can be used to improve temporal resolution
a half-scan reconstruction is usually used in cardiac CT
what is the calculation for temporal resolution with oen source and two source
one source is rotation time/2
two sources is rotation time/four
what is aliasing artifact from
insufficient sampling, so there is not enough angualr sampling or detector sampling overall
what is quarter detector offset
it is detector center is offset by a quarter of the detector cell width with respect to the isocenter. After 180 degrees rotation, the sample rays will itnerleave the previous sample rays, and this means double sampling can be achieved
what is a Z-flying focal spot
it nudges back and forth so that it can cover more area
what are some noise induced artifacts
not enought photons in all projections so there is startvation or not enough photons in some of the projections
what is photon starvation
it is fark shadows, rings and nisty- not enough phtons in all projections, the patient is too big, and the technique is too low so electronic noise dominates.
what is the solution to photon starvation
it is not enough ohotons in some projections so there are streaks along those projections. A solition to this is adapt tube output based on attenuation (AEC) or projection based on adapter filter
what are partial volume artifacts
object is smaller than voxel size so there is decreased subject contrast
what are beam hardening artifacts
these are polychromatic xray beam and energy dependent attenuation, which means lower-energy photons have a higher chance to be absorbed
what is the solution to beam hardening
it is water correction so you remap the projection data based on known water attenuation characteristics
what are ring artifacts
in 3rd generation CT, each detector 9ray sum) maps out a ring in the final imageThe diameter of the ring is related to the sample position along the detector array
what can a cracked bow tie filter do
lead to a spot in the center of the picture
what are helical artifacts from
it is caused by helcial sampling pattern and approxiamate reconstruction methods and increase with higher helical pitch, more obvious for high contrast objects with large variations along the z direction which are called windmill artifacts
what are motion artifacts
CT is based on teh obnject being still, but if the oject moves, then different projections yield conflicting information. Artifacts result from reconstruction algorithms attempt to reconcile conflicting data
what can help provent motion artifacts
better patient management, breathign instruction and holding still, and improve temporal resolution and freeze motion, it can also increase scanning speed
what are truncation artifacts
it is from part of the object being outside the scanning FOC so there is a bright ring on the outside
what can happen if there are foreign objects in the scan
it can lead to straks in the head
what are metal artifacts
dark bright streaks and dark shadows from inconsistency between the ideal model assumed by reconstruction and the actual CT signal contaminated by metal. Photon starvation, beam hardening, scatter, and non linear partial volume effect
what can reduce metal artifact
more mas and reduce noise, higher kVp reduce noise and beam hardening improvement on image quality and very limited. You an also use correction of projection data before reconstruction.
- adaptive filtering- smooth filer on data with photon starvation
- remove the metal projection by segmentation and interpolation
