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Imaging > CT Image Quality and Image Artifact > Flashcards

CT Image Quality and Image Artifact Flashcards

1
Q

what are some ways to look at CT image quality

A

CT number and accuracy, linearity, uniformity, high contrast spatial resolution, contrast, noise, CNR, low-contrast resolution, temporal resolution, artifacts

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2
Q

CT number is related to Kv

A

sometimes?

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3
Q

what is high contrast spatial resolution

A

scanner’s ability to resolve smallest features (high contrast), and measured in two orthogonal directions: in plane and cross plane

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4
Q

what are quantitative descriptors of spatial domain

A

point spread function and line spread function

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5
Q

what are quantitative descriptors of frequency domain

A

modulation transfer function

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6
Q

how do you determine in-plane spatial resolution in CT

A

focal spot size and shape, magnification factor, detector aperature size, reconstruction kernel, number of projections, and detector pitch

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7
Q

what is cross plane spatial resolution in the spacial domain

A

slice sensitivity profile and slice thickness

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8
Q

what is cross plane spatial resolution in the frequency domain

A

it is x modulation transfer function

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9
Q

what is the slice sensitivty profile

A

the line spreasd function along the z direction

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10
Q

how do you measure the SSP

A

a thin disk perpendicular to the z-axis, and small interval of image slices and it is in alignment with the x-y plane

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11
Q

how is CT number dtermined

A

it is based on the linerar attenuation coefficient relative to water. Cs= CTs-CTb and it is compared to water

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12
Q

how to find noise in the ct

A

the STD is the magintidie of the square root of the noise

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13
Q

what is the CNR in CT

A

a simple measurement of singal and noise and it is a useful metric to compare image quality when other factors are fixed

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14
Q

what is low contrast resolution

A

it is the smallest object that can be visulaized at a given contrast level

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15
Q

what is low contrast resolution dependent on

A

it is dependent on dose, contrast, and slice thickness, kernel and noise

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16
Q

what does a reconstruction kernel work on

A

reconstruction kernel involves the change of noise level, noise spatial correlation adn spatial resoliton of which affect the low contrast resolution

17
Q

what is temporal resoltion

A

the time span of the raw data used for reconstructing each image

18
Q

what does faster rotation time do to temporal resolution

A

it increases temporal resolitoon and this results in less motion artifacts overall

19
Q

what can be used to improve temporal resolution

A

a half-scan reconstruction is usually used in cardiac CT

20
Q

what is the calculation for temporal resolution with oen source and two source

A

one source is rotation time/2

two sources is rotation time/four

21
Q

what is aliasing artifact from

A

insufficient sampling, so there is not enough angualr sampling or detector sampling overall

22
Q

what is quarter detector offset

A

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

23
Q

what is a Z-flying focal spot

A

it nudges back and forth so that it can cover more area

24
Q

what are some noise induced artifacts

A

not enought photons in all projections so there is startvation or not enough photons in some of the projections

25
Q

what is photon starvation

A

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.

26
Q

what is the solution to photon starvation

A

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

27
Q

what are partial volume artifacts

A

object is smaller than voxel size so there is decreased subject contrast

28
Q

what are beam hardening artifacts

A

these are polychromatic xray beam and energy dependent attenuation, which means lower-energy photons have a higher chance to be absorbed

29
Q

what is the solution to beam hardening

A

it is water correction so you remap the projection data based on known water attenuation characteristics

30
Q

what are ring artifacts

A

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

31
Q

what can a cracked bow tie filter do

A

lead to a spot in the center of the picture

32
Q

what are helical artifacts from

A

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

33
Q

what are motion artifacts

A

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

34
Q

what can help provent motion artifacts

A

better patient management, breathign instruction and holding still, and improve temporal resolution and freeze motion, it can also increase scanning speed

35
Q

what are truncation artifacts

A

it is from part of the object being outside the scanning FOC so there is a bright ring on the outside

36
Q

what can happen if there are foreign objects in the scan

A

it can lead to straks in the head

37
Q

what are metal artifacts

A

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

38
Q

what can reduce metal artifact

A

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

Imaging (19 decks)

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