CT Image Quality and Image Artifact

Question 1

what are some ways to look at CT image quality

Answer 1

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

Question 2

CT number is related to Kv

Answer 2

sometimes?

Question 3

what is high contrast spatial resolution

Answer 3

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

Question 4

what are quantitative descriptors of spatial domain

Answer 4

point spread function and line spread function

Question 5

what are quantitative descriptors of frequency domain

Answer 5

modulation transfer function

Question 6

how do you determine in-plane spatial resolution in CT

Answer 6

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

Question 7

what is cross plane spatial resolution in the spacial domain

Answer 7

slice sensitivity profile and slice thickness

Question 8

what is cross plane spatial resolution in the frequency domain

Answer 8

it is x modulation transfer function

Question 9

what is the slice sensitivty profile

Answer 9

the line spreasd function along the z direction

Question 10

how do you measure the SSP

Answer 10

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

Question 11

how is CT number dtermined

Answer 11

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

Question 12

how to find noise in the ct

Answer 12

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

Question 13

what is the CNR in CT

Answer 13

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

Question 14

what is low contrast resolution

Answer 14

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

Question 15

what is low contrast resolution dependent on

Answer 15

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

Question 16

what does a reconstruction kernel work on

Answer 16

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

Question 17

what is temporal resoltion

Answer 17

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

Question 18

what does faster rotation time do to temporal resolution

Answer 18

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

Question 19

what can be used to improve temporal resolution

Answer 19

a half-scan reconstruction is usually used in cardiac CT

Question 20

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

Answer 20

one source is rotation time/2

two sources is rotation time/four

Question 21

what is aliasing artifact from

Answer 21

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

Question 22

what is quarter detector offset

Answer 22

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

Question 23

what is a Z-flying focal spot

Answer 23

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

Question 24

what are some noise induced artifacts

Answer 24

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

Question 25

what is photon starvation

Answer 25

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.

Question 26

what is the solution to photon starvation

Answer 26

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

Question 27

what are partial volume artifacts

Answer 27

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

Question 28

what are beam hardening artifacts

Answer 28

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

Question 29

what is the solution to beam hardening

Answer 29

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

Question 30

what are ring artifacts

Answer 30

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

Question 31

what can a cracked bow tie filter do

Answer 31

lead to a spot in the center of the picture

Question 32

what are helical artifacts from

Answer 32

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

Question 33

what are motion artifacts

Answer 33

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

Question 34

what can help provent motion artifacts

Answer 34

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

Question 35

what are truncation artifacts

Answer 35

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

Question 36

what can happen if there are foreign objects in the scan

Answer 36

it can lead to straks in the head

Question 37

what are metal artifacts

Answer 37

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

Question 38

what can reduce metal artifact

Answer 38

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