scan data to image formation

Question 1

define spatial resolution

Answer 1

ability to see/resolve high contrast spatial detail without blurring

Question 2

what is limiting resolution

Answer 2

the smaller object size resolved in mm

Question 3

if you are looking for something broad or bright, it unlikely you need high does so decrease dosage

Question 4

fundamental limitation of spatial resolution depends on what 2 things

Answer 4

• focal spot size
• detector element size
  (both of these relate to the general term of aperture size)

Question 5

defence of resolution on scanner design relies of aperture size and what other aspect

Answer 5

sampling frequency
- number of samples per projection
- number of different projections

Question 6

dependence of resolution on scanner designs relies overall on these 2 things:

• aperture sizer
• sampling frequency

Question 7

how does number of samples per projection and number of different projections affect image quality

Answer 7

low sample per projection (low dose per projection) increases noise

increasing number of different projection decreases noise/ makes a clearer image

Question 8

the scanner can have 3 special features for improving spatial resolution, what are theyu

Answer 8

• quarter-detector shift
• flying focal spot
• attenuating comb

Question 9

how does the flying focal spot work to improve spatial resolution

Answer 9

• focal spot position on anode rapidly varied, halving the spacing between samples

Question 10

how does the quarter detector shift work to improve spatial resolution

Answer 10

• centre of detector is offset by 1/4 of an element width from isocentre
• opposite vies are offset by half sample width
• overall doubles number of different projections acquired

(can be commingled with flying focal spot (1/8) detector shift)

Question 11

the less gaps between samples the better

Question 12

how is an attenuating comb work to reduce spatial resolution

Answer 12

• attenuating comb moves in front of detector to reduce effective detector size
• detector recovers half the rays but the decreased detector size allows for you to see finer detail
• only used for high resolution studies

Question 13

in general, special features to increase spatial resolution (1/4 detector shift, flying floating spot and attenuating comb) work by:

• achieving finer sampling through
• decreasing distance between samples OR
• decreasing effective detector size

Question 14

small focal spot reduced elective aperture width

small focus often restricted by mA or slice thickness selection

Question 15

why is it better to have short scan time

Answer 15

movement blur is reduced

Question 16

on some scanners no of projections per rotation depends on scan time

• short scan times may have fewer projections per rotation
• high resolutions may need to use longer scan time

Answer 16

mAs depends on speed of tube spinning around patient so increase time will decrease the dose (vice versa)

Question 17

difference between smoothing filter and edge-enhancing filter?

Answer 17

smoothing filter:
- low noise
- blurred edges

edge-enhancement filter:
- high noise
- sharp edges

Question 18

for high resolution image:

• small sampling apeture
• high sampling frequency
• small pixel size
• sharp reconstruction filter

Question 19

z-axis spatial resolution:

measure of sensitivity of scanner object long z-axis

also known as imaged slice thickness, Z-sensitivity

Question 20

what is the significance of z-sensitivity / z axis spatial resolution

Answer 20

• affects visualisation of small objects
• affects CT number and image contrast

Question 21

how would you use z-sensitivity to avoid the partial volume effect

Answer 21

• as partial volume effect is due to tissues of widely different absorption encompassed on the same CT voxel
• to get correct CT no, use z sensitivity that is equal to or smaller than features in scan

Question 22

partial volume artefact:

• occurs when an off centre object projects partly into scan plane
• incorrect CT number given, streak artefact occurs

Question 23

what can be done to reduce the partial volume e artefact besides using z-sensitivity

Answer 23

use narrower slice widths

Question 24

small focal spot can improve z sensitivity

Question 25

what is the effect of doubling z samping

Answer 25

• increases z-axis spatial resolution
• reduces helical artefacts

Question 26

define image noise

Answer 26

variation in CT number in image of uniform object

Question 27

a ‘smoother’ image is one where the pixel values do no vary as much from the true value

Question 28

what is the effect of noise on low contrast detectabiltiy

Answer 28

more noise = less low contrast detectability

LCD = ( a toolkit for assessing image quality of advanced nonlinear CT image reconstruction and denoising products)

Question 29

what type of relationship is there between number of photons and noise

Answer 29

invert square relationship

noise proportional to 1/no photon^2

Question 30

what is standard deviation used for in ct

Answer 30

used as measure of spread of values “noise”

Question 31

what is the ROI tool in CT

Answer 31

region of interest
- measures mean CT no and standard deviation

Question 32

higher noise = greater spread of value

Question 33

how can the efficiency of detectors affect noise

Answer 33

• solid state more efficient that gas = less noise
• scintillator material efficinecy
• geometric efficiency of carry design

Question 34

how does xray beam filtration affect noise

Answer 34

• harder beams attenuate less = less noise

Question 35

how does scanner geometry affect noise

Answer 35

• inverse square law = more photons reach detectors the short the focus detector distance

Question 36

how does image reconstruction techniques affect noise

Answer 36

• iterative reconstruction reduces noise relative to filtered back projection alone (whilst preserving spatial resolution)

Question 37

what 3 things are the number of photons produced proportional to

Answer 37

• tube current
• scan time
• slice width

(hence if u increase these 3 things, you decrease noise)

Question 38

How does low kV affect image

Answer 38

low kV = less penetration by photons = increased noise

Question 39

define LCD

Answer 39

low contrast detectability

• ability to detect an object with a small difference in attenuation coefficient from its homogeneous background

Question 40

define contrast

Answer 40

difference in CT number between an object and its surroundings

Question 41

what is the equation for calculating percentage contrast

Answer 41

% contrast = (CTa - CTb) / (CTb x CT air) x 100

Question 42

simply measuring noise in uniform phantom will give good indication of how scanners compare in low contrast resolution because:

low contrast resolution is limited by noise

it is hence dependant on all parakeets that affect noise

Question 43

what are the 3 types of contrasts used in CT

Answer 43

• inherent
• positive
• negative

Question 44

what is inherent contraast

Answer 44

• naturally occurring e.g air in lungs or kidney stones

Question 45

what is positive contrast

Answer 45

IV or oral contrast agents

Question 46

what is negative contrast

Answer 46

air and water

Question 47

compare how negative and positive contrast looks on scan compared to surrounding tisses

Answer 47

• negative contrast media (more radiolucent than surrounding tissues) (darker)
• positive contrast media (more radiopaque) (brighter)