scan data to image formation Flashcards
define spatial resolution
ability to see/resolve high contrast spatial detail without blurring
what is limiting resolution
the smaller object size resolved in mm
if you are looking for something broad or bright, it unlikely you need high does so decrease dosage
fundamental limitation of spatial resolution depends on what 2 things
- focal spot size
- detector element size
(both of these relate to the general term of aperture size)
defence of resolution on scanner design relies of aperture size and what other aspect
sampling frequency
- number of samples per projection
- number of different projections
dependence of resolution on scanner designs relies overall on these 2 things:
- aperture sizer
- sampling frequency
how does number of samples per projection and number of different projections affect image quality
low sample per projection (low dose per projection) increases noise
increasing number of different projection decreases noise/ makes a clearer image
the scanner can have 3 special features for improving spatial resolution, what are theyu
- quarter-detector shift
- flying focal spot
- attenuating comb
how does the flying focal spot work to improve spatial resolution
- focal spot position on anode rapidly varied, halving the spacing between samples
how does the quarter detector shift work to improve spatial resolution
- 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)
the less gaps between samples the better
how is an attenuating comb work to reduce spatial resolution
- 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
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
small focal spot reduced elective aperture width
small focus often restricted by mA or slice thickness selection
why is it better to have short scan time
movement blur is reduced
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
mAs depends on speed of tube spinning around patient so increase time will decrease the dose (vice versa)
difference between smoothing filter and edge-enhancing filter?
smoothing filter:
- low noise
- blurred edges
edge-enhancement filter:
- high noise
- sharp edges
for high resolution image:
- small sampling apeture
- high sampling frequency
- small pixel size
- sharp reconstruction filter
z-axis spatial resolution:
measure of sensitivity of scanner object long z-axis
also known as imaged slice thickness, Z-sensitivity
what is the significance of z-sensitivity / z axis spatial resolution
- affects visualisation of small objects
- affects CT number and image contrast
how would you use z-sensitivity to avoid the partial volume effect
- 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
partial volume artefact:
- occurs when an off centre object projects partly into scan plane
- incorrect CT number given, streak artefact occurs
what can be done to reduce the partial volume e artefact besides using z-sensitivity
use narrower slice widths
small focal spot can improve z sensitivity
what is the effect of doubling z samping
- increases z-axis spatial resolution
- reduces helical artefacts
define image noise
variation in CT number in image of uniform object
a ‘smoother’ image is one where the pixel values do no vary as much from the true value
what is the effect of noise on low contrast detectabiltiy
more noise = less low contrast detectability
LCD = ( a toolkit for assessing image quality of advanced nonlinear CT image reconstruction and denoising products)
what type of relationship is there between number of photons and noise
invert square relationship
noise proportional to 1/no photon^2
what is standard deviation used for in ct
used as measure of spread of values “noise”
what is the ROI tool in CT
region of interest
- measures mean CT no and standard deviation
higher noise = greater spread of value
how can the efficiency of detectors affect noise
- solid state more efficient that gas = less noise
- scintillator material efficinecy
- geometric efficiency of carry design
how does xray beam filtration affect noise
- harder beams attenuate less = less noise
how does scanner geometry affect noise
- inverse square law = more photons reach detectors the short the focus detector distance
how does image reconstruction techniques affect noise
- iterative reconstruction reduces noise relative to filtered back projection alone (whilst preserving spatial resolution)
what 3 things are the number of photons produced proportional to
- tube current
- scan time
- slice width
(hence if u increase these 3 things, you decrease noise)
How does low kV affect image
low kV = less penetration by photons = increased noise
define LCD
low contrast detectability
- ability to detect an object with a small difference in attenuation coefficient from its homogeneous background
define contrast
difference in CT number between an object and its surroundings
what is the equation for calculating percentage contrast
% contrast = (CTa - CTb) / (CTb x CT air) x 100
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
what are the 3 types of contrasts used in CT
- inherent
- positive
- negative
what is inherent contraast
- naturally occurring e.g air in lungs or kidney stones
what is positive contrast
IV or oral contrast agents
what is negative contrast
air and water
compare how negative and positive contrast looks on scan compared to surrounding tisses
- negative contrast media (more radiolucent than surrounding tissues) (darker)
- positive contrast media (more radiopaque) (brighter)
