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
