factors affecting CT image quality Flashcards
factors affecting CT image quality
- spatial resolution
- contrast resolution
- radiation dose
- noise
- image artifacts
how to increase spatial resolution in CT
- reduce sampling aperture
- increase sampling freq
- decrease slice thickness
- increase matrix size
- reduce RFOV
how to increase contrast resolution in CT
- increase photon flux
- increase slice thickness/ z-axis collimation
- reduce patient size
- increase sensitivity of detector
- reduce matrix size
noise explained in CT
- uniform phantom of water = uniform HU value
- if this does not occur = fluctuation = noise
what is noise level expressed as
%
how to reduce noise levels in CT
- increase mA
- increase slice thickness
- reduce helical pitch
- reduce object size
- increase pixel size
- increase number of photons reaching detectors
effect of low mAs
grainy
increase in kV
intensity of beam increases and beams ability to penetrate thick anatomical parts
- increases photon flux and could reduce noise
what is pixel size determined by
RFOV and matrix size
the effect of reconstructed algorithm
- image quality - sharpness
- noise reduction
- artifact reduction
- contrast enhancement
- spatial resolution
- artificial smoothing
- processing time
CT artefacts
- motion
- out of field view
- beam hardening
- metallic foreign bodies
- partial volume
- photon starvation
- ring
- multislice
what kind of energy photons are absorbed with beam hardening
low energy photons
they have a lower penetrating power than high-energy photons making them more likely to be absorbed by body tissues
why is beam hardening undesirable
- CT can distort images and hinder accurate diagnosis
- can cause shadowy artefacts
ways to solve beam hardening
- use appropriate energy filters that allow you to selectively remove low-energy filters
- calibration helps correct variations in x-ray beam + detector response
- preprocessing to reduce streak artifacts and post processing
- iterative reconstruction
what is volume averaging
where multiple structures with varying densities within a single voxel are averaged together leading to a loss of detail and contrast in CT image
how can volume averaging be resolved
- using thinner CT slices during acquisition process - represent smaller volumes + containing mix of different tissues in single voxel reduced
- metal decomposition - reduce effects of volume averaging by providing more accurate information about composition of tissues within each voxel
- iterative reconstruction
- post processing
what is photon starvation in CT
where there are not enough photons reaching CT detector resultign in poor image quality
- thick/ dense patient anatomy
- low x ray exposure
how can photon starvation be resolved
- have a CT system that varies the mA according to patient attenuation
- patient size adapted mA
- Z-axis mA modulation - some areas along patients length receive lower mA (low attenuation) than others
- angular modulation
what are ring artefacts
caused by defective detector and happens mostly in 3rd gen scanners
- unbalanced isocenter
how to resolve ring artefacts
- regular air calibration
- engineer
what is multi slice cone beam artefact
cone-beam ct systems have multi-slice arrays and several detector rows/ columns to capture multiple slices simultaneously
- artifacts arise through misalignment in detector components
how are multi-slice cone beam artefacts resolved
manufacturers have developed special cone beam reconstruction software algorithms to reduce this effect
