Week 10- image quality Flashcards
what affects image quality?
spatial resolution noise contrast resolution temporal resolution artefacts accuracy/ linearity/ uniformity
what factors affect spatial resolution
- Focal spot size and shape (somewhat under operator control)
- Detector cell size
- Scan geometry
- Sampling frequency
- Reconstruction (convolution) algorithm (operator controlled)
what is spatial resolution
•The ability to visualise detail (or small objects) in an image
what are the two types of spatial resolution
in-plane spatial resolution
cross-plane spatial resolution (slice thickness)
what is slice thickness affected by
- Detector size
- Collimation size
- Pitch
- Helical reconstruction algorithm
- MSCT reconstruction method
what is the slice sensitivity profile
A measure of cross plane spatial resolution
what is cone beam geometry
As scanners increase in detector rows, the beam geometry changes
Fan beam reconstruction is only relevant if there are a small number of detector rows
If there are >16 detector rows then the beam becomes more ‘cone shaped’
what is noise
A variation of signal intensity that is not due to the signal itself
what are some causes of image noise
scatter radiation
electronics
reconstruction algorithm
quantum noise
how is noise measured
uniform phantoms
what is SNR
It is a measure of the amount of signal / the amount of noise (as a ratio). It is simple measure of image quality
how do we increase SNR.reduce the effect of noise
Increase kVp
Increase mA
Decrease scan speed
Increase slice thickness
what is contrast resolution
the measure of how well objects of low contrast differences can be visualised in an image
what is temporal resolution
A measure of the ability to visualise motion correctly
What is image quality accuracy?
- A CT no / HU accurately represents the object’s attenuation values
