Lecture 14: X-ray CT Image Quality Flashcards
What is the main source of noise in x-ray CT images?
The fluctuation in the number of photons
What is the standard deviation for the number of photons detected in x-ray CT?
Sqrt(N) for N photons detected
What is the signal to noise ratio for x-ray CT?
N = number of photons
A high photon count gives a ____ signal to noise ratio.
High
What is the risk of having a high number of photons in x-ray CT?
It increases the risk to the health of patients as the dose of ionising radiation is higher.
What does the width of each voxel recorded for an x-ray CT image depend on?
Collimation/the ability to collimate: poor collimation causes the beam to diverge with distance so there is a change in voxel size.
Define partial voluming
The effect when tissue only fills part of a voxel.
What are the two main forms of partial voluming, and what is the effect of each?
Multiple tissue types in one voxel/tissue only fills part of the voxel: blurred image
Effect of interest is small relative to the voxel size: it gets averaged out so missed
What is the result of diverging beams for x-ray CT? How can this be mitigated?
It results in a slice sensitivity profile that is not ‘rectangular’ but one that diverges. Projections can be taken from both sides of the body to mitigate this.
Define beam hardening
An x-ray beam that is poly-energetic. As it passes through tissue the low energy photons are easily absorbed, hence, the average energy of the beam increases with distance through the patient and the beam ‘hardens’. This means that µ appears to decrease with depth.
