Optimisation of X-ray Image Quality Flashcards
Which regulations cover the requirement of x-ray equipment optimisation and what do they state?
IR(ME)R regulations cover the requirement for x-ray equipment optimisation. They state that the practitioner and operator involved in an exposure must ensure doses arising from an exposure are ALARP in line with the intended diagnostic purpose by considering:
- QA.
- Assessment and evaluation of patient dose.
- Adherence to DRLs.
Put simply, what is optimisation of an x-ray imaging system?
Optimisation involves ensuring dose is ALARP with image quality being adequate for the intended diagnostic purposes. Therefore, this does not necessarily always correspond to a reduction in patient dose.
Why is it important to have a quantum noise limited system for optimisation to be undertaken?
Optimisation relies on a striking a balance between increasing the number of photons to reduce noise effects and improve image quality and reducing the number of photons to reduce patient dose. If a system is not quantum noise limited, an increase in dose may have no effect on image quality and, therefore, the system can not be optimised in this way.
What are the equations for signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR)?
- SNR = pv/sigma_pv where pv is the linearised pixel value and sigma_pv is the standard deviation of the linearised pixel value in a region of interest.
- CNR = (M1 - M2)/sqrt((sigma_1^2 + sigma_2^2)/2) where M1 - M2 is the difference in pixel values between two objects of different contrast and sqrt((sigma_1^2 + sigma_2^2)/2) is the geometric mean of the standard deviations (sigma_1 and sigma_2) of the same two regions of interest.
How would we expect signal-to-noise ratio (SNR) to vary with dose when considering quantum noise only? What is the case in a real system?
- SNR is proportional to sqrt(N) where N is the number of photons (which is proportional to dose).
- In a real systems, other sources of noise mean the actual relationship between SNR and dose is different.
What are the optimisation considerations with the use of of an anti-scatter grid? Give examples of when an anti-scatter grid would/wouldn’t be used and why.
- Scatter will degrade image contrast. An anti-scatter grid will reduce scatter and, therefore, improve image quality. However, it will also correspond to an increased patient dose. The risk/benefit of increased patient dose vs improved image quality must be considered in deciding whether to use an anti-scatter grid.
- In adult abdomen and lumbar spine examinations, the scatter-to-primary ratio is large and, therefore, the increased dose associated with the use of an anti-scatter grid is justified due to the improvement in image quality which allows for diagnoses.
- Examinations of smaller regions (e.g. extremities and paediatrics) produce only small amounts of scatter. In these cases, it is likely that the more slight improvement in image quality afforded by the anti-scatter grid will not be justified when considering the increased patient dose. The increased concern regarding health effects associated with radiation exposure of paediatrics also needs to be considered.
What is an alternative to an anti-scatter grid? What are the issues associated with this?
- An air gap between the object and detector will mean scattered photons are more likely to be attenuated before reaching the detector or are more likely to scatter outside of the detector area.
- The increased magnification associated with this geometry will mean increased geometric unsharpness and, therefore, reduced resolution. The smaller focus-object distance will also correspond to a small increase in patient dose (although not as much as with the use of an anti-scatter grid).
How will increased focus-object distance affect patient dose and image quality? What other considerations are required with an increased focus-object distance?
- An increased focus-object distance will reduce patient dose and geometric unsharpness.
- However, increased tube loading will be required and longer exposure times, thus increasing the likelihood of movement blur.
Give examples of when it might be decided that optimisation should take place.
- After comparison of recorded patient doses with other centre’s LDRLs and with NDRLs.
- After reviewing QA data.
- In both of the above cases, investigations into variations in dose/image quality must be undertaken to determine whether they are justified (e.g. clinical differences, equipment differences etc.).
Give an example of a general optimisation strategy.
- Use a phantom to mimic a clinical situation, taking exposures using a range of exposure parameters.
- Ask clinical colleagues to evaluate the images blinded to the exposure parameters used.
- This will allow for the lowest dose for a clinically acceptable image to be determined and will provide a starting point of optimisation.
How do optimisation strategies differ for screen-film and digital detectors in general radiography?
- In screen-film, a certain level of dose is required to give an acceptable level of blackening and contrast. Optimisation strategies, therefore, aim for a constant level of dose/blackening under AEC.
- Due to the wide dynamic range associated with digital detectors, contrast is constant over a large range of exposures. Optimisation strategies, therefore, consider noise and image quality rather than receptor air kerma.
What is the effect of increasing kV?
- Increased tube output per mAs (in proportion to kV^2).
- Increased beam penetration.
- => Reduced contrast.
- => Increased detector dose for given entrance dose.
- => Reduced patient dose for same detector dose.
What is the effect of increasing mAs?
- Increased patient dose.
- Increased detector dose.
- => Improved SNR.
- Contrast maintained.
What is the effect of increasing filtration?
- Decreased output per mAs (maximum beam energy does not change - only reduction in lower energy photons).
- Increased mean beam energy.
- => Increased beam penetration.
- => Reduced contrast.
- => increased detector dose for given entrance dose.
- => Reduced patient dose for same detector dose.
Does increasing kV or increasing filter thickness provide an increased rate of reduction of entrance surface air kerma and, therefore, patient dose? How do the reductions in contrast compare and why?
- Increasing filter thickness results in an increased rate of reduction of entrance surface air kerma.
- The rate of reduction in contrast is also less for increasing filter thickness when compared to increasing kV. This is due to the fact that increasing kV introduces higher energy photons which have a detrimental effect on contrast. Increasing filter thickness does not change the maximum energy of the beam but reduces the number of lower energy photons resulting in a more narrow spectrum. The effect on contrast is, therefore, more limited.
