DR: Quality Assurance Flashcards
Discuss testing an x-ray tube output
Ionisation chamber placed at a known distance from the x-ray tube. The measurements of dose (using an electrometer) are made for various exposures to determine:
- Dose per mAs (output for range of exposures)
- Whether output varies with mA
- How output varies with kV
- Repeatability - whether output is consistent when same exposure repeated
- Consistency - whether output has changed since the baseline set of QA checks
Discuss testing x-ray tube kV
Potential measured using an electronic kV meter. The kV is then measured at a range of difference exposure settings. Checks:
- Is it accurate - do we get the value we have selected?
- Does it change if we change the mA range or exposure time?
- Does it vary during exposure?
Discuss testing filtration
Dose for fixed exposure measured with varying thickness of aluminium placed in the beam, The thickness that gives 50% transmission is the HVT or HVL.
Discuss testing the AEC (automatic exposure control)
The AEC terminated the exposure once the film has received an appropriate level of dose. It should produced a consistent optical density in the film for a wide range of tube potential (kV) and for a wide range of patient thickness. Perspex or water blocks are used to simulate a patient. A series of exposures at different tube voltages and using a different thickness of material is taken. The mAs and the detector dose indicator measurement is recorded.
AEC sensitivity: a 1mm copper in the beam is imaged with exposure under the AEC device control then the whole detector is irradiated. The mAs and DDi reading is then recorded.
Guarded timer operation: The guard timer terminates the exposure after a certain amount of time. It is tested by using a low kV exposure with lead blocking the AEC chambers and ensuring the exposure is terminated at the guard timer setting.
AEC consistency: The DDI and mAs is checked between the AEC chambers to ensure consistency between them
AEC repeatability: The mAs and DDI of successive repeated exposures is measured using the same AEC settings
AEC reproducibility: Testing is similar to the AEC consistency tests but a larger range of kV and thickness of phantoms is used.
Discuss light beam alignment
Edges of the light beam marked on film and exposed. Area of exposed filed then compared to light field.
Discuss focal spot measurements
Pinhole test: A pinhole a few microns across is radiographed. Taking into account the distance from the focus to the pinhole and the focus to film distance you can then calculate the size of the focal spot by measuring the image produced on the film.
Discuss testing low contrast sensitivity
The Leeds test object is used to image low contrast items
Discuss testing threshold contrast detail detectability
A test object with an appropriate filter and kV is imaged and the contrast that can be accurately imaged is measured
Discuss testing the limiting spatial resolution
A lead grating resolution bar pattern is used to assess the highest spatial resolution the system can image accurately
Discuss testing for uniformity of resolution
A fine wire mesg is imaged and checked for blurred areas and discontinuities
Discuss measuring uniformity
An image is obtained with no object in the field. An ROI is then placed over each quadrant and in the centre. The 5 values are used to calculate the standard deviation devided by the mean value.
Discuss testing for dark noise
An image is obtained without exposure or with very low exposure. This tests for noise in the system
Discuss scaling error testing
A grid and an attenuating object of known dimensions or a lead ruler are used to ensure that the scale of the image is correct
