Quality assurance

X-ray set tests

X-ray tube output

• Tested every 1-2 months
• 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
    • Remedial level = ± 10%
    • Suspension level = ± 20%
  • Consistency – whether output has changed since the baseline set of QA checks
    • Remedial level = ± 20%
    • Suspension level = ± 50%

X-ray tube kV

• Tested every 1-2 years
• Potential measured using an electronic kV meter. The kV is then measured at a range of different exposure settings.
  • 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 the exposure?
• Remedial level = ± 5% or ± 5 kV from baseline, whichever is greater
• Suspension level = ± 10% or ± 10 kV from baseline

Filtration

• N.B. not regularly measured as doesn’t change.
• Dose for fixed exposure measured with varying thicknesses of aluminium placed in the beam.
• The thickness that gives 50% transmission (half original dose) is the half-value thickness (HVT) or half value layer (HVL).
• Data is available that enables the filtration to be estimated from the HVT.

Automatic exposure control

• Some tests are done for level A and some for level B.
• The AEC terminates the exposure once the film has received an appropriate level of dose.
• It should produce 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 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
  • Tested every 1-3 months
  • A 1 mm copper in the beam is imaged with exposure under the AEC device control then the whole detector is irradiated. The mAs and detector dose indicator (DDI) reading is then recorded.
  • Remedial level = baseline ± 25%
  • Suspension level = baseline ± 50%
• Guard timer operation
  • Tested annually
  • 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
  • Tested annually
  • The DDI and mAs is checked between the AEC chambers to ensure consistency between them.
  • Remedial level = baseline ± 30%, mean ± 20%
• AEC repeatability
  • Tested annually
  • The mAs and DDI of successive repeated exposures is measured using the same AEC settings
  • Remedial level = mean ± 20%
• AEC reproducibility
  • Tested annually
  • Testing is similar to the AEC consistency tests but a larger range of kV and thickness of phantoms is used.
  • Remedial level = baseline ± 30%
  • Suspension level = baseline ± 60%

Light beam alignment

• Tested every 1-2 months.
• Edges of light beam marked on film and film exposed. Area of exposed field then compared to light field
• Remedial level = 1 cm misalignment on any side at 1 m from focal spot
• Suspension level = 3 cm

Focal spot measurement

N.B. most physics services do not measure this regularly as it doesn’t change and any faults can be picked up from the image quality and tube output tests.

• Pinhole
  • Pinhole a few microns across (smaller than the focal spot) 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.
  • Can estimate size, shape and irregularities in the focal spot with this method
• Star test object
  • An array of radiating lead spokes is imaged with a geometric magnification of approximately 3
  • The spokes at the centre will come to a point at which they can no longer be distinguished by the system due to being too small and close together.
  • The diameter of the blurred area can be used to calculate the focal spot size

CR and DR radiography

There are a few tests specific to computed and digital radiography as outlined below.

Detector dose indicator (DDI)

• The detector dose indicator measures the dose received at the detector. There are several tests performed to guarantee the accurate functioning of the DDI.
• DDI repeatability and reproducibility
  • Tested annually
  • Remedial level = baseline ± 10%
  • Suspension level = baseline ± 20%

Image quality

• Low contrast sensitivity
  • Tested every 4-6 months
  • The Leeds Test Objects Ltd is used to ensure the system is still able to image low contrast items
  • Remedial level = baseline ± 2 groups
• Threshold contrast detail detectability
  • Measured annually
  • A test object with an appropriate filter and kV is imaged and the contrast that can be accurately imaged is measured.
• Limiting spatial resolution
  • Tested every 4-6 months
  • A lead grating resolution bar pattern is used to assess the highest spatial resolution the system can image accurately
  • Remedial level = baseline minus 25%
• Uniformity of resolution
  
  • Tested annually
  • A fine wire mesh is imaged and checked for blurred areas and discontinuities
  • Remedial level = increase in blurring from baseline
• Measured uniformity
  • Tested annually
  • 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 divided by the mean value.
  • Remedial level = mean ± 5%
• Dark noise
  • Tested annually
  • An image is obtained without exposure or with very low exposure. This tests for noise in the system.
  • Remedial level = baseline ± 50%
• Scaling errors
  • Tested annually
  • 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.
  • Remedial level = > 2% difference