Quality assurance


Quality assurance is a requirement of IRR 1999 and each hospital should establish its own quality manual detailing:

  • What tests have to be done
  • How the tests should be done
  • How often the tests should be done
  • How the test results are recorded and analysed
  • What the acceptable margin of deviation from the standard is
    • If test differs by a margin that requires action to rectify it = remedial level
    • If test differs by a substantial margin that means equipment no longer fit to use = suspension level
  • What to do if a test is failed

Institute of Physics and Engineering in Medicine (IPEM) report 91: Recommended standards for the routine performance testing of diagnostic x-ray systems.

IPEM report 91 divides QA tests into two levels, which it calls level A and level B.

Level A tests:

  • Are generally quick and simple
  • Do not need expensive or complex equipment
  • Do not need detailed analysis
  • Are done frequently
  • Are usually done by the equipment user

Level B tests:

  • Take longer
  • Might require expensive or complex equipment
  • Need more analysis
  • Might be done less frequently than level A tests
  • Are often done by medical physics departments or by manufacturers’ engineers

 

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Equipment tests

Speed of film processor

Test frequency: Daily to weekly

Four values are calculated daily:

  1. Speed index
  2. Contrast index
  3. Base plus fog
  4. Dmax

Sensitometry

  1. Sensitometer exposed film to a range of different light levels (usually approx 20 steps)
  2. Densitometer measures transmission of light through the film to measure the optical density (OD)
1. Speed index:
  • Step at which OD approx 1.2 identified
  • Each time sensitometric film processed the density of this same step measured (result called the
  • speed index) and used as proxy for the characteristic curve
  • Speed index plotted on daily chart
2. Contrast index:
  • Two steps chosen with a difference in OD of 1
  • Each time sensitometric film processed the densities of these steps measured
  • The difference in the OD between the steps is the contrast index and is used as a proxy for the actual contrast (gamma) of the characteristic curve

IPEM report 91 suggested margins for both speed and contrast index:

  • Remedial level = 0.15 OD from baseline
  • Suspension level = 0.3 OD from baseline
3. Base plus fog

The OD of an unexposed area of film measured to find the base plus fog level

  • Remedial level = >0.25 OD
  • Suspension level = > 0.3 OD
4. Dmax

OD of darkest step measured.

  • Gradual Changes
    • Incorrect replenishment rates
    • Drift in developer temperatures
    • High film throughput
  • Step Changes
    • Change of film batch
    • Change of developer chemistry

X-ray set tests

X-ray tube output

  • Test 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

  • Test 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.
  • Test every 1-3 months
  • 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 OD and mAs is noted
    • Remedial level = +/- 0.3 OD relative to baseline
    • Suspension level = +/- 0.5 OD relative to baseline
  • AEC chamber consistency
    • Test every year.
    • There are usually three ionisation chambers in each AEC.
    • Remedial level = +/- 0.3 OD relative to baseline
    • Suspension level = +/- 0.5 OD relative to baseline

Light beam alignment

  • Test every 1-2 months.
  • Edges of light beam marked on film and film exposed. Area of exposed filed 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 radiographed with a geometric magnification of approx 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

Films and Intensifying Screens

Screens
  • Test every 12 months
  • May lose sensitivity and suffer mechanical damage over time.
  • Inspected for damage and cleaned every 6-12 months
  • Suspension level = sensitivity differs by > 20% from batch
Films
  • Crossover check performed when new batch of films used

Σ  Summary
  • Quality assurance is a requirement of IRR 1999 but exact schedule and test list is not specified - up to individual hospital
  • IPEM report 9.1 provides guidelines
  • Remedial level = action required to improve equipment performance
  • Suspension level = equipment should not be used anymore. Not every piece of equipment has a suspension level

Example testing timeline and summary

(scroll sideways to view whole table)

Equipment tested Frequency of testing Method of testing Performance criteria
Speed of film processor Daily to weekly

Sensitometry and densitometry

Assess:

  • Speed index
  • Contrast index
  • Base plus fog
  • Dmax

Speed and contrast index

  • Remedial level = 0.15 OD from baseline
  • Suspension level = 0.3 OD from baseline

Base plus fog

  • Remedial = > 0.25 OD
  • Suspension = > 0.2 OD
X-ray tube output 1-2 months Electronic kV meter measures kV at different exposure settings

Remedial = ±5% or ±5 kV from baseline

Suspension = ± 10% or ± 5kV from baseline

Light beam alignment 1-2 months Light beam field compared to exposed field on film

Remedial = 1 cm misalignment on any side at 1 m from focal spot

Suspension = 3 cm

Automatic exposure control (AEC) performance 1-3 months Film exposed at different tube voltages and different perspex thicknesses to ensure consistent OD

Remedial = ± 0.3 OD relative to baseline

Suspension = ± 0.5 OD relative to baseline

AEC ionisation chamber consistency Annual Expose ionisation chambers separately and compare (usually three)

Remedial = ± 0.3 OD relative to baseline

Suspension = ± 0.5 OD relative to baseline

Focal spot Annual

Pinhole

Pinhole radiographed and size, shape and inconsistencies of focal spot calculated from image produced

Star test object

Radiating lead spokes radiographed and central blurred area size used to calculate focal spot size

 
Filtration Annual

Half value thickness / layer of filter determined and compared to available data to calculate filtration 

Should be equivalent to 2.5 mm aluminium

 
Intensifying screen Annual Compare images produced at same settings using different screens Suspension = sensitivity differs by > 20% from batch
Film With every new batch Crossover check with old batch  

Next page: Mammography


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