There is a set hierarchy in terms of accuracy of dose calibrators. The Primary standard is held and maintained at the National Physics Laboratory (NPL). Secondary standards are those which have been directly cross calibrated with the primary. Tertiary standards (or field instruments) are cross calibrated with secondary standards. It is the tertiary standards that are generally held in nuclear medicine departments.
Click on the Standard quality control tests for radionuclide calibrators tab above to continue.
Click on each standard quality control test to see more details.
High voltage
The voltage applied across the ionisation chamber will obviously have a direct effect of the operation of the instrument. The high voltage will normally be a built-in self diagnostic test of the calibrator.
Zero / Background
The zero setting and background compensation needs to be monitored and adjusted to compensate for the variations in gain of the electronics and changes in the background levels of radiation.
Accuracy
Absolute accuracy of the readings needs to be checked daily and calibrated yearly. The commissioning and yearly checks should be with a range of sources and cross referenced to measurements made on a secondary standard. The daily checks can be performed using a long lived sealed source such as Ra-226 or Cs-137.
Relative responses
This is the relative response using different isotope settings. The reading in MBq is produced by converting the current across chamber into a digital readout. This incorporates a scaling factor to account for the relative sensitivity to the different energy emissions. This scaling factor needs to be checked on a daily basis using the same long lived source as the accuracy checks.
Precision
Coefficient of variation over 10 consecutive measurements. A source is measured repeatedly in the calibrator, being removed between each measurement. The same long lived source used for accuracy can be used.
Coefficient of Variation = 100*Standard deviation / mean
Linearity
The linearity of the response to low and high activities. This is checked using a high activity source which repeatedly measured over time as it decays through a number of half lives. The measurements are compared with the expected readings. The initial activity should be greater than highest activity used routinely.
Electrical safety
Standard electrical safety tests should be performed yearly as with all mains electricity equipment.
Subsidiary calibrations
Different volumes and containers will effect the sensitivity of the calibrator due to geometric and absorption effects. These effects need to be investigated at the time of commissioning by using a stock solution of activity. This can then be dispensed into different volumes and or containers using the weight of solution to calculate the activity.
Leakage radiation
The calibrator should have sufficient shielding to not be effected by movement of sources in the room or adjacent rooms. This shielding should also be sufficient to minimise the dose to the operator when assaying sources. The dose rate produced at the position of the operator when a high-energy, high-activity source should be checked at commissioning.
Reproducibility
Long term stability of the instrument
Click on the Frequency of tests and Acceptable tolerances tab above to continue.
The frequency and tolerances outlined below are taken from the IPSM (now the IPEM) report no.65 Quality Standards in Nuclear Medicine. It should be however be noted that new European guidelines are now being drafted.
Table 1: Frequency of tests
| |
Daily |
Monthly |
Yearly |
Acceptance testing |
| High voltage |
x |
x |
x |
x |
| Zero adjust |
x |
x |
x |
x |
| Background |
x |
x |
x |
x |
| Accuracy |
x |
x |
x |
x |
| Relative responses |
x |
x |
x |
x |
| Precision |
|
x |
x |
x |
| Linearity |
|
|
x |
x |
| Electrical safety |
|
|
x |
x |
| Subsidiary calibrations |
|
|
|
x |
| Leakage radiation |
|
|
|
x |
Table 2: Acceptable tolerances
| |
Parameter |
Reference Instrument |
Field Instrument |
| |
Precision |
±0.5% |
±1% |
| |
Reproducibility |
±1% |
±2% sealed chamber
±3% open chamber |
| |
Linearity |
±1% corrected |
±5% over range used |
| Accuracy |
Agreement with secondary standard |
±2% high energy
±5% low energy |
|
| Agreement with tertiary standard |
|
±5% high energy
±10% low energy |
*Low energy refers to Gamma or x-ray emitters where more than 30% of the emissions are below 100keV.
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