The key to patient protection is the establishment of the relationship between the
absorbed dose (and hence the risk) and the benefit which the patient receives as
result of the X-ray examination. The physicist contributes by developing and
implementing techniques for the derivation of patient dose by direct or
retrospective methods and advises on techniques which ensure that doses are as
low as reasonably achievable (the ALARA principle). The design and execution
of patient dose monitoring is also required.
Patient doses can be monitored by means of Dose-Area-Product (DAP) meters or
Thermoluminescent dosemeters (TLD).
DAP meters use ionisation chambers which can be mounted on the collimator.
They measure the dose and multiplied by the area irradiated, which is independent
of the distance from the x-ray source. The unit is centi-Gray-centi-meter-square
(cGycm²).
Ionisation chambers
Mounting of the DAP chamber
TLDs are tiny crystal (5mm x 5mm x 0.9 mm) which can be packed and stick on
surface of the patient. When irradiated, they will store the energy. Upon heating,
the stored energy will be released in the form of light which can be detected by a
photomultiplier tube. The amount of light is proportional to the energy (dose)
stored. The picture below shows the TLD chips and ready pack.
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