Dose Area Product (DAP), is a multiplication of the dose and the area exposed, often expressed in Gy.cm2. Modern x-ray systems are fitted with a DAP meter, able to record accumulated DAP during an examination. A schematic of the imaging chain with DAP meter is given below. How is DAP measured? An ionization chamber larger than the area of the x-ray beam is placed just beyond the xray collimators. The DAP ionization chamber must intercept the entire x-ray field for an accurate reading. The DAP-value is depending on x-ray technique factors (kVp, mA, filtration) and time of the procedure and the area of the x-ray field. For example, a 5 x 5 cm x-ray field with an entrance dose of 1 mGy will yield a 25 mGy.cm2 DAP value. If the field is increased to 10 x 10 cm, with the same entrance dose of 1 mGy the DAP increases to 100 mGy-cm2, which is 4 times the DAP for the 5 x 5 cm field. Why DAP? Dose-area product is relatively easy to measure. DAP has been shown to correlate well ...
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