CPI Canada Inc.
Interfacing and Programming
3C
Use and disclosure is subject to the restrictions on the title page of this CPI document.
CMP 200
®
X-Ray Generator Service Manual Ch # 901476-04 Rev. C
Page 3C-75
3C.11.0 DAP
CALIBRATION
The DAP device must be calibrated before use, and the calibration must be periodically checked as per the DAP device manufacturers
requirements, or as per local regulations.
3C11.1 Equipment
Required
The following equipment is required for DAP calibration.
•
An X-ray cassette and film. The speed of the film / screen is not relevant, as the film is only used to determine the area that is irradiated at
the dose-measurement plane.
An X-ray cassette and film is the preferred method to measure the irradiated area, but if this is not readily available, for example if this
is a digital only system, a procedure is given to determine the required area using the imaging system.
•
A film processor to develop the film, if required.
•
A calibrated dosimeter.
•
A ruler or tape measure with centimeter markings. This will be needed to measure the exposed area of the film. If measuring in inches,
use a calculator and multiply inches by 2.54 to obtain the measurement in centimeters.
3C.11.2 DAP Calibration Overview
The first step involved in calibrating the DAP meter in the generator is to carefully make a dose measurement at a given distance from the X-
ray source. The next step is to expose a test film at the same distance from the X-ray source as the dose measurement that was just made.
The dose-area product is calculated by multiplying the measured dose, in mGy, by the exposed area of the film, in cm
2
. This gives the
actual dose-area product, in mGycm
2
.
After the reference dose-area product is determined as described above, it is compared to the DAP reading as measured by the
generator. Adjustments are made to the
CAL. VALUE 1
parameter in the
DAP SETUP
menu such that the consoles DAP display corresponds
to the calculated dose-area product. This procedure may need to be repeated several times until the required accuracy is obtained.
The reason the DAP device (a specialized ion chamber) is able to accurately measure the dose-area product at its location at the
bottom of the collimator is that although the radiation level falls off at a rate inversely proportional to the square of the distance from the
source, the irradiated area increases as the square of the distance from the source. For example, by doubling the distance, the dose falls to
(½)
2
= ¼ of the previous dose BUT the area increases to 2
2
= 4 times the area. Therefore, the dose-area product will remain constant at a
given distance from the source. This is also the reason that care must be taken to ensure that the area measurement is done at the same
distance from the X-ray source as the dose measurement.
