Understanding NRT- Reading 1- 2 of 2- Radiogaphic Testing A

Arithmetic of Exposure
RELATIONS OF MILLIAMPERAGE (SOURCE STRENGTH), DISTANCE,
AND TIME
With a given kilovoltage of x-radiation or with the gamma radiation from a
particular isotope, the three factors governing the exposure are the
milliamperage (for x-rays) or source strength (for gamma rays), time, and
source-film distance. The numerical relations among these three quantities
are demonstrated below, using x-rays as an example. The same relations
apply for gamma rays, provided the number of curies in the source is
substituted wherever milliamperage appears in an equation.
The necessary calculations for any changes in focus-film distance (D),
milliamperage (M), or time (T) are matters of simple arithmetic and are
illustrated in the following example. As noted earlier, kilovoltage changes
cannot be calculated directly but must be obtained from the exposure chart of
the equipment or the operator's logbook.
All of the equations shown on these pages can be solved easily for any of the
variables (mA, T, D), using one basic rule of mathematics: If one factor is
moved across the equals sign (=), it moves from the numerator to the
denominator or vice versa.
Charlie Chong/ Fion Zhang