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Time-Current Reciprocity Calculations

Radiographic exposure is the product of the X-ray system current and the time of the exposure.  If it is necessary to change the current or the exposure time while maintaining the overall exposure, the Law of Reciprocity can be used.  The equation for this specific case is:


        Where:  C1 = Current 1
                      C2 = Current 2
                      T1 = Time at current 1
                      T2 = Time at current 2

Example Calculation

An acceptable radiograph is produced using a current of 5mA and an exposure time of 10 minutes.  If it is necessary to use a current of 8mA instead of 10mA, what does the new exposure time need to be to produce a similar radiograph?

First solve the equation for T2.

Substitute in the known values and solve.