| Any point source which spreads its
influence equally in all directions without a limit to its range
will obey the inverse square law. This comes from strictly
geometrical considerations. The intensity of the influence at any
given radius (r) is the source strength divided by the area of the
sphere. Being strictly geometric in its origin, the inverse square
law applies to diverse phenomena. Point sources of gravitational
force, electric field, light, sound, or radiation obey the inverse
square law. 
As one of the fields which obey the general inverse square law, a
point radiation source can be characterized by the diagram above
whether you are talking about Roentgens, rads, or rems. All measures
of exposure will drop off by the inverse square law. For example, if
the radiation exposure is 100 mR/hr at 1 inch from a source, the
exposure will be 0.01 mR/hr at 100 inches.
To calculate a new exposure maS ie one at a new distance using the
old exposure maS
New mAs = Old mAs x (New distance2/Old
distance2)
eg A Chest x-ray at 180 cm and using 5 mAS
What mAS would be needed supine on a trolley at 100cm?
New mAS = 5 [old mAS] x (10000 [New d2] / 32400 [Old d2]
New mAS = 5 x [10000 / 32400] 0.3 = 1.5 mAS
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