simply put - it is a predetermined amount of gamma radiation that passes
through the body and strikes a photo sensitive material on a plate
behind the body.  the image is created on the plate by the absorption of
the gamma rays of the body.  the more dense the body mass is, the less
radiation pass through and it leaves an underexposed spot on the plate.
putting all of this together and you have an image that the x-ray tech
can see.  

hope the one paragraph explanation helped

~ curtis

knowledge is power - growing old is mandatory - growing wise is optional