Re: electrons from medical x-ray equipment

[scherer@uiuc.edu (David Scherer)]

Kent makes an interesting observation about the range of photoelectrons from
diagnostic x-rays.  There is another detail that minimizes their importance:
most of the electrons aren't directed toward the patient.  Low energy
photoelectrons are preferentially emitted in the direction of the photon's
electric field vector, i.e. perpendicular to the photon's propogation.
Compton electrons must be more or less forward-directed to conserve
momentum.  (The photoelectric effect is an atomic/collective process, so the
momentum is taken up by the atom.)

>Simple inverse square law is the primary reason for keeping a minimum 
>distance.  Assume that a patient is 20 cm thick, and that acceptable 
>image is obtained at an exposure to the film of 100 mR.  If the focal 
>spot is 30 cm from the film the skin entrance dose is 100*900/100=900 
>mR.  If the focal spot is 100 cm from the film, then the skin exposure 
>rate is 100*10000/64000=156 mR.  Attenuation in the patient for this 
>example is a constant.  At least this is my understanding.

Also, greater distance improves spatial resolution (reduces penumbra) and
minimizes the annoying effects of magnification.

Good question; fun discussion.

Dave Scherer
scherer@uiuc.edu