Re: electrons from medical x-ray equipment

[lambert@HAL.HAHNEMANN.EDU]

Most x-ray equipment utilizes light-localized, variable apeture 
collimators.  Some equipment uses other types of beam limiting devices 
such as cones and cylinders, and apeture diaphragms.  

On Tue, 14 Nov 1995 mozley@darius.pet.upenn.edu wrote:
> Collimation is not so common any more.  There was a brief time when it was.

If the average energy from a diagnostic x-ray machine is 50 keV, then the 
average photoelectron energy would be somewhat less than this, since 
energy loss to overcome the work function and subsequent 
coulombic interactions.  The range of electrons in tissue are exceedingly 
small - 4 mg/cm^2 about the same as the depth of basal skin cells where 
the dead skin layer is thinnest.  Therefore even if they were created in 
large numbers, they would not significantly contribute to dose.

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.

> > >To: RADSAFE --INTERNET RADSAFE@ROMULUS.EH AHGOOD  --BCSC02   Alan Good
> >    FRANSELM--BCSC02   Francine Anselmo
> >
> >
> >I just can across an article that discusses the use of collimation in medical
> >x-ray. It says "It must be noted that numerous electrons are produced be the
> >interaction of photons with the collimator. In order to prevent these
> >electrons from reaching the patient the collimator position should be at least
> >15 cm above the patient's skin. If the photon beam contains a large number of
> >electrons, the energy absorbed by the skin is greatly increased and may result
> >in severe skin reactions."
> >
> >I have not heard that this is as significant a situation as the author
> >above describes.  Does anyone know more about this?      (Thanks in advance,
> >as they say) - Randy

Kent Lambert
LAMBERT@hal.hahnemann.edu