Re: electrons from medical x-ray equipment

[scherer@uiuc.edu (David Scherer)]

>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."
>
In megavoltage radiation therapy this is sometimes called electron
contamination of the photon beam.  I believe that the problem is most
important in that application, not in diagnostic radiology.  The higher
energy photons have a point of maximum dose below the skin surface, so there
is a skin-sparing effect.  However, these electrons can negate that effect
unless precautions are taken.  I believe Dr. Harold Johns, a prominent
Canadian medical physicist, first reported this in 1952.  You can read about
it in most medical physics texts, e.g. Kahn, _The Physics of Radiation
Therapy_ or Johns and Cunningham, _The Physics of Radiology_.

In diagnostic radiology, the photon energies are very low and the
collimators are constructed of high-Z materials (usually W). Photoelectric
absorption dominates over Compton scatter in the collimator jaws.  Also, the
maximum dose is reached very near the surface, so there is no skin-sparing
effect.  Of course, there are other, geometrical reasons for positioning the
patient at some distance from the focal spot (and, hence, the collimator).

Dave Scherer
scherer@uiuc.edu