Re[2]: Lead Aprons in Dentistry

[tdc@ehssun.lbl.gov]

A few minor corrections to this thread:

On current dental units the tube housing IS very well shielded BUT the cone
is NOT!  The beam is indeed collimated to be congruent with the open area
at the tip of the cone (long plastic tube) - thus APPEARING to be a
shielded tube - but it is not.  IF it were and the beam not so well
collimated - there would be too much scatter from the tube and that would
adversely affect image quality.

Old dental units (pointer cone) WERE collimated - it is just that they
USUALLY had three or four possible collimation choices from about 1.5 inch
to about 4 inch diameter and were always left in the widest position.  The
image quality from the old "short cone" techniques (about 4") was much
poorer than the current "long cone" techniques (about 20") due to lessened
scatter and parallax distortion.  Also for a given dose to the film - skin
dose is considerably less being further out on the inverse square curve.

X-ray exposure at the lap position from scatter does indeed measure in the
mR/hr range - but exposures are typically well less than one second each
and maybe only a second or two accumulated for a full mouth series (28
exposures).  THUS exposures ARE in the microR range

OPINION:

The apron probably doesn't save that much exposure and probably needn't be
required by law.  BUT since it isn't really very expensive and no big deal
to do - it makes a good ALARA practice!  I certainly consider it a
reasonable thing to do.

Ted de Castro