Fwd: Off-focus radiation

[Ccja@aol.com]

---------------------
Forwarded message:
From:	zamenhof@MIT.EDU (Robert G. Zamenhof)
Sender:	medphys@LISTS.WAYNE.EDU (Medical Physics Listserver)
Reply-to:	MEDPHYS@LISTS.WAYNE.EDU (Medical Physics Mailing List)
To:	MEDPHYS@LISTS.WAYNE.EDU (Multiple recipients of list MEDPHYS)
Date: 97-07-07 11:23:48 EDT

There was a very good paper on off-focus radiation in Medical Physics about
5-7 years ago by (I think) the inimitable Joel Gray.

As far as I can recall, the author stated that in normal x-ray tubes 10-20%
of the radiation output is in fact "off-focus".

I agree with Dr. Cameron that the transparency of the collimators is not an
issue here. What happens is that the "primary" or "real" image on the
receptor is overlaid with the off-focus image.  Since USUALLY the off-focus
image is relatively fuzzy (since it emanates from more than one off-focal
area) it rarely seems to add confusing detail, only degrade the contrast of
the primary image.
I have seen films with ghost-like tissue contours extending outside of the
primary radiation field, which the radiographers have dubbed "scatter".  I
believe these are the most obvious manifestations of off-focus radiation.
It also seems to become worse at high mA.

Robert G. Zamenhof, Ph.D.
Director, Section of Radiological Physics
Department of Radiology
Beth Israel Deaconess Medical Center, Harvard Medical School
1 Deaconess Road, Boston, MA 02215, U.S.A.
Tel: (617) 667-0175  Fax: (617) 975-5233  E-Mail: zamenhof@mit.edu or
rzamenho@bidmc.harvard.edu
*** "Non iligitimati carborundum est" ***