From: DACBR131@aol.com
Date: Mon, 9 Nov 1998 11:01:37 EST
Subject: Re: In office Xray - was Re: moving forward

In a message dated 11/9/98 Jeffrey R. Cates, DC writes:

<< can one test a cassette to see if the screens need replacement or should
  we simply replace or refurbish cassettes every 7 to 10 years >>

Those most simple test would be to obtain from your x-ray service company an
identical cassette and screen to the one you are using. Put some of your film
in each of the cassettes. 

Place the old cassette on the table or floor next to the new cassette/screen.
You will need a symmetrical object like a stepwedge (borrow one from the
service company) that will rest partly on the new cassette and partly on the
old cassette. Be sure to align the stepwedge with the axis of the tube to
minimize anode heel effect.

Now make an exposure of the object thereby exposing part of your current
film/screen and part of the new film/screen. You will have to play with kV and
mAs to get a reasonable image. Develop the films. Put the images of the
stepwedge next to one another on the same view box.

If you can see darkening (density) differences when comparing the same step on
each image (and I would predict that your old equipment results in the lighter
of the two images) then it is time to change your screens. If you can see any
difference it is equal to about 15 to 30% mAs. In other words, you are using
15 to 30% excess mAs to darken the film because your screens are "tired". If
you had new screens, you would use less mAs to get the same darkening effect.

Hope this helps.

Gary M. Guebert, D.C., DACBR
President, American Chiropractic College of Radiology