Experaments with Scatter

We know that CR image plates are more responsive to scatter and background radiation than film/screen systems... so we decided to see exactly how sensitive they would be to an exposure within the x-ray room.

A phantom was set up on the tabletop to expose a lateral lumbar spine. We used 75 kVp, 200 mAs at a 40" SID. We did not actually expose an image plate for the lumbar spine, but we wanted to mainly focus on the scatter produced by that exposure.

So we measured 8 feet from the spine phantom and set up a 10x12 cassette vertically with a cassette holder. A hand phanotm was placed in front of the cassette and the exposure was made. The following image resulted on Kodak CR with an exposure index of 1190.



You can see that the scatter was enough to penetrate soft tissue and bone enough to see medulary canal.



We decided to re-create the experiment with the same spine phantom, but this time we placed a chest phantom upright next to a 14x17 cassette, approximately 6 feet away from the spine phantom at about the same height. We ended up with an exposure index of 980.

It was definitely enough to penetrate the lung fields, but may not have been enough to penetrate the shoulder or neck region. 6 feet is the minimum safe distance we should be on all portables... hopefully, this will make us think twice about wearing a lead apron when we shoot portables (especially on anatomy that requires a lot of technique).

As a side note, the chest image really allows us to visualize penumbra... or geometric unsharpness produced at the periphery of the anatomy. We know that the smaller the focal spot, the less the penumbra. We have essentially created a gigantic focal spot because the source of most of the radiation hitting the chest phantom was scattered from different points within the lumbar spine phantom on the table... approximately a 27cm focal spot size!