RE: Simple radiography explanations?

David Beattie — Fri, 01 Dec 2017 10:22:02 GMT

I don't have a link, but my (non-expert) take in a nutshell:

If you have digital system then, other than exposure time which might cause blurring (i.e. you want low "s" [seconds] for anything that might move like thorax), it doesn't really matter much what your kV and mA are so long as they are sufficient to provide enough exposure of the film for the computer to interpret (check the required exposure index for your system and see what exposure indices you are actually getting). Effectively, you can underexpose a digital film but can't really over-expose it (hence checking exposure index that not using more radiation to get pictures than you actually need). If you are getting black images, with poor contrast, then this you need to make changes to the computer processing of the image obtained (eg manually cropping to exposed field, changing the look-up algorithm for that exposure etc.) rather than taking a repeat exposure with lower settings!

For a non-digital set-up with automatic processor, you may want to gain higher contrast by increasing the mA settings. If your "mA" and "s" are linked together, then you may use lower values for chests and things that move to reduce exposure index "s" and blur (think of taking a photograph at a 100m sprint), otherwise you are mainly trying to maximise the mA which will give better contrast. Increasing the kV gives increased overall exposure of the film (more blackness), but adds little to the contast seen between fat and soft tissue opacities. Contrast just means how easy it is to tell apart 2 different shades of grey - if the fat-opacity-grey and soft-tissue-opacity-grey are not that different (less contrast) then it's harder to see a clear picture.

Dave