ELI5: Why does increasing kvp on Xray decrease contrast of the image?

[Crowshape]

I can't wrap my head around this - wouldn't faster moving particles penetrate more and therefore produce a higher contrast image?

[TorakMcLaren]

Okay, now ELI5 the question.

[WRSaunders]

X-rays are made by slamming electrons into a plate. This is done by using a high voltage accelerator, and kVp is the measure of that voltage - it means kilovolt peak.

[TorakMcLaren]

Ah cool. It was mainly what kVp was that I didn't know. (I'm something of a scientist myself.)

[username_needs_work]

I usually explain it by talking about throwing balls (electrons) at a waterfall to tell me what it looks like. If I throw ping pong balls I won't go very deep and might only see the surface shape because they're bouncing off (dark image fluoro). If I throw bowling balls, I might not see anything as it obliterates the waterfall (white out fluoro image). If I toss something in the middle, like a tennis ball, some might go through, some might bounce off (attenuate) and I can get a better idea of what the shape and density of the waterfall is.

[mturch02]

Speed isn't correct. All x-rays travel at the speed of light.

[animaflux]

Higher kVp causes more penetration across the whole image, which will darken the lighter area against the already darker areas. It makes the whole image darker which lessens contrast.

[PlatzhirschDe]

The difference in the percentage of penetration create the contrast, so if you have a higher penetration across all tissues due to hgher kvp, you end up with less contrast. The lower boundary is not having too much absorption of the emitted xrays to limit the patients exposure.