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I know the basics, that by measuring how much x-ray signal reaches each 'pixel' on the receiver we can measure how much has been absorbed. But this gives only a single channel of information, e.g a greyscale image. However I'm sure I've seen images where there are opaque white parts (bones) and opaque black parts (metal)... this seems to imply color and opaqueness are two separate channels and I cannot see how that works.

Can anyone go beyond the basics to describe the real way a fluoro scan ends up as a picture I can look at? Better references than wikipedia would also be welcome.

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I'm not sure the assumptions of your question are true. For example, in this picture, both the bones and the metal ring appear black. Can you link to examples of the white bones/black metal x-ray images you describe?

There are ways to obtain contrast with x-rays that do not rely on absorption, such as scattering. However, I think most medical x-rays are based purely on absorption.

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Not just scattering but also fluorescence and neutron spallation, but both these techniques would represent far too much dose to use in a medical setting. – dmckee Mar 9 '11 at 21:50
Your image is a very old, crude image. I'm talking about modern digital fluoroscopy/x-ray. – Mr. Boy Mar 11 '11 at 10:14
Then perhaps you could link to an example? It's much easier to discuss a specific image. – Andrew Mar 11 '11 at 12:00
@John, actually the old analog ones make it easier to understand the physics: digital images may have been heavily post-processed, and you'd need details of what was done before you could analyze the image for physics. – dmckee Mar 11 '11 at 20:31

By saying that bones and metal are opaque, you actually mean that they are opaque in the visible wavelength of light. They may or may not be opaque in other wavelengths such as X-ray. Thus the color in a X-ray image is not related to the opacity as observed through our eyes. But if your eyes were adapted to X-ray's instead of ordinary light, the X-ray image would kind of look like the negative of what you would see.

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""you would actually observe opaque bones and transparent metals (as seen in the X-ray image)."" Transparent metals ? Which kind of metal do You have in mind? – Georg Mar 9 '11 at 18:58
@Georg, John seems to have seen "opaque black parts (metal)". By this, I assume that he has see a X-ray image in which some metal appeared black. If your eyes where adapted to X-rays, such metals would look transparent. – Manu Mar 9 '11 at 20:45
As I explained I know how X-rays work. Metal is (quite) opaque to X-rays, far more so than tissue. – Mr. Boy Mar 11 '11 at 10:14

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