Does it have to do with the density of the bone, like are x rays only absorbed by dense matter?
The intensity of the image is directly proportional to the density of the medium. X-rays show a lot of things, such as tumours, cysts, tendons, blood vessels, etc.. to various degrees. In a picture of a surgically repaired break, for instance, the screws and plates show up brighter than the bone, and the bone in turn brighter than the surrounding tissues.
If you select the x-ray energy spectrum to be just above the calcium K x-ray edge, the contrast for calcium is maximized relative to soft tissue. See plot and table, and sharp increase in x-ray attenuation between 4 and 5 KeV in
Very roughly the K-shell binding energy in calcium (Z=20) is
E = 13.6Z·2 eV =202· 13.6 eV = ~5440 eV
Thus the L-shell to K-shell transition energy is ~5440(1-1/22) = ~4080 eV.
Using differential x-ray energy subtraction measurements will enhance bone (calcium) contrast even more.
Cool, Bob. I didn't know that you can "tune" them.
Oh yeah, just adjust the voltage on the x-ray tube anode (actually I think the cathode voltage is adjusted). The x-ray spectrum includes both a continuous spectrum (bremsstrahlung) and characteristic x-rays from the anode material.
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