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My brother asked the simplest question ever: why do X-rays not penetrate the bone?

I know it is something to do with the compact molecular structure of bone. What I really need to know is, what is the reason for this? What is the atomic distance between bone atoms that prevents X-rays going through them?

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I'm not sure whether it's true that the x-ray wavelengths used for medical x-rays can't penetrate bone. That's not necessarily the case. All that is required for imaging is that there is a contrast between the x-ray absorption of bone and that of human tissue. And I believe that the x-ray absorption factor of a material depends not so much on the distance between the individual atoms of the material but more on the electron density of the material. – Samuel Weir Mar 19 at 3:10

Bones absorb more xrays than soft tissue because of the Calcium in the bones (and the high density helps too, but the same densities of, say, Carbon or Silicon, wouldn't absorb as many x-rays). The high atomic number of Calcium (20), dramatically increases the photoelectric effect, which is the main mechanism of xray absorption by bones.

If you go through the other elements common in the body, you'll see that most have a lower atomic number. The other common elements: Carbon (6), Oxygen (8), Hydrogen (1), etc, just can't compete with Calcium. This also explains why Iodine (atomic number 53) works as a contrast agent (along with being relatively safe and water soluble).

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And how high atomic number element increases the photoelectric effect? – Sulav Sigdel Mar 19 at 4:10
    
@SulavSigdel it makes electron density higher because the nucleus has larger charge. – Ruslan Mar 19 at 13:22
    
@Ruslan: also very important is the higher binding energy of the electron which greatly increases the cross-section, which goes as approx $Z^2$. It's not just increased density because overall the photoelectric effect goes as approx $Z^3$. – tom10 Mar 19 at 14:10
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That it's not just the number of electrons (or mass density) is clear if one thinks about a hip or abdominal xray. There because there's so much more soft tissue thickness relative to bone thickness, if it were just the number of electrons, the contrast would be really low. But there's still good contrast because calcium electrons absorb a disproportionate amount of xrays compared to lower Z elements. This is even more vivid in the example of iodine filling a vein. – tom10 Mar 19 at 14:14
    
There's more to it than just density. A port film is a radiogram made with high-energy X-rays (> 1 MeV) in order to verify the position of a patient before a radiation therapy treatment. The contrast between the bones and the soft tissue is very low. – james large Mar 19 at 19:46

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