Excellent question, John. The short answer: because the minerals (mostly calcium and phosphate) in bone absorb the X-rays and don't let them get to the film (or the imaging chip) used to collect the image. The slightly longer answer: X-rays are really just a very energetic, very short wavelength form of light. Instead of bouncing off the surface of most solid things the way visible light does, X-rays can penetrate. They just plow right through, and don't often interact with atoms. But the more mass (matter) you put in the way, the more likely it is that a given X-ray photon (that's one light "particle" or quantum) will be blocked, either absorbed or scattered off its track, by an atom somewhere along its path. If enough matter is in the way, none of them will get through. Bones have more mass than the surrounding soft tissues for 2 reasons. First, the mineralized part of the bone, the solid white part, is very densely packed with atoms stacked in tight little crystals called hydroxyapatite, mixed in with protein fibers and saturated with water. Second, the atoms themselves that make up the mineral crystals (as I said, calcium and phosphorus) have heavier nuclei made of more protons and neutrons than most of the other atoms in your body (mainly hydrogen, oxygen, carbon, and nitrogen). The bigger nuclei make bigger targets for the X-rays to hit and the higher number of them, due to the tight crystal stacking, make more targets to hit. The end result of this is that X-rays have a higher chance of running into something that can stop them when they try to pass through bone than they do when going through the softer tissues that make up the rest of you. Now, as you may know, blood has iron in it, and so you might think it should block X-rays like bone does. Well, it turns out that every atom of iron in your blood is outnumbered by thousands and thousands of lighter atoms, so it doesn't really amount to much, only about enough to make one medium-sized nail out of all the iron that's spread throughout your body! The X-ray technology we use today took years of work and experiment to fine-tune. You have to give just the right dose of X-rays to penetrate soft tissues but still be blocked by the bones to get a good image. Sometimes patients are given special liquids that absorb X-rays so other tissues, like the digestive tract or circulatory system, can be made visible on X-rays. Nowadays, films are made to be much more X-ray sensitive than in years past. That, along with computer-aided detection with solid-state "cameras" have helped reduce the dose needed to give a good picture of what's inside. Why does this matter? Because the other thing that X-rays can do as they pass through your body is break DNA and other molecules they run into. The very low doses commonly used these days for medical and dental purposes cause minimal damage easily handled by your body. But at high doses, X-rays can eventually cause real damage to tissues, killing cells and even causing cancer. That's why people who take X-ray pictures all day long, like dental assistants and radiology technicians, leave the room when they take pictures with X-rays. They don't want to accumulate enough damage to cause any problems for them later on, so they follow safe procedures to protect both you and them from any harm while getting the benefits of being able to see exactly what's going on in your bones without having to open you up to look!! I hope this answers your question, and let me know if you need more information. Paul Odgren, Ph.D. Cell Biology UMass Medical School Worcester
Try the links in the MadSci Library for more information on Anatomy.
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