Re: Why does an x-ray photograph show only the bones?

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