Saturday, 8 November 2008

How an X-ray image is formed

The best way to discuss the interaction X-rays with your body (and therefore how we get X-ray images) is to take a look at all the possible options when electromagnetic radiation reaches you. (Electromagnetic radiation was given brief coverage on the previous post.)

Atomic nuclei are surrounded by a cloud of electrons, as I'm sure you remember from school. The ones orbiting closer to the nucleus are at a relatively low energy level, whereas the ones further out are much more energetic. This is sometimes represented as on the right, with the electrons in their orbitals of varying energies.

Electromagnetic radiation is carried in the form of photons, which have a specific amount of energy. Gamma ray photons have the most energy, followed by X-ray photons, and so on through visible light all the way to radio waves, whose photons carry very little energy at all.

So what happens when a photon reaches an atom? (Remember, this is the same thing as asking what happens when X-rays/visible light/radio waves/etc. reach matter.) If conditions are just right, an electron orbiting the atom can absorb the photon, and this extra energy it incorporates bumps it up an energy level - it now temporarily orbits in a higher orbital. This unnatural state usually lasts just a fraction of a millisecond, and the electron soon emits an photon back. This means that it now has less energy again, and so it drops back to its original orbital.

A familiar example of this may be seen in what often happens when something is heated - it starts to glow. (Think think of coals in the fire, for instance.) In this case, the thing that has increased the energy of the atoms isn't usually a photon of any sort, it is heat. Nonetheless, the resulting process is the same. As the heat transfers just enough energy to the atoms, their electrons are raised by an energy level. As they fall backwards again, they emit a photon, which you see as the red-orange glow.

But there's a catch. This sort of interaction is only possible if the precise amount of energy carried in the photon matches the precise amount of energy required to bump an electron up an orbital. Too little or too much and the photon will just pass through the matter.

Thus the poor, rather energyless, radio waves pass through most matter without disturbing it at all. As I'm writing this, I'm sitting inside, listening to my radio. The radio waves happily pass through my windows, my walls, and even me (!), on their way to my radio. Are radio waves dangerous to my cells? Will I suddenly develop strange cancers thanks to the local radio station? Not at all, they don't have enough energy to do anything to me; they pass harmlessly through.

As we approach the part of the electromagnetic spectrum that our eyes are sensitive to, things start to change dramatically, though. The atoms that commonly make up our bodies (and most other things on the planet) tend to have electron orbitals at just the right distances (energetically) for the above absorption to take place. For instance, an apple appears red because it is absorbing all the other parts of the visible spectrum that hits it. Only red isn't that well absorbed, and some of this particular wavelength's light is reflected back to us; we thus see the object as being "red".

What about X-rays? The energy their photons carry is usually too high to be absorbed by most the atoms that constitute us (mainly hydrogen, carbon and oxygen), and so in these cases they will pass on through. This was what astonished Roentgen, and the rest of the world, with his cardboard, and later his wife's hand). But the bigger the atoms, the larger the differences between their orbital's energy levels. A big enough atom would still be able to absorb X-rays. So, are there any places in our bodies that are rich in larger atoms?

Well, our bones contain a great deal of calcium, don't they? And, weighing in at an impressive three times as big as carbon, calcium is just large enough for our needs. Sure enough, X-rays hitting carbon interact as readily as visible light interacted with our apple earlier. The same goes with slightly larger atoms too, which is why the coin your nephew has just managed to lodge in his oesophagus will be visible on his chest X-ray in the emergency room.

This is basically why X-rays produce the images they do. If you put black-and-white photographic film behind me and then shone a bright light at me, you would get a nice image of my shadow. No light would get through me, since my atoms interacted well enough with the visible spectrum to absorb any part of it headed my way. Since photographic film turns dark when exposed to light, my shadow would look white. (Most photographers prefer to reverse this negative, since the image is then closer to what we see with our eyes, but there's no reason why this should necessarily be done.)

The same goes for X-rays, except that now we are obviously using X-ray film instead of standard photographic film. The X-rays that get past me entirely will make the film black. Those that pass through more or less intact, but with a few X-rays having been absorbed, will leave the film a shade of grey. And parts that, like the visible light above, have been absorbed more or less entirely will leave a white shadow on the film. X-ray images are quite literally shadows - except ones made after X-rays, rather than visible light, have passed by.

The above image shows all this wonderfully. It is an image of a patient's knee joint - the patient was suffering from osteoarthritis. The bones are beautifully illuminated in white. Near the top of the image you can see a grey smudge on either side of the bones - that's the soft tissues (muscle, fat, skin, etc.). And the blackness at the image's peripheries is where there was nothing at all, and so the X-rays thudded harmlessly into the X-ray film. (The arrows point to some of the X-ray features of osteoarthritis - click here if you're interested.)

That ought to do it for now. I'll write one more post on X-rays tomorrow, dealing with why they can be bad for you.

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