How X-Ray Vision Exposes Our Atoms

X-ray / Tomasz Walenta. A trip to the radiologist provides a reminder that we’re mostly made up of six elements, and just one reflects X-ray...

X-ray / Tomasz Walenta.

A trip to the radiologist provides a reminder that we’re mostly made up of six elements, and just one reflects X-rays strongly enough to form a clear picture. It would have been a lousy scene in an action movie. 

A radiologist retreated behind a transparent screen, and all my eyes and ears could detect was the clicking of her mouse and the red laser cross lined up on the back of my hand. But for a couple of seconds, she really had X-ray vision, capable of spying on my hidden internal architecture without me being able to feel it. I felt seen. And that, of course, was the point.

We are so used to being opaque that we don’t really think about it. The light that our eyes can see covers a relatively narrow range of wavelengths, which barely double as you go from violet, the shortest, to red at the long end. It’s a useful range because it passes easily through air and other gasses but is halted or redirected by most solids and liquids, helping us to tell where and what they are. But there’s no rule that all light has to do that, and over time we have learned that there are plenty of types of light that we can’t see.

When X-rays were discovered back in 1895—and named for their initially mysterious nature—it didn’t take long to realize that they whizzed straight through lots of solid things, but not all of them. Rather than distinguishing among materials of different colors, X-rays provide information about the fundamental types of atoms that different objects are made of.

That is extremely useful if, like me, you have recently slammed the heel of your hand into a very solid piece of metal at an extremely painful speed while opening an old wooden sash window. The clicks of my radiologist’s mouse set off a cascade of electrons playing bumper cars inside the X-ray equipment.

The human body is complicated, but from an atomic point of view, the overall recipe is simple. About 99% of our mass is made up of just six elements: oxygen, carbon, hydrogen, nitrogen, calcium and phosphorus. Almost all of the calcium and phosphorus is in our bones, woven together into a mineral scaffolding of enormous strength.

This is fantastically convenient because calcium is the largest atom of the six, with phosphorus second, and it is packaged into a very dense form in bone. Our smaller atoms barely interact with X-rays, so muscle, ligaments and other soft tissues only cast a faint shadow. But those densely-packed, large calcium atoms are very effective at stopping X-rays. So if you use X-rays to look at a human body, you are largely looking at where the calcium is.

The clicks of my radiologist’s mouse set off a cascade of electrons playing bumper cars inside the X-ray equipment. The electrons were accelerated by an electric field and then smashed at high speed into a high-density target made of the rare metal tungsten, spewing out a small number of X-rays with a huge side order of heat. The X-rays spent their brief existence whooshing through my hand, with most of them avoiding the heavy atoms of my bones to reach the other side.

It’s very hard to focus X-rays with lenses like we do with visible light, so the image appears as a shadow rather than resembling a photograph. Still, when the doctor showed me my image, I was surprised by how sharp it looked. My forearm and finger bones were neatly outlined in white because each bone has the densest, strongest material in its outer layer. The smooth pebbles of my wrist bones occupied the gaps in between, delightfully intact.

I was quietly proud of my wrist innards for being so pretty. As the doctor poked and prodded my wrist to explore the soft tissue damage, it already seemed to hurt less. It wasn’t what I had planned for my Sunday afternoon, but at least I got to see myself–my atomic self—in a new light.