The Physics Of X-Ray Imaging

So here is Part One of my series of the “Physics Of” medical imaging. First up is the most recognizable: X-ray Radiography.

Radiography (which uses x-rays, but the images are generally called “X-Rays”) are the most common form of medical imaging, and are incredibly useful. Thousands of images are performed everyday and medicine was revolutionized when this non-invasive means to study the body was discovered.

But how exactly do we get x-rays and use them for imaging?

Lets start with a bit of history. The first X-ray image was created by a guy named Wilhelm Rontgen in 1895.

Wilhelm Rontgen

Rontgen called them “X” rays because they were an “unknown” type of radiation, and the name kind of stuck.

The first image was of Rontgen’s wife’s hand, and is pretty cool because you can actually make out her wedding ring.

First image using X-rays of Wilhelm Rontgen's wife's hand

I actually find this a bit funny. I just picture a crazy looking physicist saying “Honey! C’mere! Stick your hand in front of this radiation for a second!”

Luckily for Mrs. Rontgen, x-rays, in small doses, are not very dangerous. So what exactly are x-rays?

X-rays are electromagnetic waves just like visible light, radio waves and microwaves. They have a wavelength range of roughly 0.01 to 10 nanometers (1 nanometer = 1 billionth of a meter).

When talking about x-ray imaging, however, its easier to think of x-rays in terms of photons. Photons are like tiny wave “packets” and electromagnetic waves can be described as a big collection of photons.

X-rays are generated in an x-ray tube (unsurprisingly). Basically, a bunch of electrons are shot at a piece of metal (usually tungsten, the same metal used in old school incandescent light bulbs). Now what happens next is a little complicated, but really cool…

So the electron travels at a certain speed toward the piece of tungsten; it has kinetic energy, which is the energy of motion. But as it gets close to the Tungsten it will run into an electric field produced by the metal, and will actually slow down.

X-ray Tube

Now, in physics there is principle called the conservation of energy. Basically this just says that energy can never be created or destroyed, it can only change form. So when the kinetic energy (energy of movement) of the electron drops (when it slows down) that lost energy has to go somewhere. Where it goes, in fact, is in the generation of an x-ray. The electron will actually emit an x-ray when it gets slowed down by the tungsten. Pretty sweet eh?

Schematic of X-ray tube. Electrons come in from the bottom, strike the tungsten target (the anode) and emits x-rays

This is actually a type of radiation called Bremsstrahlung, which is German for “braking radiation”.

Schematic Diagram of Bremsstrahlung

Ok, so now we got x-rays, how do we make an image?

Well, if we fire x-rays at, oh lets say, YOU! the x-rays will interact with your body. How you ask?

Well when an x-ray passes through the body, it may get absorbed or scattered by the body. An x-ray gets absorbed when the x-ray hits an electron in our body, and the electron “jumps” out of the atom. This is called the photoelectric effect.

The Photoelectric Effect

The x-ray may also get scattered. This just means that the x-ray will get close to the nucleus of an atom and get kind of turned in another direction due to the electric field of the nucleus. This is known as Compton Scattering.

Compton Scattering Effect

In spots of our body that very dense like bones, the x-rays have a much higher chance of getting absorbed or scattered than if they pass through muscle or fat, which are less dense. So if we were to stick a piece of film which is sensitive to x-rays behind someone getting a radiograph, you would get lots of x-rays hitting the film when they pass through muscle or fat, but very few pass through bones (or metal, if you’re really unlucky).

So on the radiograph muscles and fat show up dark, and bones show up white. BAM! Radiograph!

Chest Radiograph

See, now that wasn’t so bad was it? Pretty interesting if you ask me.

The next installment of my “Physics Of” medical stuff  series will be something that takes x-rays to the next level: Computed Axial Tomography, commonly called “CAT” scans.