Editor’s note: This post got a little long and technical, particularly the section on physics. If physics is not your thing, feel free to skip to the third section (Smile, it’s time for your photon bombardment!).
An X-ray, or diagnostic radiograph, is a shadow, pure and simple. Whereas visible light creates shadows we can observe with the naked eye, X-rays are invisible, and their “shadows” take a bit of work to see. The esteemed physicist and hipster-ahead-of-his-time Dr. Wilhelm Röntgen first produced and detected X-rays in the late 19th century (further reading here), and the basic principles of radiography—taking X-rays—has not changed much in the last 100 years.
Sit back, relax, and let’s talk physics
Might I suggest pouring a glass of adult beverage of your choice, because it’s time to discuss the physics of radiology. We are entering a world of photons, electrons, and gamma rays, produced right under our noses in nearly every hospital and doctor’s office in the country. Regardless of your feelings about physics, it’s probably not a bad time to have your first sip right now.
X-rays are invisible beams of high-energy electromagnetic radiation. What is electromagnetic radiation? First let me answer the easier question of where it is. EVERYWHERE. At every moment of every day, it moves all around and through us, ubiquitous on Earth and throughout the universe. The so-called electromagnetic spectrum includes gamma rays, X-rays, ultraviolet light, visible light, infrared light, microwaves and radio waves, a seemingly hodgepodge and loosely-related collection. Common among these waves is how they propagate through space: via pure-energy oscillations in electric and magnetic fields. It is the variable size of these different types of waves which accounts for their disparate behaviors. Oh, they also travel at the speed of light.
X-rays sit near one end of the electromagnetic spectrum: the badass end. As we just learned, electromagnetic radiation travels as invisible waves through space, with the energy of the waves dependent on wavelength (the distance between the peaks of adjacent waves) and frequency (the number of waves passing per second). Perhaps counterintuitively, electromagnetic waves are more powerful if they are small and closer together, i.e., short in wavelength. X-rays, near the badass end of the spectrum, have incredibly small wavelengths of about 0.1-10 nanometers—about the diameter of an atom—and resultant very high energy.
Go on. Have another drink. It’s helpful, isn’t it?
To further complicate matters, electromagnetic radiation sometimes behaves more like a discrete particle (called a photon) rather than a pure-energy wave—deemed the wave-particle duality. So when you hear X-rays sometimes described as photons and sometimes as radiation, remember it’s all just those crazy X-rays.
You may be wondering how we safely produce these high-energy juggernauts in hospitals and office parks, each and every day, all around the world? And how do we use them to create an image?
Tungsten, what is it good for? X-ray production1
X-rays are generated by an X-ray tube—a heated tungsten filament inside a glass-enclosed vacuum—with superficial similarities to an incandescent light bulb. When the metal filament is heated by an electric current, it creates a surrounding cloud of electrons, tiny negatively-charged particles. A separate current is then applied from one end of the tube to the other, directing the negatively-charged electrons to fly across the tube toward a positively-charged plate of (again) tungsten, called the anode. The electrons strike the tungsten anode and, via the magic of physics, X-rays are born! Oh, we could talk for pages about how exactly this occurs—including my personal favorite German physics term: bremsstrahlung (braking radiation)—but we should probably move on.
Smile, it’s time for your photon bombardment!
Now that we are experts in physics (PhDs really), we can entertain precisely what occurs when we fire radiation at someone in the radiology department. A modern X-ray system has three components: X-ray tube, collimator, and image receptor. Actually, the fourth and most critical “component” is you, the patient!
- X-ray tube: As we just discussed, the lightbulb-like X-ray tube produces our X-rays, which exit in a certain direction due to the orientation of elements in the tube combined with protective housing which prevents radiation from scattering elsewhere (where we don’t want it).
- Collimator: Think of a collimator like the focusing lens of a camera, except instead of controlling the light entering into a camera, a collimator controls the amount and shape of X-rays exiting the tube by opening or closing lead blades located on all sides. Collimators also use a mirror and regular light bulb to shine a “practice” beam of light onto the patient, simulating where the X-ray beam will hit when the exposure is taken.
- Image receptor: Ugh, my head hurts after 45 minutes of refreshing my knowledge about image receptors, so I will severely summarize a complex topic. After being produced in the tube and passing through the collimator, X-rays continue through and around the patient, projecting a “shadow” made of X-rays on the image receptor, located immediately behind the patient. With digital radiography, the most state-of-the-art system currently used, X-rays are directly detected by the receptor, and the number of X-rays hitting each part of the receptor panel form an image—the patient’s shadow. (Other systems, still in widespread use, need an intermediary step in which X-rays are converted into visible light using a “photostimulable phosphor,” and that light is subsequently used to create the X-ray image. The latter setup utilizes a large plastic cassette as the image receptor, which is fed into a machine and processed after the X-ray exposure is taken.)
Some of you may wonder why I haven’t talked about developing X-ray images on hard copy film—the kind you can actually hold in your hand—as is often depicted by TV or movies. This technique is essentially not used anymore, but if you are interested in a little history and further explanation, see footnote #2.
Why are bones white?
While this may seem like a silly question—of course bones are white, haven’t you ever had babyback ribs?—remember we are not talking about how bones appear to the naked eye, but how they appear on an X-ray image. It is quite coincidental that bones look white in real life and on X-rays; fat tissue, for example, looks yellow to the naked eye, but dark gray on X-ray. So, if the color of bones or fat does not account for the X-ray appearance, what does? It is the composition and physical density of the tissue.
As X-rays pass through the body, three things can happen: absorption into tissue, scattering (deflection in different directions), or passage straight through. When X-rays enter the chest, for example, they encounter bones, fat, fluid (blood and otherwise), and air (in the lungs), among other things. If the physical density of the tissue is high—it is packed together tightly—X-rays are more likely to run into the atoms and molecules of that tissue, and get stopped/absorbed. Tissue composition, in the form of the atomic number of its internal elements, also affects absorption (atomic number=number of protons in the nucleus). A higher atomic number, such as that of calcium in bones, results in more absorption of X-rays. If X-rays are scattered, they generally don’t provide useful information (just trust me on this), and you can think of them as contributing to the “blurriness” of an X-ray. Finally, X-rays that pass straight through tissue strike the image receptor and create our shadow image of the patient.
Back to our original question: why are bones white? Well, bones are dense, and have a lot of calcium, so they absorb a lot of X-rays and allow just a few to pass straight through. On the other end of the spectrum is air, in our lungs and around our bodies, through which almost all X-rays pass without being absorbed. Other types of tissue—fat, fluid, organs, and muscles—are somewhere in between, with fat generally allowing more X-rays to pass through than the others in that list.
Hold on, almost there. Here’s a picture of my cute dog.
The last piece of the puzzle is what happens when X-rays strike the image receptor. If most X-rays pass through (air and lungs) and strike the receptor, it is registered as black on an X-ray image. If most X-rays are absorbed (bone), it is registered as white. In between are shades of gray.2
X-rays: A step by step guide
After that long set up, let’s walk you through the process of getting an X-ray, elaborating on a few concepts.
- When a patient visits the hospital or imaging center, the X-ray technologist—an individual trained to take and process X-rays—will ask for some basic information, read the prescription for the X-ray, and lead the patient into the X-ray suite.
- In the X-ray suite, the technologist positions the patient to image the appropriate body part, which may mean standing up, lying down, or otherwise contorting one’s body. Pillows or sandbags are sometimes used to help.
- The patient is positioned directly in front of the image receptor, with the X-ray tube and collimator aimed at the patient.
- The X-ray technologist shines a rectangle-shaped light from the collimator onto the body part being imaged, to simulate where the X-ray beam will hit.
- The exposure button is pushed, and the patient and image receptor are bombarded by X-rays, creating a “shadow” of the patient on the image receptor.3 This takes a few seconds at most.
- Steps 2-5 are repeated for additional views, if necessary. For example, a chest X-ray is often taken from the front and the side.
- Depending on the type of image receptor, the X-ray image is either directly created, or the imaging cassette is processed by the technologist to create the image.
- Finally, the image is uploaded to the computer system for the radiologist to interpret.
Who needs X-rays?
X-rays are the most commonly performed study in radiology, and are taken of all parts of the body—chest, abdomen, spine, and extremities most commonly. Your doctor may order an X-ray for many different reasons, including these common indications: looking for pneumonia on a chest X-ray; evaluating for kidney stones or bowel blockage on an abdomen X-ray; assessing the degree of arthritis on a spine X-ray; or looking for broken bones on an extremity X-ray.
Several specialized uses of X-ray bear mentioning. Mammograms take pictures of breast tissue as a screening method to look for breast cancer. Fluoroscopy uses X-rays in real time—short movie clips—to aid in procedures, and is often performed by a specialist radiologist called an interventionalist. A CT (computed tomography) scan is also based on X-ray technology, a topic that I will address in an upcoming post.
As we conclude our journey through the radiant world of X-rays, let me address a few salient points and misconceptions to remember, if you should ever need an X-ray.
- X-rays are painless.
- X-rays are relatively easy and fast to obtain, and as a result are often the initial imaging test used to evaluate a problem.
- While very useful for certain pathologies, X-rays may not see everything that needs to be seen. By their nature, X-rays are a two-dimensional representation—a shadow—of a three-dimensional object—the patient. Sometimes, we need to see the body in three dimensions, in which case CT or MRI can be much more effective.
- Radiation dose, while not completely insignificant, is exceedingly small for any single X-ray.
- Some patients joke that they will be “glowing” from the radiation received during a procedure, but I assure them they will not set off any radiation detectors at the airport. The radiation in the X-ray suite stops immediately after the X-ray is taken, and is not carried in the body.
That was a lot of information, but now it’s time to take the last sip of that drink! Do you have questions/comments about this article or X-rays in general? Please let me know below.
- Bonus points if you get this song lyric reference. Seinfeld fans will have an advantage. Drumroll: “War, what is it good for? Absolutely nothing!” from “War” by Edwin Starr.
- Some of you may point out this doesn’t quite answer the question of why, so here is the detailed answer. When less X-rays strike the receptor, that part of the image is registered as white (like bone). But why white, and not green or blue? The answer harkens back to the days of early radiography, when X-rays were developed on large sheets of film, using chemicals in a dark room (only rarely done today). Traditional X-ray film consists of a emulsion of gelatin containing fine silver bromide granules. When these granules are hit by X-rays, they become more easily “developable” when exposed to the chemical developer solution, which converts silver bromide to metallic silver. Parts of the film not exposed have less metallic silver after development. The final step, called “fixing”, washes away the silver bromide, and leaves behind the metallic silver. Following this logic, areas on the film exposed to less X-rays (bone) retain more silver bromide during development, and that silver bromide is washed away during fixing. The effect on the physical sheet of film is to make these areas—which correspond to bone—more transparent, allowing more light to shine through from behind, ultimately making these areas appear more “white”; areas exposed to a lot of X-rays block the light and appear black. Modern digital X-rays are displayed in the same way, more by convention than necessity. Some radiologists actually like to invert the image, i.e., make bones appear black, to make things more conspicuous in certain situations.
- To prevent an overexposed or underexposed image (this can occur, just like with a photograph), automatic exposure controls (AECs) are built into the image receptor. When the number of X-rays hitting the AECs reaches a predetermined threshold, the X-rays beam shuts off.