What Are X-Rays? And How Does Radiography Work?
X-rays (often written x-rays or xrays) are a type of electromagnetic (EM) radiation. The term X-ray is shorthand for X-radiation, so named simply because it was an unknown form of radiation when discovered. Its discoverer was Wilhelm Röntgen, which is why X-rays are sometimes called Röntgen rays in other parts of the world. Röntgen was the recipient of the first Nobel Prize in Physics for his discovery.
X-rays have a shorter wavelength and higher frequency than ultraviolet radiation, but lower frequency and longer wavelength than gamma rays.
If you're wondering what EM radiation is, what terms like frequency and wavelength really mean, and how X-rays allow people to make important medical images as well as help us in a host of other endeavors, then you've arrived at a good place. Here you can explore any or all of those subjects and, by doing so, arrive at a better understanding of X-rays and how we use them in our lives.
On this page you'll find:
- Basics of electromagnetic force and radiation
- Where do X-rays fit into the EM spectrum?
- How do people produce X-rays?
- X-rays in medical imaging
- Advantages and disadvantages of the medical applications of X-ray
- Non-medical applications
Basics of electromagnetic force and radiation
One of the four fundamental forces in nature (along with gravity, weak nuclear force, and strong nuclear force), electromagnetic force was discovered in the 1800s when scientists realized that electricity and magnetism were not two separate forces, but instead were the result of the same underlying force. We’re all pretty aware of the effects of gravity on a daily basis (some of us may experience painful reminders more often than others), but believe it or not, gravity is actually the weakest of those four forces.
LET'S BACK UP EVEN FURTHER - WHAT IS FORCE?
Force is essentially a pull or push on something by something else. In an interaction where I punch a boxing bag, I am imparting a force on that bag (and probably hurting myself in the process). But force can be imparted without anything touching at all. Gravitational force between celestial bodies compels our planet to orbit the sun and compels the moon to orbit us. And magnets exert force without touching as well.
Within an atom, there are positively charged particles (protons) and negatively charged particles (electrons), as well as neutral particles (neutrons). Neutrons and protons are found within the nucleus of the atom, which is why they are called nucleons. Electromagnetic force attracts or repels objects, depending on whether objects are oppositely charged or charged the same. Within the atom, the positive charge of the nucleus (thanks to protons) keeps the negatively charged electrons bound around it in the atom. And during a chemical reaction, these same electromagnetic forces bind atoms and molecules together.
If like particles repel one another, you may wonder, what keeps the protons within an atom’s nucleus from repelling each other so much that the atom rips apart? The answer is strong nuclear force, which is therefore even stronger than EM force! Together, the strong nuclear force and EM force keep an atom intact. Weak nuclear force, meanwhile, bring stability into the picture by enabling radioactive decay of particles (neutrons breaking down into protons and electrons).
Every chemical reaction is a result of electromagnetic force behind the interaction of electrons in different atoms and the resultant momentum of those electrons as they subsequently move. We ought to be just as aware of electromagnetic force as we are of gravity in our daily lives – electromagnetic processes surround us!
Some electromagnetic processes create a type of energy called electromagnetic radiation. It’s called ‘radiation’ because this energy radiates away from its origin. “Electromagnetism” is a term for the study of EM force. According to classical electromagnetism, electromagnetic force takes the form of waves. These electromagnetic waves vary tremendously in wavelength and frequency, creating an entire spectrum of electromagnetic radiation traveling maximally at the speed of light. In a medium, light typically travels at a speed less than the “speed of light” – 300,000,000 meters per second – and different types of light may travel at different speeds as well.
WAIT A MINUTE - WHAT KIND OF WAVE ARE WE TALKING ABOUT HERE?
These waves are known as ‘transverse waves’ because they oscillate perpendicularly to the direction in which they are radiating. EM waves are the product of electric and magnetic oscillations perpendicular to each other as well as to the direction the wave is traveling. Measurable properties of these waves include wavelength, frequency and energy.
- Wavelength is the measurement of distance that a wave travels before a repeat in the wave cycle.
- Frequency is the number of wave cycles that would pass by in a given amount of time – measured in hertz, where 1 hertz (Hz) equals one wave cycle per second. With speed of light as a constant, frequency and wavelength have an inverse relationship to each other, such that shorter wavelengths will mean higher frequencies and vice versa.
- The higher the frequency of the waves, the higher the energy they carry. Quantum theory tells us that these waves consist of photons, which are particles that have no mass, but carry energy. Photons are the quanta of EM radiation, and their energy is measured in electronvolts (eV).
Where do X-rays fit into the EM spectrum?
The electromagnetic spectrum spans EM waves with frequencies ranging from about 3 hertz on the low end (extremely low frequency waves) to around 300 exahertz (or 300,000,000,000,000,000,000 Hz) for some gamma rays. These waves vary in wavelength from over 100,000 kilometers all the way down to a picometer, which is a trillionth of a meter. For perspective, the smallest atoms are about 62 picometers in diameter, and the diameter of the earth is 12,742 kilometers.
On the very low frequency end of this spectrum are radio waves. The lowest frequency radio waves are produced in nature by such things as lightning and the earth’s magnetic fields, but we humans also contribute them:
- Intentionally, for use in submarine and mining communication for instance, because these waves can travel through sea and earth. We also generate higher frequency radio waves for communications, broadcasting and radar.
- Unintentionally, as a byproduct of electrical power grids.
Moving up from there in the direction of higher frequency and energy, we successively find:
- Microwaves - We cook food with these.
- Infrared – So named because infrared is lower frequency than red visible waves. Infrared is what we often associate with heat, even though all EM waves can confer some amount of heat.
- Visible light – Red, orange, yellow, green, blue, indigo and violet, in that order from lowest to highest frequency. All the colors of the rainbow!
- Ultraviolet (UV) – So named because ultraviolet is higher frequency than visible violet waves. We wear sunblock to protect our skin from the harm UV can inflict.
- X-rays – The overarching focus of this page, and how we might see a broken bone in the body.
- Gamma rays – Produced by a particular kind of radioactive decay called gamma decay, as well as some other things like lightning for instance.
X-rays span a range of wavelength, frequency and energy:
- Wavelength ranges from 10 nanometers to 0.01 nanometer.
- Frequency ranges from about 30 petahertz to 30 exahertz (30,000,000,000,000,000 Hz to 30,000,000,000,000,000,000 Hz)
- Energy ranges from 100 electronvolts (eV) to 100 keV (100,000 eV).
We humans have historically carved up the electromagnetic spectrum in this way not because there are fundamental differences between these waves beyond their different wavelengths, frequencies and energies, but instead for a number of other reasons that at times seem rather arbitrary or misleading; by any method of delineating them, the borders can get a bit fuzzy. In some cases, we define them based on their differing effects on us and the physical world around us. At other times, we may define them based on their origins. However, at various times these modes don’t completely succeed. What do I mean? Here are a couple examples!
- Infrared gets its name because it is lower frequency than “red” EM waves in our visible portion of the spectrum. But although it occupies a part of the EM spectrum outside of the “visible” portion, people can actually see some infrared waves (just the highest frequency, shortest wavelength portion). Should we redefine the border between visible and infrared waves?
- There isn’t a defining difference between X-rays and gamma rays that achieves total agreement among scientists. One mode of differentiation historically was by source, with gamma waves coming from the nucleus of an atom during radioactive decay and X-rays coming from electrons. But in reality there are other processes by which the radiation is produced, and the process isn’t always even discernible. Distinguishing one from the other happens often based on a sort of combination of context, history, source and application.
How do people produce X-rays?
An X-ray tube is a device for producing X-rays. In it, a cathode releases electrons, and the tube’s high voltage sends those electrons racing toward a piece of metal, the anode (often tungsten, though the type of metal for the anode depends on the application). When electrons hit the metal, the tube produces X-rays. The energy level of this radiation is limited by the tube’s voltage. Though the purpose of this device is to produce X-rays, actually about 99 percent of the energy produced takes the form of heat.
Another way of creating X-rays – particle-induced X-ray emission (or PIXE) – sends positive ions (rather than electrons) racing toward a substance, the result being X-ray emissions from the substance. The X-ray emission varies by substance, creating a recognizable signature of sorts, which is very useful to a fascinating array of professionals, from geologists to art historians.
X-rays in medical imaging
Our oldest use of X-rays (literally dating back to the month after Röntgen discovered them) is in medical imaging; this application is known as medical radiography. A radiograph is the image produced using X-rays. The professionals who operate the necessary equipment are known by a variety of titles, though radiologic technologist is considered the most accurate title today. Often in different facilities they may be called radiologic technologists or technicians; X-ray technologists or technicians; and also radiographers.
This is the type that first comes to mind for many people. Here are the basics of how it works.
- There is an X-ray generator, a device that produces the radiation by means of an X-ray tube as described above.
- There is a device that detects the X-rays – nowadays this will typically be a digital X-ray detector (such as a flat panel detector), but could perhaps be a photo film. This device will essentially convert the X-rays into visible light so as to produce images that the human eye can see.
- Between the X-ray generator and the detector, the medical team will position the object or body part that is the subject of the imaging.
- The X-ray emitting device sends the X-rays toward the object, which casts varying “shadows” upon the X-ray detector behind it (hence the archaic term, skiagrapher).
The relative variance of the shadows depends upon the density of the materials within the object or body part. Dense, calcium-rich bone, for instance, absorbs X-rays to a higher degree than soft tissues that permit more X-rays to pass through them en route to the detector, making X-rays very useful for capturing images of bone; bone is subsequently said to be a radiodense substance, in contrast to the softer tissues in the body that are considered more radiolucent. But X-ray is also very good for capturing images of certain pathologies that affect other types of body tissue as well – for instance, breast and lung cancers. What makes X-rays so good at capturing images from our bodies is that the human body is not a homogenous lump of equally dense substances – it is our bodies’ inconsistencies in density and composition that make radiography work well.
In projection radiography, there is much room for adjusting the energy level of the X-rays depending on the relative densities of the tissues being imaged and also how deep through a body the waves must travel in order to achieve the imaging.
- Images of bone – for instance, to examine a fracture or for diagnostic measures related to bone conditions like osteoarthritis or certain cancers – require high-energy X-rays because of the high density of bone.
- Images of soft tissues like lungs, heart and breasts (both chest X-rays and mammography are very common diagnostic applications of X-rays) require relatively less energy from the X-rays in order to penetrate properly and achieve excellent images.
- In order to achieve these different energies, technologists use X-ray generators of different voltages and equipped with anodes made of different metals.
When the diagnostic work is complete, a radiologist (the doctor in a radiology/diagnostics department) will review the images along with the referring doctor, such as an orthopedist, sports medicine doctor, oncologist, primary care doctor or cardiologist.
Fluoroscopy produces X-ray ‘videos’, as it were, allowing for the analysis of tissues or substances as they move within the body, as well as useful supportive imaging for surgeons during procedures. Additionally, these ‘videos’ can now be recorded and stored, of course, whereas in the past they were created in live time but never kept.
Thomas Edison coined the term fluoroscopy while experimenting with X-rays. In fluoroscopy, the same principles of projection radiography were combined with a fluorescent screen technology, and the result traditionally was a live visualization with movement. Nowadays, the medical community uses the same digital detector screen technology for fluoroscopy as it uses to achieve standard projected radiography, significantly reducing the amount of radiation absorbed by the patient in the process.
Angiography is a process in which contrast agents are injected into the bloodstream via a guided catheter in order to enable X-ray imaging of our circulation. The resulting angiogram can be either film (capturing movement, like fluoroscopy) or still images. The contrast agent will be a higher density than our blood, enabling high quality images of our blood vessels. Although the term historically implies the use of projected radiography, nowadays CT angiography has emerged as another form of angiography (read more about computed tomography below). Angiography has many applications – among them, detecting a clot (or thrombus) in a vein; evaluating coronary artery disease; locating an aneurysm; or guiding the placement of a stent during a procedure.
CT (COMPUTED TOMOGRAPHY)
Also sometimes called a CAT (computerized axial tomography) scan, the CT scan is a powerful diagnostic tool that involves many X-ray images of a body part captured simultaneously from various angles. While this is a form of radiography, typically the term isn’t used when referring to computed tomography.
The CT scanner is shaped kind of like a doughnut, with the patient inside the doughnut hole during the scan. The data from all of these X-rays gets processed and combined by a computer to produce very detailed, segmented images. In effect, the CT scan creates the image equivalent of a series of surgical cross-sections in a desired plane, without a single cut to the body. Today, computers can further combine these segments to produce highly detailed, three-dimensional depictions as well.
CT scans can help doctors diagnose a variety of medical problems.
- A CT scan of the head, for example, proves useful in detecting tumors, tissue death, bleeding in the brain and damage to the skull. They can be performed so rapidly that paramedics often use a CT scanner in their ambulances when transporting a patient who may have suffered a stroke or injury to the head.
- Computed tomography provides higher resolution images than traditional two-dimensional radiography; the varying densities of body tissues enable X-rays to provide the medical team with good 2D images, but sometimes tissues are so close in density that the traditional projected radiography fails to convey the differences. In these cases, CT scan can provide images with higher contrast resolution.
- Certain parts of the human anatomy, like the alveoli and bronchioles within the lungs, could not be captured by 2D radiography in ways that would provide effective diagnostic aide.
- Although computed tomography does rely on high-energy X-rays, new developments may ultimately lessen the amount of radiation exposure to patients in certain circumstances; CT colonography, for example, could possibly subject a patient to lower radiation than a traditional barium enema when examining the large intestine for polyps and colon cancer. Investigation of these possible breakthroughs continues.
- The use of IV contrast agents in conjunction with computed tomography provides valuable imaging of pulmonary vessels when diagnosing pulmonary embolism, as well as vessels in the brain.
X-rays are not only used for diagnostic purposes, but also treatment itself. In fact, radiation therapy (or radiotherapy) is nearly as old as radiography; soon after Röntgen’s discovery, a Chicago-based physician tried to cure a person of breast cancer using X-rays.
Radiation therapy is the application of X-rays as treatment for a variety of cancers. In it, X-ray beams are directed from multiple angles to converge upon the site of the cancer, ensuring that the bulk of the radiation exposure focuses on the malignancy instead of healthy tissue. Some tissues immediately surrounding the cancer will be deliberately subjected to heightened radiation, though, in cases where there is concern of spread (metastasis) or to preclude the possibility.
Most major types of cancer are receptive to some form of radiation therapy, and it is often prescribed in conjunction with chemotherapy. However, different cancers vary in their radiosensitivity (the term used to describe the degree to which they are susceptible to destruction from high-energy radiation). If a type of cancer (such as lymphoma) is very radiosensitive, that means relatively low radiation doses will kill the cancer pretty quickly. Other cancers are considered radioresistant because they are not easily killed by exposure to radiation, requiring higher doses that pose increasing risk to the patient. Radiation therapy is shaped partly by the radiosensitivity of the particular cancer as well as its location, the stage of the cancer and the state of the patient.
Advantages and disadvantages of X-rays in medicine
The strengths of X-ray medical imaging are abundant, with medical teams reaching countless diagnoses in efforts to treat patients suffering from numerous conditions. Fractures, tumors, blood clots, arthritis and osteoporosis are just a few examples of the debilitating or life-threatening conditions that X-rays have helped medical professionals visualize and diagnose in order to provide patients with the most effective care.
However, with these great benefits also come significant risks of over-exposure. X-rays are part of the spectrum of EM radiation known as ionizing radiation, because their energy is so high that they can strip electrons from an atom. When we think of ionizing radiation popularly, radioactive substances and nuclear power plant accidents may come to mind, because the energy given off by these is enough to ionize atoms as well. In reality, X-rays, gamma rays and the upper frequency portion of UV radiation all have the high energy necessary to ionize atoms. This is why no one should dismiss the danger of sunburns and the subsequent need to protect our skin from overexposure.
What does that mean for human health? The greater the exposure to ionizing radiation like X-rays, the greater the risk will be for cell damage, cancer and possibly death. It’s important to put the exposure in proper context; some procedures deliver higher amounts of radiation than others. CT scans deliver a particularly high amount; one CT scan of the chest would be roughly equivalent to 5 years worth of normal, background radiation. A chest X-ray, meanwhile, exposes us to the same amount of radiation that we’d normally absorb in about 10 days. When you visit the dentist, and sometimes the dental assistant administers radiography, you’re exposed to about a day’s worth of typical background radiation during that procedure – far less than other types of radiography.
Risk of developing cancer in a person’s lifetime due to high-energy X-ray exposure, such as from CT scans, varies with age as well – the younger a person is exposed, the greater the risk in that person’s lifetime. As of 2007, 4 million of the roughly 62 million annual CT scans administered in the United States were for children.
Medical teams try to balance the beneficial applications of X-rays with the potential health risks from exposure to this high-energy form of EM radiation. Because of the need to weigh the risks of these procedures alongside the risks of a particular medical condition, there are well-recognized guidelines in place (based on statistical analysis) for the use of CT scans on certain areas of the body (such as the lungs and brain) when certain pathologies are suspected. Thankfully, developments in digital detection technology for radiography reduce the amount of radiation exposure to patients during projected radiography and fluoroscopy. Not only that, but it’s also become clear that radiography isn’t the best diagnostic tool for certain situations where previously it may have been used. For instance, MRI provides greater diagnostic power than a CT scan when examining the brain to determine the cause of headaches, which also spares the patient from possible risks of exposure to ionizing radiation. When possible, doctors rely on other diagnostic procedures, such as ultrasound, that provide excellent imaging in many circumstances and do not carry the risks posed by X-rays.
Aside from the risk of exposure to ionizing radiation, roughly half of CT scans involve the use of contrast agents, which pose a certain risk to patients. Possibly up to seven percent of people who are given contrast agents (whether for CT scan or MRI) suffer kidney damage as a result of it.
Non-medical applications of X-rays
X-ray hair removal notwithstanding, humanity has found many useful applications and purposes for detecting X-rays throughout the past century. Here are a few examples of how X-rays are either applied or detected in order to aid us in various endeavors:
- Industrial radiography and CT. Similar in concept to medical radiography, X-rays help to identify any flaws in the solidity or structural composition of industrial parts.
- X-ray crystallography. This is a process wherein X-rays are shot through a substance, revealing its unique crystalline structure via the diffraction patterns of the exiting X-rays. This application has proven extraordinarily important throughout the past century in a wide variety of ways – the study of DNA, the measurement of atoms, the study of chemical bonds, our understanding of vitamins and amino acids, and more.
- X-ray astronomy. Ever since the 1940s, astronomers have known that the Sun emits X-rays, though these emissions are roughly a million times less than the visible light radiation it emits. It wasn’t until the 1960s that astronomers led by Riccardo Giacconi discovered another source of X-rays from space, in the Scorpius constellation. Unlike our Sun, this source emitted far more X-ray radiation than it did visible light. Since that discovery, a whole field of X-ray astronomy emerged, using special telescopes in space rather than terrestrial telescopes. These X-ray telescopes can detect objects at longer distances than traditional telescopes. Astronomers now recognize that X-rays are a feature of black holes and neutron stars.
- X-ray microscopes. These microscopes use lower-frequency X-rays to create images of objects that would be too small for light-based microscopes to capture in such detail. As an added benefit, the fluorescence caused by X-rays allows for analysis of the chemical composition of the substance being visualized.
- Security. Airports and border crossing locations use X-rays to inspect luggage and cargo (and sometimes human beings, using a certain type of full-body scanner).