The X Factor: 125 Years of X-Ray Innovation
It was 125 years ago this month (November 8, 1895 to be precise) that German physicist Wilhelm Conrad Roentgen (1845-1923) discovered the existence of X-rays. He was in his lab conducting experiments with a cathode-ray tube (then called a “Crookes tube”) consisting of a glass bulb encapsulating positive and negative electrodes in a vacuum.
The cathode tube was covered with heavy black paper, so Roentgen was surprised to see an incandescent green glow emitting from it when he applied electric current. The glow was reflected on a screen nine feet away. On further experimentation he found that the light would pass through most substances, including human tissue, but not solid objects such as metal or bone. Because he didn’t know what these mysterious light rays were, he called them “X” the mathematical symbol for an unknown variable.1
Wilhelm Roentgen (left); one of the first radiographic images, an X-ray of Roentgen’s wife’s hand, showing her wedding ring.
This discovery took the world by storm and within months medical radiography machines had been built and put into use—for example, by battlefield physicians to locate bullets in wounded soldiers.2 Roentgen was awarded the first Nobel Prize in Physics in 1901.
Three-quarters of a century later, in 1972, British engineer Godfrey Hounsfield and South Africa-born physicist Allan Cormack developed a new imaging method called computed tomography (CT), also called X-ray computed tomography or “CAT scan.” CT produces 3-dimensional images of an object derived from a series of flat X-ray images. This method revolutionized diagnostic imaging, and Hounsfield and Cormack were awarded the 1979 Nobel Prize in Physiology or Medicine.
Computed tomography images of the human brain, from the base of the skull to the top (Image Source: Wikimedia Commons, view scrollable image series).
X-Rays and the Electromagnetic Spectrum
Like all light waves emitted by the sun—visible and invisible—X-rays are part of the electromagnetic spectrum. X-rays, and their near neighbor gamma rays, have the shortest wavelengths by orders of magnitude: visible light wavelengths are in the region of 6000 angstroms, X-rays are in the range of one angstrom, and gamma rays are around 0.0001 angstrom.
Frequency and wavelength of the electromagnetic spectrum, including visible light and, X-rays (adapted from http://mynasadata.larc.nasa.gov/science-processes/electromagnetic-diagram/).3
Because X-rays (and gamma rays) have very short wavelengths and a high energy level, they can penetrate materials that light cannot, and they have the power to break the chemical bonds in those materials. Referred to as “ionizing radiation,” these wavelengths can ionize—or remove electrons—from an atom. Ionizing radiation can be dangerous; when it penetrates the cells of living organisms, the cells may mutate or die.
For example, excessive radiation exposure can cause human or animal cells to become cancerous. But while this power—to pass through materials that are impervious to visible light waves—can be dangerous, it is also what has made X-rays a valuable tool for a broad range of applications from enabling life-saving medical imaging to helping us understand deep space.
Medical Diagnostics. X-rays are the key diagnostic tool in multiple areas of medicine, including inspecting damaged bones, finding foreign objects inside the body, and identifying cavities in our teeth. A photo-sensitive film is placed behind the patient and X-rays are passed through the tissue, creating a shadow image of bones, metal, and other substances that X-rays cannot penetrate to a greater or lesser extent. Because repeated or extended exposure to radiation can cause damage to humans, typically a lead apron or shield has been used to protect parts of the body not being imaged (although recently medical experts are re-evaluating the efficacy of lead shielding4 to minimize exposure).
Cancer Treatment. Physicians have found a way to leverage the tissue-damaging properties of ionizing radiation as a weapon against cancer. Using controlled dosages of radiation (often X-rays but sometimes also protons or other energy waves) doctors can pinpoint specific areas of cancerous tissue with radiation to damage or destroy malignant cells.
Airport Security. X-ray scanning machines were introduced into airports in the 1970s, enabling checked luggage to be scanned for explosives or dangerous items. In 2007, Amsterdam’s Schiphol airport became the first to introduce full-body scanning machines, replacing the simple metal-detector wands that had been used previously to scan passengers for any weapons.
Today, roughly 500 scanners are in use at U.S. airports; about half use millimeter-wave (microwave) imaging technology that does not produce ionizing radiation, and the other half use backscatter X-ray systems using a very low-dose of ionizing radiation that presents “truly trivial risk”—one scan provides less than 1% of the total radiation a passenger typically will receive while in an airplane for 6 hours at cruising altitude, due to being in thinner atmosphere that allows more of the sun’s radiation through.5
Examples of backscatter X-ray body scan images, which can detect weapons, drugs that have been ingested, and other metal and objects for airport security. (Images Copyright © Unival Group)
Non-Destructive Materials Testing. Although medical applications took off immediately after the discovery of X-rays, industrial applications followed within the decade. Radiographic imaging can be used to understand the internal structures of a vast array of substances beyond just living organisms.
The field of non-destructive testing (NDT) emerged during the 20th century using not only X-rays but also gamma rays, ultrasound technology, and infrared imaging to inspect manufactured materials and components. Beyond its use as a medical diagnostic tool, X-ray CT allows testing of materials including metal, plastic, and composites to identify defects and analyze characteristics such as dimension, shape, and density.
X-ray images of a driver-side airbag (top), passenger airbag (center) and side-impact airbag (bottom) being inspected prior to vehicle installation to verify correct placement of the ignition components that will ensure the airbags inflate properly during an accident. (Image: Copyright ©Adaptive Energy)
Detecting Counterfeit Artwork. Many great artists reused canvases, painting over an older artwork. Clever art forgers may use an old but worthless used art canvas to paint a counterfeit version of a famous work (since a new canvas can be easily detected with ultrasonic testing). But with X-ray imaging, investigators can reveal the layers of paint underneath to detect the forgery. For example, X-ray might reveal that a supposed 17th century masterpiece covers a 19th century hack artwork, or it can establish the existence of an earlier painting underneath the visible one by an artist who was known to only use fresh canvases.
An art scientist uses X-ray technology to help evaluate a painting’s authenticity. (Photo credit: Richard McCoy via Wikimedia Commons)
Innovations in X-Ray Technology
As X-ray imaging advances into the future, scientists and innovators continue to explore new techniques. For example, medical technology experts anticipate that combining artificial intelligence with radiology could improve the contrast and spatial resolution of diagnostic images and help eliminate visual noise.
A recent breakthrough is the emergence of color X-rays, after 125 years of black-and-white images. Through a decade of development, scientists have demonstrated color X-ray imaging using hybrid pixel-detectors based on advanced chip technology developed at CERN, Europe’s leading physics research organization.6
A 3D image of a wrist with a watch showing part of the finger bones in white and soft tissue in red. (Image: Copyright © MARS Bioimaging Ltd).
More recently, a research team at Florida State University announced the development of a new type of material that could be used to make X-ray detectors that are “greener” and more affordable. X-ray scintillators are a commonly used component of an X-ray detector that converts the radiation emitted by an X-ray into visible light. The researchers discovered that the chemical compound organic manganese halide could be used to create scintillators that are less harmful to the environment, enable flexible X-ray detectors, and cost less than existing technologies.7
What scientists haven’t found a way to do yet, however, is give humans the power of X-ray vision using just our eyes, like Superman. But stay tuned, you never know . . .
- “History of Medicine: Dr. Roentgen’s Accidental X-Rays”, Columbia University Irving Medical Center Department of Surgery. (Retrieved October 28, 2020)
- “History of Radiography”, NDT Resource Center. (Retrieved October 28, 2020)
- Tang, J., “Unlocking Potential for Microwaves for Food Safety and Quality”, Journal of Food Science, August 2015. DOI: 10.1111/1750-3841.12959
- Jaklevic, M., “That Lead Apron on the X-Ray Room? You May Not Need It”, The New York Times, January 14, 2020.
- “Are full-body airport scanners safe?” Harvard Health Letter, June 2011.
- Muller, R., “First 3D colour X-ray of a human using CERN technology”, CERN news release, July 17, 2018.
- Liang-Jin Xu, Xinsong Lin, Qingquan He, Michael Worku, Biwu Ma. “Highly efficient eco-friendly X-ray scintillators based on an organic manganese halide.” Nature Communications, 2020; 11 (1) DOI: 10.1038/s41467-020-18119-y