German physicist Wilhelm Röntgen (1845–1923) was experimenting with electrical current when he discovered that a mysterious and invisible "ray" would pass through his flesh but leave an outline of his bones on a screen coated with a metal compound. In 1895, Röntgen made the first durable record of the internal parts of a living human: an "X-ray" image (as it came to be called) of his wife’s hand. Scientists worldwide quickly began their own experiments with X-rays, and by 1900, X-ray was widely used to detect various injuries and diseases. In 1901, Röntgen was awarded the first Nobel Prize for physics for his work in this field.

An X-ray is a form of high-energy electromagnetic radiation with a short wavelength capable of penetrating solids and ionizing gases. X-rays are emitted from an X-ray generator and directed toward a specially treated metallic plate placed behind the patient's body. As the beam of radiation passes through the patient, it forms shadows of tissues and structures inside the body on the X-ray plate. X-rays are slightly impeded by soft tissues, which show up as gray on the X-ray plate, whereas hard tissues, such as bone, essentially block the rays, producing a light-toned "shadow." Thus, X-rays are best used to visualize rigid body structures such as teeth and bones. A contrast medium, like a radiopaque barium solution, can be ingested to help visualize soft tissues of the upper digestive tract to show the contours of the stomach and intestine.

Like many other high-energy radiations, X-rays are capable of damaging cells and causing mutations that can lead to cancer. Despite their widespread use, this danger of excessive exposure to X-rays was not fully appreciated for many years. Today, the impact of radiation on patients and operators is attenuated by proper shielding and limiting exposure.

Refinements and enhancements of X-ray techniques have continued throughout the twentieth and twenty-first centuries. Although often supplanted by more sophisticated imaging techniques, the X-ray remains a standard in medical imaging, especially for viewing fractures and dentistry.

This text is adapted from Openstax, Anatomy and Physiology, Section 1.7: Medical Imaging