Radiography

Radiography is a method for seeing inside an object by using X-rays, gamma rays, or other forms of ionizing and non-ionizing radiation. It is used in medical diagnosis and treatment, industrial testing, and airport security, where body scanners often rely on backscatter X-ray technology.

In conventional radiography, an X-ray generator produces a beam that is aimed at the object. How much radiation is absorbed depends on the object's density and composition. The radiation that passes through is captured by a detector—either photographic film or a digital device—behind the object. This process of creating flat, two-dimensional images is called projectional radiography.

In computed tomography (CT), an X-ray source and its detectors rotate around the subject as it moves through a conical X-ray beam. Every point in the subject is crossed by many beams from different directions at different times. The data on how much each beam is attenuated is collected and computed to produce two-dimensional images on three planes—axial, coronal, and sagittal—which can then be processed into a three-dimensional image.

The origins of radiography and fluoroscopy both date to November 8, 1895, when German physicist Wilhelm Conrad Röntgen discovered X-rays. He noticed that this radiation could pass through human tissue but not through bone or metal. Because he did not know what type of radiation it was, he called it "X." He later received the first Nobel Prize in Physics for this discovery.

Röntgen's lab notes were burned after his death, so accounts of his discovery are based on biographers' reconstructions. While studying cathode rays with a Crookes tube wrapped in black cardboard and a fluorescent screen coated with barium platinocyanide, he saw a faint green glow from the screen about one meter away. He realized invisible rays from the tube were passing through the cardboard and making the screen glow, and that they could also pass through opaque objects to affect film behind them.

Röntgen first demonstrated the medical use of X-rays by making an image of his wife's hand on a photographic plate. This was the first X-ray photograph of a human body part. When she saw it, she said, "I have seen my death."

The first clinical use of X-rays occurred on January 11, 1896, when John Hall-Edwards in Birmingham, England, radiographed a needle stuck in an associate's hand. On February 14, 1896, he became the first to use X-rays during a surgical operation.

The first medical X-ray in the United States was made using a discharge tube designed by Ivan Pulyui. In January 1896, after reading about Röntgen's discovery, Frank Austin at Dartmouth College tested all the discharge tubes in the physics lab and found that only Pulyui's tube produced X-rays. This was because Pulyui had included an oblique mica "target" inside the tube for holding fluorescent samples. On February 3, 1896, Gilman Frost, a professor of medicine, and his brother Edwin Frost, a physics professor, exposed the wrist of Eddie McCarthy—whom Gilman had treated for a fracture weeks earlier—to X-rays. They captured the image of the broken bone on gelatin photographic plates obtained from local photographer Howard Langill.

X-rays were used for diagnosis very early on. Alan Archibald Campbell-Swinton opened a radiographic laboratory in the United Kingdom in 1896, before the dangers of ionizing radiation were known. Marie Curie later promoted radiography for treating wounded soldiers in World War I. Initially, many types of hospital staff—physicists, photographers, physicians, nurses, and engineers—performed radiography. Over time, the medical specialty of radiology developed around the technology. As new diagnostic tests emerged, radiographers were trained to use them. Today, radiographers also perform fluoroscopy, computed tomography, mammography, ultrasound, nuclear medicine, and magnetic resonance imaging. Although a nonspecialist dictionary might define radiography narrowly as "taking X-ray images," this has long been only part of the work of X-ray departments, radiographers, and radiologists. Early radiographs were called roentgenograms, and the term skiagrapher (from Greek words for "shadow" and "writer") was used for radiographer until about 1918. In Japanese, the word for radiograph is rentogen (レントゲン), sharing its etymology with the original English term.

Because the body contains substances of different densities, ionizing and non-ionizing radiation can reveal internal structures by highlighting differences in attenuation—or, for ionizing radiation, the absorption of X-ray photons by denser materials like bone. The study of anatomy through radiographic images is called radiographic anatomy. Radiographers typically perform medical radiography, while radiologists analyze the images, though some radiographers also specialize in image interpretation. Medical radiography includes many modalities that produce different types of images, each with distinct clinical uses.