How Do X-Rays Work? Seeing Bones Through Skin
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In November 1895 a German physicist experimenting with electric discharges in a glass tube noticed that a screen across the room glowed when the tube was on, though the tube was wrapped in black card, and within weeks he had photographed the bones of his wife's hand, who said on seeing it that she had seen her own death. Wilhelm Rontgen called the rays X because he did not know what they were. They are light, of a wavelength ten thousand times shorter than the visible kind, and the discovery went from laboratory to hospital in under a year, faster than any medical technology before or since.
What they are
X-rays are electromagnetic radiation, the same phenomenon as radio, visible light and ultraviolet, at wavelengths between about ten nanometres and ten picometres, which means each photon carries tens to tens of thousands of times the energy of a photon of visible light. That energy is what lets them through. Visible light is absorbed by the electrons in almost any solid within a fraction of a millimetre; an X-ray photon is energetic enough to pass most electrons by, and it is stopped only by chance encounters, more of them in material with more electrons per volume. Bone, rich in calcium, has many; muscle and fat, mostly carbon, hydrogen and oxygen, have fewer; air has almost none. An X-ray image is a shadow, recording how many photons got through at each point, and the bones show white because they cast the deepest shadow.
How they are made
An X-ray tube is a vacuum bulb in which electrons boiled off a hot filament are accelerated by tens of thousands of volts into a tungsten target. When they slam into it two things happen. Most of the electrons are decelerated by the tungsten nuclei and give up their energy as a spread of X-ray photons, braking radiation, and a few knock electrons out of the inner shells of tungsten atoms, whose neighbours fall into the gaps and emit X-rays of specific energies. Ninety-nine percent of the energy becomes heat, which is why the target spins and the tube is oil-cooled. The beam passes through the patient to a detector, once photographic film and now a flat panel that converts X-rays to light and light to a digital image, and a lead shutter limits it to the region of interest.
What they can and cannot show
A plain X-ray shows anything dense against anything less dense, which makes it the first tool for a set of questions and useless for others:
- •Fractures, bone disease, joint spaces and dental decay, the original and still the commonest use
- •The chest: lungs full of air are dark, so fluid, infection, collapse and tumours show against them, and the heart's outline is visible
- •Foreign bodies, swallowed coins and bullets, and the shadow of a kidney stone
- •Soft tissue barely at all: the brain, the liver and a torn ligament are invisible unless a contrast agent that absorbs X-rays, barium in the gut or iodine in the blood, is introduced
The dose
The energy that lets X-rays through tissue also lets them knock electrons out of molecules on the way, and a broken DNA strand is occasionally a cancer decades later. The early radiologists, who calibrated their tubes on their own hands, lost fingers and lives before the danger was understood. Modern doses are small: a chest X-ray gives about 0.1 millisieverts, roughly ten days of the natural background radiation everyone receives from rocks, cosmic rays and their own bodies, and a dental X-ray a fiftieth of that. A CT scan of the abdomen is about 10 millisieverts, some three years of background, and it is CT, whose use has grown enormously, that accounts for most of the medical radiation in rich countries and that doctors are asked to justify each time. The principle in the profession is that every exposure should be as low as reasonably achievable, and the lead apron over the parts not being imaged is the visible sign of it.
From shadow to slice
A single X-ray flattens the body into one plane, so that everything along the beam's path is superimposed. Computed tomography, invented by Godfrey Hounsfield at EMI in 1971, at a company famous for the Beatles' records, solves that by taking hundreds of X-ray shadows from all round the patient as the tube spins on a ring, and having a computer reconstruct from them the density at every point in a cross-section, which is then displayed as a slice and stacked into a three-dimensional picture. The mathematics had been worked out by an Austrian in 1917 with no application in mind. CT shows soft tissue that a plain film cannot, and it is the tool for stroke, trauma, cancer staging and much else; MRI, which uses magnetism and radio waves rather than X-rays, does better for the brain and joints without any dose at all. Beyond medicine, X-rays inspect welds and luggage, read the structure of crystals, which is how the shape of DNA was found, and, from telescopes in orbit, image the hottest gas in the universe.
Does an X-ray make you radioactive
A common worry is that having an X-ray leaves you radioactive, and it does not. An X-ray tube produces rays only while the electricity is flowing, like a light bulb, and the moment it is switched off there is no radiation left in the room or in you. The photons that pass through your body either come out the other side or are absorbed, and absorbing an X-ray photon does not turn the atoms of your body into radioactive ones. You can hug someone straight after their scan without any risk to either of you.
This is different from nuclear medicine, where a patient is given a small amount of a radioactive substance that emits gamma rays from inside the body, and where there can be brief precautions afterwards. X-rays and gamma rays overlap in energy and are the same kind of light; the names describe where the radiation comes from rather than a strict wavelength. X-rays come from electrons, as in a tube or from very hot gas in space, while gamma rays come from changes inside atomic nuclei. The real risk from a scan lies in the dose delivered during those few moments, not in anything that lingers.
From shoe shops to Vermeer
The early enthusiasm for X-rays ran well ahead of any understanding of the risk. From the 1920s many shoe shops in Europe and North America installed shoe-fitting fluoroscopes, cabinets in which a customer stood with their feet in a slot and looked down through a viewer at a live X-ray image of their toes wriggling inside new shoes. Children loved them, and the machines were sold as scientific fitting. They delivered doses far higher than any modern foot X-ray, to customers and to the staff who stood beside them all day, and as the hazard became clear they were banned or abandoned in most countries from the 1950s onward.
A happier use is in art galleries. Lead white, the main white paint of European painting for centuries, absorbs X-rays strongly, so an X-ray of a canvas shows the lead-rich layers beneath the visible surface, including changes the artist made and earlier compositions painted over. In 1979 an X-ray of Vermeer's Girl Reading a Letter at an Open Window, in Dresden, revealed a large painting of Cupid hanging on the wall behind the girl, hidden under a layer of paint. Later analysis showed that the covering paint had been added after Vermeer's death, and conservators removed it, revealing the Cupid again by 2021.
The X-ray photograph behind DNA
X-rays do more than show bones. When a beam of X-rays passes through a crystal, the regularly arranged atoms scatter it into a pattern of spots, and from that pattern scientists can work out how the atoms are arranged. This technique, X-ray crystallography, has revealed the structures of thousands of molecules, from salt to proteins.
Its most famous image is Photograph 51, taken in 1952 by Rosalind Franklin and her student Raymond Gosling. Its distinctive X-shaped pattern showed that DNA has a helical structure, and when James Watson saw it without Franklin's knowledge, it helped him and Francis Crick build their double-helix model in 1953. Franklin died in 1958, and the Nobel Prize for the structure in 1962 went to Watson, Crick and Maurice Wilkins.
The takeaway
X-rays are high-energy light that passes through flesh and is partly stopped by bone and other dense material, so that a detector behind the patient records a shadow in which bone is white and air is black. They are made by firing electrons into tungsten, they carry a small cancer risk that modern doses keep near the level of natural background, and a CT scanner turns hundreds of shadows taken from every angle into a computed slice through the body.