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 hundreds to hundreds 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, funded partly by the Beatles' record sales, 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.
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.