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X-Ray Imaging

Seeing through the body

Date
1895
Era
Electric age
Rebuild stage
V Electric age
Difficulty

Plate

Legend
  1. 1Induction coil
  2. 2Concave cathode
  3. 3Tilted platinum anticathode
  4. 4Glass globe
  5. 5Photographic plate

Anatomy

Platinum anticathode: the electrons stop here and the X-rays startInduction coil: tens of thousands of volts from an 8 V batteryPlate as it would develop: dark where the rays arrive, pale under boneThe concave cathode focuses the electrons on the platinum, which stops them: at 50 kV about 0.4% of their energy is turned into X-rays there, and the rest into heat. At 30 kV, a soft tube, 16% of the X-ray energy gets through 20 mm of flesh and 1.2% through a finger bone with its flesh, so the bones print almost clear, but at the same current the exposure takes 10 times as long as at 60 kV. At 90 kV the shares are 49% and 28% and the shadow goes flat. Each exposure is timed so that the bare plate comes out equally dark. In a gas tube the voltage rose as the tube hardened with use. Early operators often tested their tubes on their own hands and suffered burns. Bone path: 6 mm of bone in 20 mm of hand (estimates); attenuation from NIST data. The hand is drawn in outline so that its shadow shows.

X-ray imaging sends invisible, penetrating rays through the body onto a plate or detector, so that bones and dense tissues cast shadows. Wilhelm Röntgen discovered the rays in Würzburg on 8 November 1895, and by early 1896, weeks after he announced them, doctors in Europe and America were using them to find fractures and bullets.

What it is

X-rays are electromagnetic waves like light, but with much shorter wavelengths, roughly 0.01 to 10 nanometres against 400 to 700 for visible light, which lets them pass through materials that stop light. Dense matter absorbs more of them than light matter, so an object placed between an X-ray source and a sensitive plate leaves a shadow picture of its inside: bone, which contains calcium, stands out clearly against the flesh around it.

The apparatus on the plate is the kind used in 1896 and the following years. An induction coil turns the low voltage of a battery into tens of thousands of volts. Two wires carry that voltage to a “focus” tube, a glass globe pumped almost empty of air, with a concave aluminium cathode at one end and a small platinum plate, the anticathode, tilted at 45 degrees in the centre. Below the tube lies a photographic plate wrapped in black paper, with the object to be pictured on top of it.

How it works

The tube still holds a trace of gas. The high voltage ionises it, and the positive ions, striking the cathode, knock electrons out of the metal, which the voltage then drives across the tube. These cathode rays, as they were then called, are invisible streams of electrons, and the amount of gas left in the tube governs how they behave. The concave cathode focuses them onto a small spot on the platinum anticathode. When the fast electrons are stopped in the metal, a small part of their energy comes out as X-rays; nearly all of it becomes heat. X-rays leave the target in many directions; because it is tilted, a useful beam comes out sideways through the glass toward the photographic plate, while the rest spreads round the room unless it is shielded.

A Coolidge X-ray tube: a glass bulb with a coiled tungsten filament in a metal cup at one end and a heavy angled tungsten target at the other The Coolidge tube of 1913: a heated tungsten filament supplies the electrons, so the output can be set precisely.

The rays travel in straight lines from that small spot, which is why it matters: the smaller the spot, the sharper the shadows. Where the rays pass easily, through air or flesh, they blacken the photographic emulsion; where bone or metal absorbs them, the plate stays lighter. Printed or viewed directly, the plate shows the skeleton of a hand, a coin in a purse or a bullet in a limb.

Early gas tubes were erratic: their behaviour changed as the remaining gas was used up. In 1913 William Coolidge of General Electric made a tube with a hard vacuum and a heated tungsten filament as the cathode. The current, and with it the amount of X-rays, could now be set by the filament temperature and their penetrating power by the voltage, which made doses repeatable. Modern tubes still follow that design.

How to build it

An X-ray apparatus cannot be built safely at home, and in most countries operating one needs a licence. X-rays damage living cells without being felt; the early workers learned this through burns, amputations and fatal cancers. This section describes how the first machines were made in laboratories, then gives two simple experiments that show the same principles.

How it was done in 1896. Instrument makers wound induction coils of many kilometres of fine silk-covered wire over an iron core, fed them from batteries through a vibrating interrupter, and connected them to tubes made by glassblowers. The tubes were evacuated with mercury pumps over hours, sealed, and seasoned until they gave steady rays. A plate holder wrapped in black paper lay under the tube; exposures for a hand took several minutes at first, sometimes far longer, and less as tubes improved.

See shadows with light. In a dark room, fix a bright torch about 2 m from a wall and hold a pencil halfway. Tape a card with a 5 mm hole over the torch, then take it off so the whole lens shines: the small source gives a sharp shadow, the large one blurred edges, just as a small focal spot does in an X-ray tube. Then press the torch against a fingertip: it glows red, because red light gets through a centimetre of flesh.

See fluorescence. Röntgen noticed the rays because a screen coated with barium platinocyanide glowed. With an adult in charge and following the lamp’s safety instructions, shine a low-power ultraviolet torch of the 365 nm “UV-A” kind on a glass of tonic water in a dark room: the quinine in it absorbs the invisible light and glows blue. Highlighter ink and some washing powders do the same.

Safety. Never connect old vacuum tubes or cathode-ray tubes to a high voltage: when electrons strike the glass or metal inside, they can emit X-rays. Induction coils and other high-voltage supplies can deliver dangerous or fatal shocks. Ultraviolet light harms eyes and skin: never look into the torch, keep the beam off eyes and skin, wear UV-blocking glasses and choose a low-power torch sold for UV-A only. Medical X-rays are given by trained staff, with the dose kept as low as the examination allows.

History and evolution

A glass tube wrapped in black card on a laboratory table, and a small screen glowing yellow-green an arm’s length away Röntgen’s discovery: a tube wrapped in black card, and a fluorescent screen nearby that glows all the same.

On the evening of 8 November 1895, Wilhelm Conrad Röntgen, professor of physics at Würzburg, was studying an electric discharge in a tube wrapped in black card. A paper screen coated with barium platinocyanide, lying some distance away, glowed each time the tube was switched on, although no light could leave the card. Over the next weeks he worked alone and showed that the unknown rays passed through paper, wood and flesh but were absorbed much more strongly by bone and by metals such as lead. On 22 December he photographed the hand of his wife, Anna Bertha, showing the bones and her ring. His paper “On a new kind of rays” appeared at the end of the month.

An early X-ray plate of a left hand, the bones pale against a dark ground, with a needle lodged in the palm From early 1896, an X-ray plate could show a surgeon where a needle or a bullet lay before he cut.

The news spread at extraordinary speed, and about a thousand publications on X-rays appeared within a year. In early 1896 Herbert Jackson in London and others built focus tubes with a concave cathode and an angled metal target, which gave much sharper images. Surgeons used X-rays to locate bullets and needles; hospitals set up X-ray rooms. Röntgen received the first Nobel Prize in Physics in 1901.

The danger was not understood at first. Burns and hair loss were reported in 1896, and many early operators suffered disfiguring injuries; Clarence Dally, Thomas Edison’s assistant, died in 1904 after years of exposure. Lead screens, dose limits and training followed. In 1912 Max von Laue’s team showed that X-rays are waves by diffracting them through crystals, which opened the study of crystal structures. Computed tomography, which builds cross-sections from many X-ray measurements, earned Godfrey Hounsfield and Allan Cormack the Nobel Prize in 1979.

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