An X-ray is a shadow made of light you can't see. An X-ray machine heats a tungsten filament until electrons boil off, then pulls them across a vacuum with 40,000 to 150,000 volts. They slam into a spinning tungsten disc, and the sudden braking throws out X-ray photons.
An X-ray is a shadow made of light you can't see. Fire electrons at spinning tungsten, send the beam through a body, and catch what gets out. Build the machine in 3D and make your own X-ray picture.
XRayClearOpened 23 Jul 202615 min to playFree · no sign-up
In 60 seconds
A tube that makes invisible light
An X-ray machine heats a tungsten filament until electrons boil off, then pulls them across a vacuum with 40,000 to 150,000 volts. They slam into a spinning tungsten disc, and the sudden braking throws out X-ray photons.
Almost all of it becomes heat
Less than 1% of the electrons' energy turns into X-rays; the rest heats the anode. That is why the disc spins at thousands of rpm, spreading the heat round a ring. The top photon energy in keV equals the kVp, and a 60 keV photon carries about 26,000 times the energy of green light.
The picture is a shadow
Each photon either gets through you or it doesn't: I = I₀·e^(−μx). Calcium-rich bone stops several times more per centimetre than muscle, and air in the lungs stops almost nothing. Fewer photons means whiter, so bones look white.
Harder X-rays, less contrast
Turn up the voltage and the photons punch through bone and muscle more equally, so the picture flattens. Scattered photons fog it, so a grid of thin lead strips under the patient blocks the slanting ones.
From film to flat panel
Film gave way to laser-read CR plates (1983) and then flat panels: a caesium iodide layer turns each X-ray into thousands of flashes of light, and a grid of photodiodes about 140 µm apart reads them out in seconds.
Small doses, big uses
A chest X-ray is about 0.02 mSv, a few days of the 2.4 mSv of natural background we get each year; a CT scan of the abdomen is about 8 to 10 mSv. The same rays check airport bags, find cracks in welds, and in 1952 revealed the helix of DNA.
The history
130 years from a strange glow in a darkened lab to the digital X-rays taken billions of times a year.
A tube that fires electrons at metal, a beam, a patient and a detector.
An X-ray machine is a tube that makes invisible light, pointed at a detector, with you in between. Everything else is there to aim the beam, keep it safe and read the picture.
Inside the tube is a glass vacuum envelope. At one end a tiny coil of tungsten wire, the filament, glows white-hot inside a focusing cup. At the other end spins a heavy tungsten anode disc, turned by a rotor and the stator coils outside the glass. The whole insert sits in a steel housing filled with oil and lined with lead, with one small window where the beam gets out.
Under the window, the collimator uses lead shutters to trim the beam to size, and a lamp and mirror shine a light field on the patient so the radiographer can see exactly where it will land. A thin aluminium filter soaks up useless weak X-rays. Under the table, a grid of lead strips blocks stray rays and a flat-panel detector records the picture. The radiographer picks kVp, mA and time at the console.
Boil electrons off a hot wire, fling them at tungsten, and 1% of the crash comes out as X-rays.
Heat a thin tungsten wire until it glows and electrons boil off its surface. That is the filament. Now put a huge voltage, 40,000 to 150,000 volts, between it and a block of tungsten, the anode. The electrons race across the vacuum and hit the anode at more than half the speed of light.
When a fast electron swerves past a tungsten nucleus it brakes hard and flings out a photon. Germans called this bremsstrahlung, "braking radiation". A gentle swerve gives a weak photon, a head-on stop gives the strongest one. So the beam holds a smooth spectrum of energies, and the top energy in keV equals the kVp. Sometimes an electron knocks out an inner electron of a tungsten atom instead, and the atom gives off a sharp characteristic line, like Kα at 59 keV.
A 60 keV X-ray photon carries about 26,000 times the energy of a photon of green light (2.3 eV). That's why it can go through you. But making them is wasteful: about 99% of the energy becomes heat. So the anode spins at thousands of rpm, spreading the heat round a ring called the focal track, and an aluminium filter soaks up the weakest X-rays, which would only be absorbed in your skin.
An X-ray picture is a shadow: bone blocks more of the beam than muscle, and air blocks almost none.
An X-ray picture is a shadow. Each photon either gets through you or it doesn't, and the detector counts the ones that make it. Where fewer arrive, the picture is white. So it is a negative: bones white, lungs dark.
How much gets through follows a simple rule: I = I₀ × e−μx. Every centimetre of material (x) lets through the same fraction, set by its attenuation coefficient μ. Bone is dense and full of calcium, a heavier atom than the carbon, oxygen and hydrogen in soft tissue, so at 30 to 60 keV it stops several times more per centimetre than muscle. Air in the lungs stops almost nothing.
Higher kVp means harder photons that sail through everything, so bones and muscle look more alike: less contrast. Lower kVp gives more contrast but needs more dose to get enough photons through. Some photons don't go straight: they scatter sideways and fog the picture. A grid of thin lead strips, lined up with the beam, lets straight photons through and blocks most of the slanting ones.
X-rays are turned into light, then into a picture: chemically, with a laser, or with millions of tiny photodiodes.
For about a hundred years, X-ray pictures were made on film. Film on its own needs a lot of X-rays, so it sits between two intensifying screens: phosphor layers that glow when an X-ray hits them. The light darkens tiny grains of silver halide in the film, and chemicals turn them into black silver. That is why bones look white: fewer X-rays, fewer black grains.
In 1983 Fujifilm brought out computed radiography (CR). A reusable plate of barium fluorobromide traps the X-ray energy in the crystal. A red laser sweeps the plate and each trapped spot gives out a flash of blue light, which is measured and turned into numbers.
Today most hospitals use flat-panel (DR) detectors. On top is a layer of caesium iodide grown as tiny needles, about half a millimetre thick. Each X-ray it stops becomes thousands of flashes of green light, and the needles guide the light straight down like optical fibres. Underneath, a grid of photodiodes turns light into electric charge, and a thin-film transistor (TFT) at each pixel lets it out, row by row. The pixels are about 100 to 150 µm across, the width of a hair, and the picture is ready in seconds.
Doses are measured in millisieverts. A chest X-ray is about the same as a few days of natural background.
X-rays are ionising radiation: they carry enough energy to knock electrons off atoms, and in large doses that can damage cells. So doctors use as little as they can. The effective dose, measured in millisieverts (mSv), adds up the dose to each organ, weighted by how sensitive it is.
We all get radiation from nature every day: from rocks, soil, radon gas, space and even our own bodies. The world average is about 2.4 mSv a year (UNSCEAR). A chest X-ray gives about 0.02 mSv, a few days' worth of that background. A dental X-ray is about 0.005 mSv, a mammogram about 0.4 mSv, and a CT scan of the abdomen about 8 to 10 mSv, because it takes hundreds of pictures from all round.
For the staff, the rule is ALARA: As Low As Reasonably Achievable. The main danger is X-rays scattered off the patient, and they spread out like light from a bulb. Double your distance and the dose drops to a quarter: the inverse-square law. Staff stand back or behind a lead screen, and wear lead aprons when they must stay close. Before X-rays of the belly or pelvis, patients are asked whether they might be pregnant, because a growing baby is more sensitive. For X-rays of other parts, such as the chest or teeth, the dose to a baby is tiny.
Airport scanners, cracks in welds, and the shapes of molecules like DNA.
At the airport, your bag rides a belt through a thin fan of X-rays and a line of detectors builds the picture strip by strip. Most scanners are dual-energy: they measure each point with weaker and stronger X-rays. Light elements (carbon, oxygen) block the weak X-rays much more than the strong ones, while metals block both. From the ratio the machine guesses the material and colours it: orange for organic things like food and paper, green for mixtures and light metals, blue for metals.
In factories, X-rays check welds in pipelines, boilers and aircraft without cutting them open. A gas bubble or a crack means less steel, so more X-rays get through and it shows up dark on the picture.
X-rays also reveal the arrangement of atoms. Their wavelength is about the same as the spacing of atoms in a crystal, so a crystal bends them into a pattern of spots. In 1912 Max von Laue showed this, and William and Lawrence Bragg worked out the rule: rays bouncing off neighbouring layers add up only when 2d sin θ = nλ. In 1952 Rosalind Franklin and Raymond Gosling took Photo 51, an X-ray pattern of DNA whose cross shape pointed to a helix.
So electrons can fly from the filament to the anode without hitting air. Air molecules would stop and scatter the electrons. In a vacuum they fly freely across the gap.
What does the collimator do?
Trims the beam to just the part being X-rayed. Its lead shutters shape the beam, and its lamp shows the radiographer where the beam will land.
Where is the detector?
Under the patient, in the table. The beam passes down through the patient onto a flat panel in a tray under the table top.
A tube runs at 100 kVp. What is the most energetic X-ray photon it can make?
100 keV. An electron that falls through 100 kV carries 100 keV. At most, all of it goes into one photon.
Roughly how much of the electrons’ energy turns into X-rays?
About 1%. Less than 1% becomes X-rays at these voltages. The other 99% heats the anode.
Why does the anode spin?
To spread the heat round a ring so the spot does not melt. The electrons keep hitting a fresh part of the track, so the heat is shared over a ring instead of one tiny spot.
Why is bone white on an X-ray picture?
Bone blocks more X-rays, so fewer reach the detector there. The picture is a negative: where fewer photons arrive, it is brighter. Dense, calcium-rich bone stops the most.
What happens to contrast when you raise the kVp?
It goes down. Harder photons get through bone and muscle more equally, so they look more alike.
What does an anti-scatter grid block?
Photons scattered sideways inside the body. Its lead strips line up with the straight beam, so slanted scattered photons hit the lead instead.
What does the caesium iodide layer in a flat panel do?
Turns X-rays into flashes of light. It is a scintillator. Each X-ray it stops becomes thousands of flashes of green light for the photodiodes.
How are CR plates read?
Scanned with a laser that releases the stored energy as light. A red laser makes each trapped spot give out blue light, which is measured point by point.
Why does a smaller pixel pitch give a sharper picture?
More pixels fit in the same area, so finer detail can be recorded. Detail smaller than about two pixels cannot be seen. At 140 µm, a 43 cm panel has about 3,000 pixels across.
About how much natural background radiation does a person get in a year, on average worldwide?
2.4 mSv. UNSCEAR puts the world average at about 2.4 mSv a year, from rocks, radon, space and food.
A radiographer moves from 1 m to 2 m from the patient. What happens to the scattered dose?
It drops to a quarter. Inverse-square law: twice the distance spreads the same radiation over four times the area.
Which gives the biggest dose?
A CT scan of the abdomen. A CT scan takes hundreds of views from all round the body: about 8 to 10 mSv, hundreds of times a chest X-ray.
On an airport scanner, what colour are metal objects usually shown?
Blue. Metals have heavy atoms that block both the weak and strong X-rays, and the screen colours them blue.
How does a gas bubble in a weld show up?
As a dark spot, because more X-rays get through. Less steel means less blocking, so more X-rays reach the film or detector there.
What did Photo 51 (1952) help reveal?
The helical shape of DNA. Its X-shaped diffraction pattern was a tell-tale sign of a helix.
Words worth knowing
X-ray
Light with photons thousands of times more energetic than visible light, able to pass through soft materials.
Bremsstrahlung
"Braking radiation": X-rays made when fast electrons swerve and slow down near atomic nuclei.
kVp
The peak voltage across the X-ray tube. It sets the top photon energy (in keV) and how penetrating the beam is.
mAs
Tube current times exposure time. It sets how many photons are made.
Attenuation
The weakening of the beam as matter absorbs or scatters photons, following I = I₀·e^(−μx).
Contrast
How different neighbouring parts of the picture look, such as bone against muscle.
Anti-scatter grid
Thin lead strips lined up with the beam that block photons scattered sideways in the body.
Scintillator
A material, like caesium iodide, that turns an X-ray into a flash of visible light.
Millisievert (mSv)
The unit of effective dose, weighting radiation by how much harm it can do to each organ.
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