How do X-ray machines look inside objects?

1. Sam bumped his wrist, and the doctor wants a look at the bones inside. You cannot see bones from the outside. So Sam's teddy goes first, to show how an X-ray works.
2. Ordinary light bounces off Teddy, so he blocks it completely. X-rays are a kind of light with much more energy. They slip between the atoms of soft things and come out the other side.
3. But something dense is hiding inside Teddy. X-rays that hit it are stopped. Behind it there is a gap in the beam, a shadow shaped exactly like the hidden thing.
4. Behind Teddy stands a flat plate called a detector. Where lots of X-rays land, the picture turns dark. In the shadow it stays white. A computer shows the result: a key!
5. Now it is Sam's turn. He rests his hand in front of the plate and keeps still. Click! It takes less than a second, and he cannot feel it at all. The machine uses only a small, careful amount.
6. On the screen are the bones of Sam's hand. Bones block more X-rays than the soft parts around them, so they look whiter. Now the doctor can see exactly how to help.

Tap each part. Follow the path: tiny particles fly across the tube, X-rays come out, and the plate catches them.

• Hot wire. A small wire is heated until it glows. The heat shakes loose tiny particles called electrons, ready to be fired across the tube.
• Metal target. A very strong electric push shoots the electrons across the empty tube. They smash into a metal target and stop suddenly. That sudden stop makes X-rays.
• Shielded case. The tube sits in a heavy case lined with lead, a metal that stops X-rays very well. X-rays can leave only through one small window, pointing at the thing to be pictured.
• Beam shutters. Sliding metal shutters shape the beam so it covers only the part to be pictured, such as one hand, and nothing more.
• Detector plate. The flat plate is covered in thousands of tiny sensors. Each one measures how many X-rays landed on it.
• Computer and screen. A computer collects the readings from every sensor and draws the picture: white where few X-rays arrived, dark where many did.

X-ray each thing and compare the pictures. Which materials let X-rays through? Which ones stop them?

Experiment: Five blocks are all the same thickness: wood, water, bone, aluminum and steel. Which makes the whitest patch in the X-ray picture?

1. X-ray the teddy, the backpack or the hand. Which parts are white?
2. Now X-ray the five test blocks.
3. Compare the patches from darkest to whitest.

What we found: The denser the material, the more X-rays it stops, and the whiter its patch. Wood is nearly see-through, bone is pale gray, and steel blocks almost everything.

Turn on JavaScript to try it yourself.

One box hides the key. X-ray them to look inside, and open the right box first time. One box is metal: it needs more power.

X-rays are a kind of light, but your eyes cannot see them. Each little packet of X-ray light carries much more energy than ordinary light. That extra energy lets X-rays pass through things that ordinary light bounces off, such as cloth, cardboard, skin and muscle.

Not everything lets them through equally. The more tightly packed and the heavier the atoms of a material are, the more X-rays it stops. Stuffing and cloth stop almost none. Water and muscle stop some. Bone, which contains calcium, stops more. Metals such as steel stop nearly all of them.

An X-ray machine has three main parts. A tube makes the X-rays by shooting tiny particles called electrons at a metal target. The thing to be pictured stands in the beam. Behind it, a flat detector measures how many X-rays arrive at each spot. A computer paints the result: dark where many X-rays arrived, white where few did. So it really is a shadow picture, like our shadow puppet show. The difference is that these shadows come in many shades of gray, not just black.

Too many X-rays are not good for living things, so the machines are built to use as few as possible. A picture of a hand takes less than a second, cannot be felt, and uses less radiation than everybody gets from nature, from rocks, food and space, in one ordinary day. The people who take X-ray pictures step behind a screen because they take pictures all day long.

Key words

X-ray

A kind of invisible light with enough energy to pass through soft things.

Radiograph

A shadow picture made with X-rays. Dense parts look white.

Detector

The flat plate that measures how many X-rays arrive at each spot.

Density

How much stuff is packed into a space. Denser materials stop more X-rays.

Absorb

To take in and stop. Bone absorbs more X-rays than skin does.

Dose

The amount of X-rays used. Machines use the smallest dose that gives a clear picture.

Shielding

Material, often lead, placed to stop X-rays from going where they are not needed.

Check your thinking

1. Why do bones look white in an X-ray picture?

   Show the answer

   Bones stop more X-rays, so fewer reach the detector behind them. Where few X-rays arrive, the picture stays white. Bone blocks more than the soft parts around it.

2. What is it like to have an X-ray picture taken of your hand?

   Show the answer

   You hold still for less than a second and feel nothing. The exposure is over in a fraction of a second, cannot be felt, and uses only a small, careful dose.

3. A teddy has a metal key hidden in its stuffing. What does the X-ray picture show?

   Show the answer

   A faint teddy with a bright white key inside. Stuffing lets most X-rays through, so the teddy is faint. The metal key blocks them and shows up white.

4. Why is the case around an X-ray tube lined with lead?

   Show the answer

   Lead stops X-rays very well, so they only come out where they are needed. Lead has very heavy, tightly packed atoms. A thin layer blocks nearly all the X-rays.

5. Why do radiographers keep the number of pictures and the size of the beam as small as they can?

   Show the answer

   X-rays are ionizing radiation, so the dose is kept as low as reasonably achievable. X-rays can damage cells, so each exposure should be justified and limited to the area needed, even though a single dose is very small.

6. Which of these lets the most X-rays through?

   Show the answer

   A block of wood. Wood is the least dense of the three, so it stops the fewest X-rays.