Ultrasound: an image made of echoes
An ultrasound machine sees nothing. It sends out a beep no ear can hear, waits, and measures **how long it takes to come back**. The whole image —the baby, the kidney, the tumour— comes out of timing echoes and dividing by two.
What you are looking at
- The white bar going down the column is a single pulse. It does not stop at the first boundary: at each one part of it bounces and the rest keeps going.
- Each coloured echo travels back up and, when it arrives, a line appears on the axis to the right. That is the only data the machine has: an instant in time.
- At the end, watch the marks that appear to the left of the column. They are not placed there: they come from halving each time and multiplying by the speed of sound. They land on the boundaries because the arithmetic works, not because they were drawn there.
01A beep nobody hears
The probe —that cold thing the doctor presses against your skin— is a crystal that deforms when electricity passes through it. Send it a very short electrical pulse and the crystal knocks, and that knock enters the body as sound.
Not just any sound: between 2 and 15 million vibrations per second, far above what a human can hear (which tops out around 20,000). That very high frequency is not a whim — it is what lets you tell small things apart, for the same reason you cannot measure a bean with a metre stick.
And the same crystal that emitted stays listening. It emits for less than a microsecond and then listens for hundreds. Ultrasound is, above all, waiting.
02Wherever the tissue changes, part of it bounces
Sound travels along undisturbed as long as the material does not change. The moment it crosses a boundary —fat to muscle, muscle to fluid, fluid to bone— a fraction comes back and the rest goes on.
How much bounces depends on how different the two sides are. Between two soft tissues less than 1% returns, which is very little and is exactly what makes ultrasound possible: if a lot bounced at the first boundary, there would be no sound left to see what lies deeper.
With bone and with air the opposite happens: almost everything bounces. That is why ultrasound is no use for looking inside an adult skull or through a lung, and why the gel is not a detail — a millimetre of air between probe and skin would send the whole pulse back before it got in.
03The only sum the machine does
Sound travels through soft tissue at about 1540 metres per second, which is 1.54 millimetres per microsecond. If an echo comes back at 65 microseconds, it covered 10 centimetres — but there and back. The boundary is at 5.
That is all of it: `d = c·t/2`. The division by two is because the sound makes the trip twice, and it is the classic mistake of anyone explaining this in a hurry.
The picture on the hospital screen is this same line, repeated. The probe sweeps in a fan —hundreds of lines, dozens of times a second— and paints each echo as a dot, brighter or dimmer according to its strength. Those grey shapes are not photographs of anything: they are maps of where the tissue changes.
04What the machine assumes, and why it is sometimes wrong
That 1540 m/s is an assumption. Fat carries sound at about 1450 and muscle at about 1580; bone, at over 3000. The machine does not know and uses the same number for everything.
The consequence is that depths come out slightly shifted, and that behind a region of fat, structures appear displaced. An experienced sonographer recognises those artefacts and discounts them.
It is a clean example of something that turns up in every indirect measurement: the result is not what you measured, it is what you measured plus the model. Change the assumption and the image changes without anything in the body having changed.
Where this is used
Pregnancy scans
The use everyone knows, and the reason it is sound and not X-rays: ultrasound ionises nothing. A pregnant woman can have several without accumulating a dose of anything.
Doppler scans of the carotid arteries
If, as well as timing the echo, you measure how much its frequency shifted, you get the speed of the blood. That is where this meets the [Doppler effect](/en/concepts/doppler): the same shift as the siren, applied to red blood cells, and it is how a blocked artery is found without opening anyone up.
A ship's sonar, and a bat's
Exactly the same sum with a different speed: 1500 m/s in seawater, 340 in air. A bat solves `d = c·t/2` dozens of times a second and catches insects in the dark with it.
Finding cracks without breaking the part
Ultrasonic inspection checks bridge welds, ship hulls and turbine blades. An internal crack is one more boundary, and it returns its echo earlier than it should: the defect is located to the millimetre without cutting anything open.
Explaining it to a child
The idea, in plain wordsWhen you shout in the hills and the echo comes back, if you counted the seconds you could work out how far away the hillside is. An ultrasound machine does the same thing, with a beep nobody can hear and a very fast stopwatch.
- Stand facing a big, distant wall —a sports court, a hillside, a building— and clap loudly. Count how long the sound takes to come back.
- Sound travels about 340 metres in a second through air. If the echo took one second, the sound covered 340 metres: 170 there and 170 back. The wall is 170 away, not 340. That division by two is the same one ultrasound does.
- Try it from closer and from further away. The further away, the longer it takes: the time IS the distance, and that is all the machine needs to know.
The question to leave them withWhy does the doctor put cold gel on you before pressing the probe down?
For the classroom
Ten minutes with the animation, ready to take into a room. Free under CC BY 4.0, like everything else here.
- Go outside, facing a distant wall, and clap loudly. Have them count how long the echo takes.
- At 340 m/s, a one-second echo means 340 metres travelled. Ask how far away the wall is. Almost everyone will say 340.
- There is the whole lesson: the sound went and came back. The wall is at 170.
- Play the animation. It is exactly the same with 1540 m/s instead of 340, a thousand times a second.
An echo returns at 65 microseconds. At 1540 m/s, how deep is that boundary?
Answer: 5 cm. In 65 µs sound covers about 10 cm, but there and back — half of that is the depth.
Why does the doctor put gel on you before pressing the probe down?
Answer: Because between air and body almost all the sound bounces. A millimetre of air would send the whole pulse back before it got in. The gel takes the air out of the way.
The machine assumes 1540 m/s for the whole body, but fat carries sound at 1450. What follows from that?
Answer: That behind a region of fat, structures appear slightly shifted from their true depth. It is a clean example that the result of an indirect measurement is what you measured PLUS the model you used.
Almost everyone skips the division by two, adults included. It is not carelessness: "how far did it travel?" and "how far away is it?" sound like the same question. The echo in the yard is the cheapest way to pull them apart.
FísicaEterna classroom guide · CC BY 4.0 · fisicaeterna.cl
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Suggested attributionUltrasound: an image made of echoes — FísicaEterna (fisicaeterna.cl), CC BY 4.0