Seeing inside the body with sound

A baby scan can feel like magic: a grainy picture of someone who hasn't been born yet. But the machine is doing something simple. It sends sound into the body and times the echoes. It's sonar, made small.

Where it started

In June 1958, two Glasgow doctors who looked after pregnancy and birth, Ian Donald and John MacVicar, and an engineer, Tom Brown, published a paper in The Lancet called "Investigation of abdominal masses by pulsed ultrasound". It described scans of more than 100 patients. Their machine had grown out of an industrial flaw detector, a tool for finding hidden flaws in metal. It was put together from whatever they could find, including a disused flaw detector, bits of Meccano (a construction toy) and a hospital bed table. Doctors soon saw that it could also show a baby growing in the womb.

Sonar, made small

The probe that rests on your skin is a speaker and a microphone in one. It sends short pulses of sound at millions of vibrations a second (megahertz, or MHz), far above anything we can hear. Each time the sound reaches a boundary between two kinds of tissue, a little of it bounces back. The scanner notes when each echo arrives and, from the speed of sound, works out how deep each boundary is. Line by line, those depths become a picture.

The trips are tiny. An echo from 5 cm under the skin is back in about 65 millionths of a second.

Why the gel?

Sound passes from one soft tissue to the next fairly easily, so most of it carries on deeper at each boundary. Air is different. Water's acoustic impedance, a measure of how hard it is to set a material moving with sound, is about 3,600 times air's. So where water meets air, about 99.9% of the sound bounces straight back.

Our soft tissues are mostly water, so even a thin layer of air between the probe and your skin would block the view. The gel keeps that air out.

The speed it has to assume

To turn a time into a depth, the scanner needs the speed of sound. It doesn't know the real speed inside you, so it assumes one: 1,540 m/s, a typical figure for soft tissue. Real tissues are a little different.

Sound is slower in fat, so echoes that pass through it arrive a little late. The scanner then draws whatever is beneath a thick layer of fat slightly deeper than it really is. Sound in air has a version of the same problem: its speed changes with the temperature. That's the story of why sound is faster on a warm day.

Sharp or deep

Scanners work at about 2 to 15 MHz. Higher pitches have shorter waves, and shorter waves show finer detail. At 2 MHz the wavelength in soft tissue is about 0.8 mm, and at 15 MHz it's about 0.1 mm. But high pitches fade faster as they travel, so they don't reach as deep. That's why parts of the body near the surface get a high pitch, and deeper ones a lower one.

Four ways to show the echoes

• A-mode (A for amplitude) is the simplest: one line straight in, with each echo drawn by how strong it is.
• B-mode (brightness) is the familiar grey picture. The probe scans a flat slice through the body, and every echo becomes a dot. The stronger the echo, the brighter the dot.
• M-mode (motion) takes one line and draws it again and again as time goes by. Time runs across and depth runs down, so anything that moves draws a wave. It's used to watch small moving parts, like the flaps of a heart valve.
• Doppler uses the Doppler effect to measure movement, usually of blood: how fast it flows, and which way.

Try it with Sonar Ruler

Sonar Ruler's Live Wall works the same way, in air. Its chirp sweeps from 1 to 15 kHz, about a thousand times lower in pitch than a scanner, and its echoes come back in thousandths of a second rather than millionths. Its waterfall is an M-mode of your room: distance runs along the tape, and time runs down the screen, with the newest echoes at the top.

1. Stand about 2 m from a wall and point your phone's bottom edge at it.
2. Step slowly in and back out, a few times.
3. Watch the wall's line. It draws your movement, the way an M-mode draws a heart valve. Tap it to track it as you go.

Sources

• University of Glasgow, Marking 50 Years of Ultrasound (2008)
• NIBIB (NIH), Ultrasound
• AIP Scilight, Gradient impedance metafluids enable broadband water-air sound signal transmission
• Radiopaedia, Propagation speed
• Radiopaedia, Ultrasound frequencies
• Radiopaedia, M-mode
• Wikipedia, Medical ultrasound