Ultrasound images rest on a guess about the speed of sound
An ultrasound machine never measures depth directly. It times how long each echo takes to come back and converts that into distance by assuming sound always travels at 1,540 metres per second. Real tissues carry sound at quite different speeds, so the familiar grainy picture is a clever estimate rather than a true cross-section of the body.
The sound itself is far above human hearing, which tops out near 20,000 vibrations a second. A handheld probe sends short pulses into the body, and each boundary between tissues bounces some back, stronger or weaker depending on how the materials differ. There is a trade-off in pitch: shallow structures such as tendons, the thyroid or a newborn's brain are scanned at 7 to 18 megahertz for sharp detail, while the liver and kidneys need 1 to 6 megahertz to reach deeper, at the cost of resolution.
Machines display echoes in several ways. The simplest, A-mode, plots echo strength along a single line. B-mode sweeps an array of elements across a plane to build the two-dimensional grey picture most people know, and stacking many such planes produces 3D images. M-mode fires pulses down one line again and again and lays the results side by side, much like streak photography, which makes it ideal for tracking a beating heart's moving walls.
Doppler methods add motion. Blood cells shift the pitch of returning echoes, letting machines measure flow in arteries and veins, and a technique called B-flow instead highlights moving reflectors while muting still tissue. Tiny probes on catheters can even travel inside blood vessels to image their walls from within.
Obstetric scanning was one of the earliest clinical uses, developed by Ian Donald in the late 1950s and 1960s. Ultrasound has limits, though: gas in the bowel blocks the waves and fat weakens them. One experimental hybrid adds a laser pulse that makes haemoglobin vibrate and give off its own sound, producing 3D images of blood and tissue with neither magnets nor ionizing radiation.
Source: Medical ultrasound
