How Ultrasound Works: Piezoelectricity, B-mode Imaging & Doppler

A crystal that both speaks and listens

Every ultrasound probe is built around a piezoelectric crystal — usually lead zirconate titanate (PZT). Piezoelectricity is a two-way conversion: an applied voltage deforms the crystal mechanically (transmit mode, generating the pulse), while returning mechanical strain generates a voltage (receive mode, detecting the echo). The crystal resonates at f = v_crystal/(2t), where t is its thickness — medical imaging uses 2–15 MHz, corresponding to crystals just 0.13–1.0 mm thick. A brief ~200 ns electrical shock excites a short burst of 2–3 wavelengths; a tungsten-rubber backing layer damps the ringing so the pulse stays short, and a matching layer roughly λ/4 thick reduces reflection at the crystal-tissue boundary.

Timing the echo: depth from the speed of sound

Sound in soft tissue travels at a fairly constant c ≈ 1540 m/s. If a pulse leaves the probe and its echo returns after time t, the reflecting structure sits at depth d = c·t/2 — halved because the round trip covers the distance twice. Attenuation grows with frequency, roughly 0.5 dB/cm/MHz, so the scanner applies time-gain compensation (TGC) to boost the fainter signals from deeper structures automatically.

d = c·t / 2                    c ≈ 1540 m/s (soft tissue)
Reflection coefficient: R = ((Z₂ − Z₁) / (Z₂ + Z₁))²
Soft tissue → bone   (Z: 1.63 → 7.0 MRayl):  R ≈ 41%  (bright echo)
Soft tissue → air    (Z: 1.63 → 0.0004):     R ≈ 99.9% (near-total reflection — why gel is essential)

live demo · a pulse-echo wavefront returning from a boundary● LIVE

B-mode: stacking echoes into a picture

B-mode (brightness mode) builds a 2-D grayscale image one beam angle at a time. Each direction produces an "A-line" — echo amplitude versus depth after envelope detection and log compression to squeeze a 50 dB dynamic range into an 8-bit display. Placing 256–512 of these lines side by side, refreshed roughly 30 times a second, reconstructs the familiar wedge-shaped ultrasound image. Resolution has two independent axes: axial resolution (along the beam) is set by pulse length, typically 0.1–0.5 mm; lateral resolution (across the beam) is set by beam width at the focus and is usually coarser, 1–3 mm.

Doppler: measuring motion, not just position

Moving red blood cells shift the frequency of the reflected pulse — the acoustic Doppler effect. A cell moving toward the probe compresses returning wavefronts, raising the received frequency by an amount proportional to its velocity along the beam:

Δf = 2·f₀·v·cos(θ) / c
Example: f₀ = 5 MHz, v = 0.5 m/s (normal carotid artery), θ = 60°
Δf = 2 × 5×10⁶ × 0.5 × cos(60°) / 1540 ≈ 1623 Hz  (audible!)

Colour Doppler paints this shift as red-toward/blue-away overlays on the B-mode image; spectral Doppler plots velocity against time to characterise valve stenosis via the Bernoulli relation ΔP = 4v². Modern probes replace the single crystal with 128–512 elements fired with sub-microsecond time delays, steering and focusing the beam electronically — the same phased-array trick used in radar — enabling real-time 3-D (4-D) imaging.

Frequently asked questions

Why does ultrasound gel matter so much?

The reflection coefficient at a soft-tissue-to-air boundary is about 99.9% because acoustic impedance is nearly zero for air — almost no sound would ever enter the body without gel. Gel removes the air gap between probe and skin, letting the pulse couple into tissue instead of bouncing straight back.

How does B-mode turn echoes into a picture?

Each beam direction produces one A-line: echo amplitude versus time, converted to depth via d = c·t/2. Hundreds of these lines, one per angle, are placed side by side as columns of brightness to build the 2-D grayscale image, refreshed roughly 30 times a second.

Why can Doppler ultrasound measure blood flow but not tissue position alone?

B-mode only measures how long an echo takes to return, which gives distance, not motion. Doppler additionally measures the frequency shift of the returning echo — moving red blood cells compress or stretch the reflected wavefronts, and that shift, Δf = 2f₀v·cosθ/c, is directly proportional to flow velocity along the beam.

Try it live

Everything above runs in your browser — open Ultrasound Imaging and trade frequency for resolution versus penetration while you build up an A-mode and B-mode scan. Nothing is installed, nothing is uploaded.

▶ Open Ultrasound Imaging simulation