Generating and Detecting Ultrasound

Ultrasound imaging depends on a single, elegant physical trick: a crystal that turns electricity into sound, and sound back into electricity, using exactly the same mechanism running in both directions. Understanding that mechanism is the key to everything ultrasound imaging can do.

What is ultrasound?

Ultrasound is simply sound — a mechanical, longitudinal pressure wave — at frequencies above the upper limit of human hearing, roughly above 20 kHz. Diagnostic medical ultrasound typically uses frequencies in the range of a few megahertz, far beyond anything audible, chosen because higher frequencies give shorter wavelengths and therefore finer image resolution, while still being able to penetrate soft tissue to a useful depth. As a mechanical wave, ultrasound needs a physical medium to travel through, and it propagates through the body's tissues, reflecting at boundaries between different structures in ways that later lessons in this topic use to build up images.

Before any of that imaging can happen, however, a beam of ultrasound first has to be generated, sent into the body, and then detected again once it returns as a faint echo. Both of these jobs — generating and detecting — are done by the same physical effect, working in two opposite directions.

The piezoelectric effect

Certain crystals (and some ceramics engineered to behave similarly), called piezoelectric materials, have a special property linking their mechanical shape to their electrical state. This relationship works both ways, and both directions are essential to how an ultrasound transducer operates.

• The direct piezoelectric effect. When a piezoelectric crystal is mechanically deformed — squeezed, stretched, or struck — by an external force, it generates a small voltage across itself. Mechanical stress in, electrical signal out. This is exactly what happens when a returning ultrasound echo strikes the crystal: the tiny mechanical vibration of the returning pressure wave deforms the crystal slightly, and that deformation produces a small but measurable voltage that can be amplified and processed electronically.
• The converse (reverse) piezoelectric effect. Conversely, when a varying voltage is applied across a piezoelectric crystal, the crystal physically deforms — it mechanically vibrates in response to the changing electric field. Electrical signal in, mechanical vibration out. This is exactly what allows ultrasound to be generated in the first place: applying an oscillating voltage at a chosen frequency makes the crystal vibrate at that same frequency, and those mechanical vibrations are transmitted into the body as a genuine sound wave.

The crucial point is that these are two directions of the same underlying physical effect, occurring in the same type of material. A single piezoelectric crystal, or a matched pair, can therefore act as both the source and the detector of ultrasound, simply by switching which direction of the effect is in use at a given moment.

Pulse generation and detection

In practice, an ultrasound transducer uses its piezoelectric crystal in a rapid two-stage cycle:

• Transmit. A short burst of oscillating voltage is applied across the crystal. Via the converse piezoelectric effect, the crystal vibrates at the driving frequency for the duration of the burst, generating a brief pulse of ultrasound that is transmitted into the body.
• Receive. Once the pulse has been sent, the crystal switches to acting as a receiver. Any returning echo — ultrasound reflected back from a boundary between tissues inside the body — strikes the crystal and mechanically deforms it very slightly. Via the direct piezoelectric effect, this produces a small voltage signal, which is amplified and processed to build up information about what lies inside the body.

This transmit-then-receive cycle happens for every single pulse sent into the body, and is repeated many times per second to build up a full scan. The time delay between sending a pulse and receiving its echo is what carries the depth information used throughout ultrasound imaging — a relationship developed fully in the A-Scans and B-Scans lesson later in this topic.

Transducer design

A real medical ultrasound transducer (the handheld probe placed against the patient's skin) is more than just a bare piezoelectric crystal. Two further design features are needed to make the pulse-echo cycle work well in practice:

• Backing material. A layer of dense, highly absorbent material is bonded to the back of the crystal. Without it, the crystal would continue vibrating (ringing) for some time after the driving voltage burst ends, just as a struck bell keeps ringing after the strike. The backing material rapidly damps out this vibration, so the crystal stops transmitting and is ready to detect a returning echo almost immediately, rather than still being lost in its own residual vibration.
• Matching layer. A thin layer is often placed on the front face of the crystal, between it and the skin. Its role is to reduce the large mismatch in acoustic properties between the piezoelectric crystal and body tissue, which would otherwise cause much of the pulse's energy to simply reflect at that boundary rather than entering the patient. This idea — acoustic impedance mismatch and how much of a pulse reflects at a boundary — is developed fully with its own formula in the next lesson.

Bringing it together

A single physical effect, working in two directions inside the same crystal, is what makes pulse-echo ultrasound imaging possible: the converse piezoelectric effect turns an electrical drive signal into the outgoing pulse, and the direct piezoelectric effect turns the returning echo back into an electrical signal. Everything else in ultrasound imaging — how far a boundary is, how strongly it reflects, and how a full 2D image gets built from many such pulses — is built on top of this basic transmit-receive cycle, made practical by careful transducer design.

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