24.1 Production and Use of Ultrasound

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Understand that a piezo-electric crystal changes shape when a p.d. is applied across it, and that the crystal generates an e.m.f. when its shape changes.
  2. Understand how ultrasound waves are generated and detected by a piezoelectric transducer.
  3. Understand how the reflection of pulses of ultrasound at boundaries between tissues can be used to obtain diagnostic information about internal structures.
  4. Define the specific acoustic impedance of a medium as Z = ρc, where c is the speed of sound in the medium.
  5. Use I_R / I₀ = (Z₁ – Z₂)² / (Z₁ + Z₂)² for the intensity reflection coefficient of a boundary between two media.
  6. Recall and use I = I₀ e^(–μx) for the attenuation of ultrasound in matter.

1. The Piezoelectric Effect

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What is ultrasound?

Ultrasound is sound with a frequency above the range of human hearing — that is, above 20 000 Hz (20 kHz). In medical applications, frequencies can reach into the megahertz (MHz) range.

What is a piezoelectric crystal?

A piezoelectric crystal (e.g. quartz, made of silicon dioxide, SiO₂) is a special material with two remarkable and linked properties:

  • Property 1 — Applied voltage causes shape change: When a potential difference (p.d.) is applied across the crystal, the crystal changes shape — it either expands or contracts (compresses). If the p.d. is reversed, the crystal deforms in the opposite direction. If an alternating p.d. is applied, the crystal rapidly vibrates back and forth at the same frequency as the alternating voltage.

  • Property 2 — Shape change generates an e.m.f.: When the crystal is mechanically deformed (squeezed or stretched by an external force), the arrangement of positive and negative charges inside shifts. This charge separation creates an electromotive force (e.m.f.) — that is, a voltage — across the crystal. This is the piezoelectric effect.

Why does this happen inside the crystal?

In a quartz crystal, silicon atoms carry a partial positive charge and oxygen atoms carry a partial negative charge. When the crystal is in its normal, unstressed state, these charges are distributed symmetrically — they balance out and no voltage is produced. When the crystal is compressed, the centres of positive and negative charge shift relative to each other, creating a net charge separation and therefore a voltage. When the crystal is stretched, the charges shift in the opposite direction, producing a voltage in the opposite direction.

In summary:

  • Apply a p.d. → crystal changes shape (vibrates)
  • Deform the crystal → crystal produces an e.m.f.

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