Piezoelectric devices Principles of generation and detection of ultrasound…

The idea

A piezoelectric crystal, often lead zirconate titanate, changes shape when a voltage is applied and produces a voltage when deformed. In an ultrasound transducer, a brief alternating voltage at the resonant frequency makes the crystal vibrate and emit a short ultrasound pulse. The same crystal then acts as a detector: returning echoes deform it and generate a small alternating voltage. A backing material damps vibrations so the pulse is short, improving axial resolution. The transducer is coupled to skin with gel to reduce reflection at the air boundary. The time between emission and echo gives depth using d = vt/2, and echo amplitude indicates the size of the acoustic impedance change.

What earns the marks

• A piezoelectric material converts an applied alternating voltage into mechanical vibration, generating an ultrasound pulse at the driving frequency.
• The same piezoelectric crystal detects returning echoes because incident sound deforms it and generates a small alternating voltage.
• The transducer is driven at its resonant frequency to produce maximum amplitude output, and damping shortens the pulse to improve resolution.
• Coupling gel excludes air between transducer and skin because the large acoustic impedance mismatch at an air boundary would reflect nearly all the ultrasound.
• The time delay between pulse emission and echo detection gives reflector depth using d = vt/2, where v is the speed of sound in the medium.
• Echo amplitude depends on the acoustic impedance difference at a boundary, so larger mismatches produce stronger echoes.

Common mistakes and how to put them right

• Thinking a separate transmitter and receiver are always needed.

  Put it right by explaining that one piezoelectric crystal can both generate and detect ultrasound.

• Ignoring the factor of one half in depth calculations.

  Put it right by using d = vt/2 because the pulse travels to the boundary and back.

• Assuming coupling gel improves the piezoelectric effect.

  Put it right by stating that gel provides acoustic coupling and reduces reflection at the air–skin boundary.

Exam tips

• Describe the sequence emission, propagation, reflection, detection and timing, so the role of the piezoelectric crystal is clear at each stage.
• When calculating depth, identify the total travel time and divide by two before multiplying by speed, or use d = vt/2 directly.
• Use the term acoustic impedance mismatch when explaining echo strength, and link it to reflection at tissue boundaries.