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Chapter 8 — Chapter 8 — Medical Physics
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📘Easy summary
Examples of radiations used in medical physics are X-rays, gamma rays and positrons (from radioactive tracers). Ultrasound imaging operates on the piezoelectric effect, central to the ultrasound transducer. 'Piezein' (Greek) means 'to press'. When mechanical pressure is applied to certain crystals (e.g. quartz), they generate an electric charge — the direct piezoelectric effect. When an alternating voltage is applied, the crystal vibrates and produces sound waves — the converse piezoelectric effect. It is possible because of the unique internal structure of some crystals. The transducer plays a dual role: it emits ultrasound (converse effect) and then switches to receive mode and detects echoes (direct effect).
⭐Key points
- Direct: pressure → voltage (detection).
- Converse: voltage → vibration (emission).
- Same crystal emits and receives.
🔬Diagram
🌿Real-life example
An AC voltage at 5 MHz on the crystal makes it vibrate at 5 MHz, emitting ultrasound.
💡Exam tip
- Always name both effects when explaining the dual role of the transducer.
✅Quick check
1. Ultrasound transducers work on:
2. Applying voltage to make a crystal vibrate is the:
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