Medical Physics — A-Level Physics Revision

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What is Medical Physics?

Medical physics is the application of physics principles and techniques to medicine and healthcare. This topic often explores various non-invasive imaging techniques. Key areas include the production and use of X-rays for diagnosis, understanding concepts like attenuation and contrast, and the principles of ultrasound scanning, including A-scans and B-scans, and the importance of acoustic impedance matching.

Board notes: Medical Physics is a common optional topic in the AQA and OCR specifications. It provides a practical application of many core physics principles, such as waves, electricity, and radiation. The level of detail required for different imaging modalities (like MRI and PET scans) can vary between boards.

Worked example

Common mistakes

• 1Confusing the mechanisms of X-ray and ultrasound imaging. X-ray imaging depends on the differential absorption of photons by different tissues, while ultrasound imaging depends on the reflection and transmission of sound waves at tissue boundaries.
• 2Misunderstanding acoustic impedance. Acoustic impedance (Z = ρc) is a property of the medium, not the ultrasound wave. A large difference in acoustic impedance between two media causes strong reflection, which is essential for forming an image but can also create artefacts.
• 3Forgetting the role of the coupling gel in ultrasound. The gel is used to eliminate the air gap between the transducer and the skin. Since the acoustic impedance of air is very different from that of skin, most of the ultrasound would be reflected without the gel, preventing a clear image from being formed.

Medical Physics exam questions

Exam-style questions for Medical Physics with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• How are X-rays produced for medical imaging?

  X-rays are produced in an X-ray tube when fast-moving electrons, accelerated by a high voltage, are suddenly decelerated by hitting a metal target (usually tungsten). Their kinetic energy is converted into heat and X-ray photons.

• What is the difference between an A-scan and a B-scan in ultrasound?

  An A-scan (Amplitude scan) is a one-dimensional scan showing the amplitude of reflected pulses against depth, used for measuring distances. A B-scan (Brightness scan) uses multiple A-scans from different angles to build up a two-dimensional image, where the brightness of each dot corresponds to the echo strength.