X-ray
Medical Imaging - OCR A-Level Physics
- X-rayHigh-energy electromagnetic radiation (wavelength ~10⁻⁸ to 10⁻¹³ m) used in medical imaging and crystallography.: fast, cheap, good for bones and dense structures. Uses ionising radiation. Cannot distinguish between soft tissues well.
- CTComputed tomography — an imaging technique that takes multiple X-ray projections and reconstructs cross-sectional (and 3D) images.: excellent cross-sectional detail, good soft-tissue contrast. High radiation dose. Expensive. Not real-time.
- UltrasoundSound waves with frequencies above 20 kHz (beyond human hearing). Medical ultrasoundSound waves with frequencies above 20 kHz (beyond human hearing). Medical ultrasound typically uses 1-15 MHz. typically uses 1-15 MHz.: no ionising radiation (safe for pregnancy). Real-time imaging. Portable and inexpensive. Poor image quality compared to CT/MRI. Cannot penetrate bone or air-filled organs well.
- PETPositron emission tomography — an imaging technique that detects gamma rays from positron–electron annihilation events.: shows metabolic activityThe number of nuclear decays per unit time. Measured in becquerels (Bq), where 1 Bq = 1 decay per second. (not just structure). Can detect cancers before structural changes appear. Requires radioactive tracer injection. Low spatial resolutionThe smallest change in a quantity that an instrument can detect. For example, a ruler has a resolution of 1 mm. alone - combined with CT for anatomical detail. Expensive.
Examiner Tips and Tricks
- Comparison questions always require you to link the physics to the clinical application.
- Do not just list advantages - explain WHY using the underlying physics (e.g. 'UltrasoundSound waves with frequencies above 20 kHz (beyond human hearing). Medical ultrasound typically uses 1-15 MHz. is safe in pregnancy because it uses non-ionising sound waves, unlike X-rays which can damage fetal DNA').