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An endoscope transports light from inside the body; an MR scanner detects radio-frequency signals from nuclei. Both avoid ionising radiation but form images by different physical mechanisms. This lesson is part of the Medical Physics option; study it if this is your centre’s chosen Paper 3 Section B topic.
What you'll be able to do
An optical fibre has a core of higher refractive index than its cladding. A ray inside the core undergoes total internal reflection when it meets the boundary at an incidence angle greater than the critical angle. Measure that angle to the normal, not the fibre axis.
A flexible endoscope uses an objective to form an image at the end of a coherent fibre bundle. Fibres maintain the same relative arrangement at both ends, preserving the image. An illumination bundle need not preserve arrangement and may be non-coherent. Here “coherent bundle” describes spatial ordering, not a fixed optical phase relationship.
Hydrogen nuclei have magnetic moments and precess in a strong magnetic field. There is a small excess in the lower-energy alignment, producing a net magnetisation; it is inaccurate to say every proton has exactly the same alignment. Radio-frequency pulses at resonance excite nuclei and disturb the net magnetisation.
As the system returns towards equilibrium, changing magnetisation produces a detectable RF signal. Gradient coils make field strength, and therefore resonant frequency, depend on position so signals can be assigned to regions in a section. A computer reconstructs the image from the detected signals. Detailed magnet construction and relaxation-time calculations are outside this AQA option.
Endoscopy provides a direct optical view of accessible surfaces and can allow sampling or small interventions, but insertion is invasive and does not reveal arbitrary deep cross-sections. MR provides detailed soft-tissue sections without ionising photons; scans can be slow and motion affects image quality.
The strong magnetic field creates practical restrictions involving certain implants and ferromagnetic objects. Describe these as consequences of the field, rather than saying the RF waves are radioactive. For a comparison question, connect each advantage or limitation to the structure being investigated.
Try each problem before opening the worked solution. State the physical model, show substitutions with consistent units, and explain what your answer means.
Equation recap
Common mistakes to avoid
Key takeaways
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.