A-LEVEL 9702 · A2 · TOPIC 24

Medical physics

The chain of the topic: three imaging methods reuse earlier physics to see inside the body. Ultrasound exploits the reflection of sound at acoustic-impedance boundaries, X-rays exploit photon attenuation through tissue, and PET exploits positron annihilation into a back-to-back gamma pair. Around the hexagon are the three techniques; above is what they build on, below is where the detection ideas lead.

TOPIC 24: MEDICAL PHYSICS CAMBRIDGE A-LEVEL PHYSICS 9702 · PATHWAYS TheLucidSTEM · thelucidstem.com BUILDS ON T7Waves: intensity, reflection, I = I₀e−μx T22Quantum physics: photon energy hf = eV T23Nuclear physics: β⁺ decay, E = mc² 24.1 24.2 24.3 image TOPIC 24 MEDICAL PHYSICS 1 · ULTRASOUND Pulse-echo: reflect sound off tissue boundaries. A piezoelectric crystal vibrates in an a.c. field, and deforms to make an e.m.f., so it sends and detects. A pulse partly reflects at each boundary; echo time gives depth, echo size gives the boundary type. Z = ρc    Iᵣ/I₀ = (Z₁−Z₂)²/(Z₁+Z₂)² I = I₀ e−μx transducer tissue boundary Z₁ | Z₂ incident pulse reflected echo 2 · X-RAYS & CT Shadow imaging by differential attenuation. Fast electrons hit a metal target; on stopping they emit X-ray photons, with a few keV up to hf = eV. Dense bone absorbs more, so it casts a shadow; contrast media raise μ in soft organs. CT spins the source, then rebuilds 2D slices into 3D. I = I₀ e−μx source patient (thickness x) I₀ I detector 3 · PET SCANNING Map a tracer by back-to-back annihilation photons. A β⁺-emitting tracer is taken up in active tissue. Each positron meets an electron and annihilates, giving two 0.51 MeV gamma photons, 180° apart. A detector ring times the pair; the difference fixes the point on the line, and slices build the image. Eᶖ = m₀c² = 0.51 MeV each e⁺ + e− γ γ detector ring records the pair 4 · COMPARING THE IMAGES Pick the method to match the question. Ultrasound: no ionising dose, real-time, soft tissue. X-ray and CT: sharp bone and dense structure detail. PET: function, not just shape; shows metabolic uptake. Resolution improves as more slices or angles combine. 2D slices stack into a 3D image LEADS TO T25Astronomy: detecting and imaging distant EM radiation T23Nuclear physics: dose, decay and detector revisited T22Quantum physics: photon spectra and detection One idea carries forward: an image is built by detecting reflected or emitted radiation and timing or attenuating it.

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