Medical Physics · revision map · A-Level 9702, Topic 24

TOPIC 24 · MEDICAL PHYSICS
CAMBRIDGE INTERNATIONAL AS & A LEVEL PHYSICS 9702 · ONE-PAGE REVISION MAP
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MEDICAL PHYSICS24
Ultrasound

Pulse-echo: a piezoelectric transducer sends ultrasound and detects its echoes from tissue boundaries.

  • The crystal vibrates in an a.c. field, and deforms to make an e.m.f., so it both emits and receives.
  • Acoustic impedance Z sets how much reflects.
  • Echo time gives depth; echo size gives boundary type.
  • A-scan: spikes vs depth. B-scan: a 2D image.
Z = ρc Iᵣ/I₀ = (Z₂−Z₁)² / (Z₂+Z₁)²
transducer pulse echo boundary Z₁ | Z₂
X-rays & CT

Shadow imaging by differential attenuation through the body.

  • Fast electrons hit a metal target; on stopping they emit X-ray photons up to hf = eV.
  • Intensity falls exponentially with depth x.
  • Dense bone absorbs more, casting a shadow; contrast media raise μ in soft organs.
  • Sharpness: a small focus and collimation. Contrast: tissues that attenuate differently.
  • CT: rotate the source, then rebuild 2D slices into a 3D image.
I = I₀ e−μx source I₀ patient x I detector
PET Scanning

Map a tracer by back-to-back annihilation photons.

  • A positron-emitting tracer is taken up in active tissue.
  • Each positron meets an electron and they annihilate.
  • This gives two 511 keV gamma photons emitted in opposite directions (180° apart).
  • A detector ring times the pair; the time difference fixes the point on the line, and slices build the image.
Eᶖ = m₀c² ≈ 0.511 MeV each

Conservation of momentum forces the two photons apart at 180°.

e⁺+e− γ γ detector ring records the 180° pair
Comparing the Images

Pick the method to match the question.

  • Ultrasound: no ionising dose, real-time, soft tissue and in pregnancy.
  • X-ray / CT: sharp bone and dense-structure detail; CT gives 3D.
  • PET: shows function, not just shape; maps metabolic uptake.
  • Resolution improves as more slices or angles combine.
  • X-ray, CT and PET use ionising radiation, so dose is limited; ultrasound does not.

Each image is built by detecting reflected or emitted radiation, then timing or attenuating it.

ultrasound · X-ray / CT · PET each trade dose, depth and detail differently
2D slices stack into a 3D image
BUILDS ON · Waves · Quantum · Nuclear · IGCSE Sound → MEDICAL → LEADS TO · Astronomy
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