Physics 9702 · syllabus 24
Cambridge A Level Physics 9702 — Medical physics topical past papers
The physics behind medical imaging: ultrasound and acoustic impedance, X-ray production, attenuation and CT scanning, and PET scanning with positron tracers.
- Questions
- 29
- Years
- 2020–2026
- With mark scheme
- 100%
- Questions 2022–2026
- 21
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What Medical physics covers
Ultrasound is produced and detected by a piezoelectric crystal, which changes shape when a p.d. is applied across it and generates an e.m.f. when its shape is changed. Pulses sent into the body reflect at boundaries between tissues, and the delay of each echo gives the depth of the boundary. How much is reflected depends on the specific acoustic impedance, Z = ρc, of the two media: the bigger the mismatch, the stronger the echo, which is why a coupling gel is used between the transducer and the skin.
X-rays are produced when fast electrons strike a metal target; the shortest wavelength corresponds to an electron giving all of its energy, eV, to a single photon. X-rays are attenuated exponentially in matter, I = I₀e^(−μx), and the different attenuation coefficients of bone and soft tissue give an image its contrast. CT scanning combines many X-ray images of one section, taken from different angles, into a cross-section, then stacks sections into a three-dimensional image.
PET scanning uses a tracer containing a β⁺ emitter. Each positron annihilates with an electron, producing two gamma-ray photons that travel in opposite directions; detecting the pair and comparing their arrival times locates the event, building an image of where the tracer has collected. This A Level topic, examined on Paper 4, draws on waves, photons from quantum physics and β⁺ decay from particle physics.
In the syllabus
The headings this topic has in the Cambridge International 2025–2027 syllabus, as the board prints them.
- 24 Medical physics
- 24.1 Production and use of ultrasound
- 24.2 Production and use of X-rays
- 24.3 PET scanning
What you should be able to do
- Explain how a piezoelectric transducer generates and detects ultrasound pulses
- Use Z = ρc and the intensity reflection coefficient, and explain why a coupling gel is needed
- Use I = I₀e^(−μx) for the attenuation of both ultrasound and X-rays
- Calculate the minimum X-ray wavelength from the accelerating p.d., and explain contrast
- Outline how CT scanning builds a three-dimensional image from X-ray images of sections
- Explain PET scanning and calculate the energy of each annihilation photon
Key ideas
- Specific acoustic impedance Z = ρc (kg m⁻² s⁻¹)
- Intensity reflection coefficient: reflected intensity ÷ incident intensity = (Z₁ − Z₂)²/(Z₁ + Z₂)²
- Attenuation I = I₀e^(−μx), with μ the linear attenuation coefficient (m⁻¹)
- Minimum X-ray wavelength = hc/(eV), where V is the accelerating p.d.
- Electron–positron annihilation at rest: each of the two photons carries mₑc² = 8.2 × 10⁻¹⁴ J (0.51 MeV)
How Medical physics has been examined
From 2022 to 2026, Medical physics was on Paper 4 · A Level Structured Questions in 11 of 11 exam series, worth about 6% of that paper’s marks.
| Paper | Questions | Share of marks | Series |
|---|---|---|---|
| Paper 4 · A Level Structured Questions | 21 | 6% | 11 of 11 |
Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 9.
Filed in Momentum under: Medical Imaging.
A worked example
An ultrasound pulse travels 3.0 cm through soft tissue (Z = 1.5 × 10⁶ kg m⁻² s⁻¹, μ = 23 m⁻¹) to a boundary with bone (Z = 7.6 × 10⁶ kg m⁻² s⁻¹), and the reflected pulse returns to the transducer. What fraction of the original intensity arrives back?
- Reflection coefficient: (7.6 − 1.5)² / (7.6 + 1.5)² = 6.1² / 9.1² = 0.449
- The pulse crosses 3.0 cm of tissue there and back, so x = 0.060 m
- Attenuation factor: e^(−μx) = e^(−23 × 0.060) = e^(−1.38) = 0.252
- Fraction returning = 0.449 × 0.252 = 0.113
Answer: About 0.11, or 11%, of the original intensity
Written by Momentum for this page, with its own numbers; it is not taken from a past paper.
Mistakes that cost marks
- Forgetting that an echo travels to the boundary and back — the depth is half the distance the pulse travels, and attenuation acts over the whole return path
- Taking the reflection coefficient as the fraction transmitted — the transmitted fraction is 1 minus the reflection coefficient
- Saying the PET gamma rays come from the tracer nuclei — they come from the annihilation of an emitted positron with an electron
- Giving each annihilation photon an energy of 2mₑc² — that is the total, shared between the two photons
- Linking the minimum X-ray wavelength to the target metal — it depends only on the accelerating p.d.
Exam technique
- In PET answers, give the sequence in order: tracer absorbed by tissue, β⁺ decay, annihilation, two photons in opposite directions, detection and arrival-time comparison, image
- When explaining contrast, compare the attenuation coefficients of the tissues rather than saying bone blocks X-rays
- Keep μ and x in matching units, m⁻¹ with m or cm⁻¹ with cm, before evaluating e^(−μx)
Practising in Momentum
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