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LearnCambridge97022025–2027Paper 4 · A Level Structured QuestionsMedical physics
Paper 4 · Module

Medical physics

3 subtopics≈ 115 min in total
Start the module: UltrasoundOpen graphing calculator

  1. Ultrasound

    Ultrasound (above 2020 kHz; medical uses 11–1515 MHz) is generated and detected by a piezoelectric crystal, which vibrates when an alternating p.d. is applied and produces an e.m.f. when squeezed by returning echoes. Pulses reflect at boundaries between tissues. The fraction reflected depends on the acoustic impedances Z=ρcZ=\rho c: IRI0=(Z1−Z2)2(Z1+Z2)2\dfrac{I_R}{I_0}=\dfrac{(Z_1-Z_2)^2}{(Z_1+Z_2)^2}. A big mismatch reflects almost everything, so gel is used to couple the transducer to the skin. Ultrasound is attenuated exponentially: I=I0e−μxI=I_0e^{-\mu x}.

    ≈ 40 min

  2. X-rays and CT scanning

    X-rays are produced when high-speed electrons hit a metal target. An electron accelerated through p.d. VV can give at most all its energy eVeV to one photon, so the minimum wavelength is λmin⁡=hceV\lambda_{\min}=\dfrac{hc}{eV}. X-rays are attenuated exponentially in matter: I=I0e−μxI=I_0e^{-\mu x}. Bone attenuates much more than soft tissue, giving contrast. CT scanning combines images from many angles to build 2D slices, then stacks them into a 3D image.

    ≈ 40 min

  3. PET scanning

    In PET (positron emission tomography), a tracer containing a β⁺ emitter, often fluorine-18 in a glucose-like molecule, is absorbed by active tissue. Each emitted positron travels a millimetre or so and annihilates with an electron, producing two γ photons of 511511 keV each, travelling in opposite directions (conserving momentum). A ring of detectors records pairs arriving at the same time; the line joining them and the tiny difference in arrival times locate the event. Many events build a map of tracer concentration.

    ≈ 35 min