A-Level Physics: Medical Imaging

Medical imaging shows A level physics at work in hospitals: X-ray tubes and attenuation, radioactive tracers and annihilation photons, and sound waves reflected at tissue boundaries. In some specifications it is compulsory content (for example OCR A includes a medical imaging section in its core modules), and in others it is part of an optional topic (for example the AQA Medical physics option). This material covers the imaging techniques and the calculations that go with them.

The quiz (12 questions) begins with X-rays: the minimum wavelength from a tube at a given accelerating p.d., exponential attenuation and the half-value thickness, the fraction of intensity passing through several half-value thicknesses, which interaction mechanisms dominate at low energies, and what happens in Compton scattering. The advantage of CT scanning over a single X-ray image is examined next. Nuclear medicine questions cover the components of a gamma camera and the origin of the paired 511 keV photons in PET scanning. Ultrasound questions calculate the fraction of intensity reflected at an air–skin boundary (and why coupling gel is essential), an intensity reflection coefficient between two tissues, the depth of a boundary from an A-scan echo time, and the Doppler frequency shift produced by flowing blood.

The flashcards (12 cards) give the maximum X-ray photon energy, the attenuation equation and half-value thickness, acoustic impedance, the intensity reflection coefficient, the Doppler ultrasound equation, the PET annihilation photons, the Z³ dependence of the photoelectric effect, the pair production threshold, the gamma camera collimator, the piezoelectric effect and the role of coupling gel.

The written work (8 questions) asks you to explain how an X-ray tube works and calculate its maximum photon energy and minimum wavelength, describe the four X-ray interaction mechanisms with their energy ranges, explain how barium and iodine contrast media work, compare CT scanning with ordinary X-ray imaging, describe the structure of a gamma camera, explain how PET locates a tracer (including a timing calculation), calculate acoustic impedances and the reflection at a tissue–bone boundary, and find the speed of blood from a Doppler shift.

The content is based on the medical imaging content of A level Physics specifications in England (for example OCR A module 6.5, medical imaging, and the AQA Medical physics option). Constants and data such as speeds of sound and impedances are given in each question.

  • Explain how X-rays are produced and calculate the maximum photon energy and minimum wavelength
  • Use I = I₀e^(−μx) and half-value thickness, and describe the four X-ray interaction mechanisms
  • Explain contrast media and compare CT scans with X-ray images
  • Describe the gamma camera and PET scanning, including annihilation and coincidence detection
  • Use acoustic impedance, intensity reflection coefficients, echo times and the Doppler equation in ultrasound imaging

Practice material written by Zestly, based on the medical imaging content of A level Physics specifications in England (for example OCR A H556 module 6.5 medical imaging and the AQA 7408 Medical physics option).

Sample question

An X-ray tube operates with an accelerating p.d. of $80\ \text{kV}$. What is the minimum wavelength of the X-rays produced? ($h = 6.63 \times 10^{-34}\ \text{J s}$, $c = 3.00 \times 10^{8}\ \text{m s}^{-1}$, $e = 1.60 \times 10^{-19}\ \text{C}$)

See the answer

$1.6 \times 10^{-11}\ \text{m}$

An electron arriving at the anode has kinetic energy $eV = 80\ \text{keV} = 1.28 \times 10^{-14}\ \text{J}$. The most energetic photon takes all of it, so $\lambda_{\min} = \frac{hc}{eV} = \frac{6.63 \times 10^{-34} \times 3.00 \times 10^{8}}{1.28 \times 10^{-14}} = 1.6 \times 10^{-11}\ \text{m}$. Using $80\ \text{V}$ instead of $80\ \text{kV}$ gives a value 1000 times too long.

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