Basic structure and workings of a photomultiplier tube and gamma camera.

The idea

A gamma camera images a gamma-emitting tracer inside a patient. A lead collimator with many parallel channels lets through only gamma photons travelling roughly parallel to the channels, so each point on the detector maps to a direction in the patient. The photons strike a large sodium iodide crystal doped with thallium, which scintillates and emits visible photons; the number emitted is proportional to the gamma energy deposited. These visible photons reach a photocathode, which releases electrons by the photoelectric effect. A photomultiplier tube then amplifies the signal: dynodes at successively higher positive potentials accelerate and multiply electrons by secondary emission, giving a measurable pulse at the anode. Signals from many tubes are processed to build an image.

What earns the marks

• The collimator is a thick lead block pierced by many narrow parallel channels; it absorbs oblique gamma photons so only near-parallel photons reach the crystal, defining the field of view and spatial resolution.
• The scintillator is a large thallium-doped sodium iodide crystal; an absorbed gamma photon produces a burst of visible photons whose number is proportional to the energy deposited.
• The photocathode releases electrons when visible photons strike it, by the photoelectric effect; the number of photoelectrons is proportional to the light intensity.
• The size of the anode pulse indicates the energy of the original gamma photon, allowing unwanted scattered photons to be rejected, and the position of the tube giving the signal locates the event.
• A gamma camera detects gamma photons emitted from within the patient, unlike a gamma-ray detector used for transmission imaging, because the tracer is inside the body.

Common mistakes and how to put them right

• Thinking the collimator focuses gamma photons like a lens.

  Put it right: It does not refract or focus them, it simply absorbs photons that are not travelling nearly parallel to the channels.

• Confusing the roles of the crystal and the photocathode.

  Put it right: The crystal converts gamma photons to visible photons, while the photocathode converts visible photons to electrons.

• Believing the dynodes emit gamma rays or that the tube detects gamma photons directly.

  Put it right: The dynodes multiply electrons by secondary emission, and the gamma photon is detected via the scintillation light.

• Stating that all dynodes are at the same potential.

  Put it right: They must be at successively higher positive potentials so that electrons gain kinetic energy between dynodes.

Exam tips

• Sketch a labelled diagram with collimator, crystal, photocathode, dynodes and anode, and annotate the energy conversion at each stage.
• Use the phrase 'successively higher positive potential' when describing dynodes, and link the anode pulse size to the energy of the incident gamma photon.
• When asked why the collimator is needed, refer to rejecting obliquely travelling photons and hence improving spatial resolution, not to focusing.
• Practise explaining the photoelectric effect at the photocathode and secondary emission at the dynodes as two distinct processes.