Photomultiplier tube

A photomultiplier tube converts a weak flash of light into an electrical pulse and amplifies the signal. It does this by using the light to release electrons, then multiplying the number of electrons before collecting them.

When photons strike the photocathode, they release electrons by the photoelectric effect. A potential difference accelerates these electrons towards the first of a series of electrodes called dynodes.

Each electron striking a dynode releases several secondary electrons from its surface. The dynodes are held at successively more positive potentials, so the released electrons accelerate towards the next dynode. Each impact releases more electrons, multiplying their number at each stage.

Finally, an anode collects the resulting electrons. Their arrival produces an electrical pulse much larger than the initial photoelectric signal.

Gamma camera

A gamma camera forms an image showing where a gamma-emitting radioactive tracer is distributed inside the body. The tracer emits gamma photons, some of which emerge from the body and reach the camera. By estimating where many photons arrive, the camera builds up an image of the tracer’s distribution.

Gamma camera: From photons to an image

Collimation: Emerging photons first encounter a lead collimator containing narrow channels. The channels preferentially allow photons travelling in particular directions to reach the detector; photons travelling in other directions are absorbed by the lead. This helps preserve information about where the photons came from.
Light production: Photons that reach the scintillation crystal, commonly made of sodium iodide, can produce small flashes of light in it.
Detection: An array of photomultiplier tubes detects the flashes and produces electrical signals. The relative sizes of the signals from different tubes are used to estimate the position of each detected event in the crystal.
Image formation: The estimated positions of many detected events are combined electronically. The resulting image shows the distribution of the tracer in the body.