How a gamma camera works
A sodium iodide crystal gives a tiny flash when a gamma photon hits it
To detect the gamma photons we use a large crystal of sodium iodide. The crystal gives a tiny flash of visible light every time a gamma photon hits it. This flash is picked up by photomultipliers which convert the flash into into an electrical signal. The electrical signals from the photomultipliers are analysed by a computer to construct an image.
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Photomultipliers convert the flashes into an electric signal
A photomultiplier works by causing an increasing avalanche of electrons. The photomultipliers are quite large and may have a diameter of a few centimetres. So if each flash of light was only detected by one photomultiplier, the resolution of the image would be very poor.
The light guide, which is made of transparent plastic, spreads the light from each flash around several photomultipliers. By comparing the intensity of each signal you can compute more accurately where on the crystal the flash occurred.
The collimator helps to focus the image
In reality each bright patch in the tumour emits gamma photons in all directions. This means each bright patch could ‘illuminate’ the whole crystal and we couldn’t tell them apart.
The collimator helps to ‘focus’ the image so that we can tell the bright patches apart. It’s just a lead sheet with holes in it. Only gamma photons travelling perpendicularly to the crystal manage to hit it. All the other photons are absorbed by the collimator.
This is quite wasteful of photons because most never cause a flash. More sophisticated gamma cameras have collimators that allow gamma photons from one point to go through several holes. Clever software and clever arrangement of the holes allows a brighter image to be reconstructed.