Gamma camera

 

TExt

The gamma camera is the device that detects the gamma rays emitted from the patient and converts them into an image. The four fundamental components of the gamma camera are:

  1. a sodium iodide crystal

  2. an array of photomultiplier tubes

  3. a collimator

  4. a computer

Schematic of a gamma camera

When a gamma ray is emitted from the patient and strikes the sodium iodide crystal, a light photon is emitted from the crystal (scintillation).  The emitted light is detected by the nearest photomultiplier tubes, resulting in an electric current that is proportional to the quantity of light that strikes each tube.  The imaging computer assesses the relative current from adjacent tubes for each scintillation, applying a formula to accurately localise the site at which the emitted gamma ray struck the crystal.


Obviously gamma rays are emitted from the patient in random directions, which could make localisation of the origin of the tracer impossible.  The collimator is a heavy lead plate with multiple holes drilled into it, perpendicular to the crystal.  It blocks gamma radiation travelling in an oblique direction, which would give erroneous localisation information.  Only radiation that is passing in a perpendicular direction to the crystal (and which therefore provides a true representation of the localisation of radioactivity in the patient) is permitted to pass through the drilled holes and strike the crystal.

Collimator

Camera head, containing the crystal adjacent to the collimator (covered by the blue part of the camera cover), with about 60 photomultiplier tubes covered by the larger white part of the head cover

Patient gantry

Gamma cameras usually have either one or two “heads”, where the head is the part of the camera composed of the collimator, the crystal and the photomultiplier tubes.  This part of the camera is brought as close as possible to the patient during imaging.  A dual-headed camera can simultaneously acquire images of the anterior and posterior aspects of the body, which is useful for example in whole body imaging.  A single headed camera could also be used for whole body imaging, but it would take twice as long, since the anterior and posterior images would have to be acquired separately.  The advantage of the single-headed camera is that it is more manouverable, making it useful for immobile patients and specific procedures like thyroid or dynamic renal scintigraphy, which require only a single view.


It is safe to enter the camera room when a patient is being imaged (unlike CT for example), so make sure you go into the room to see an examination being acquired.  You will see the image forming on the screen in the room as “counts” are accumulated by the camera.

This is the second head of this dual-headed camera, which allows image data to be collected simultaneously in two different projections (for example, anterior and posterior in a whole body bone scan)

A single -headed camera, with the head to the right side and a rectangular counterweight on the left.  This camera is much easier to position close to a patient who is unable to lie flat, for example, than the dual-headed camera shown above.

The two heads of the dual-headed camera can also be configured at right angles like this, ideal for maximising the detection of counts during a cardiac study.