3.10.6.2
The gamma camera
Medical Physics | AQA A-Level Physics
- A gamma camera detects gamma rays emitted by a radioactive tracer inside the body. It has four main components in sequence:
- 1. Collimator: a thick lead plate with thousands of parallel holes (tubes). Only gamma rays travelling perpendicular to the camera face pass through. This ensures that each point on the detector corresponds to a specific position in the body, giving spatial information. The resolution of the gamma camera is about 6 mm.
- 2. Scintillator crystal: usually sodium iodide doped with thallium (NaI(Tl)). Gamma photons interact with the crystal and produce flashes of visible light (scintillations). The number of light photons is proportional to the gamma ray energy.
- 3. Photomultiplier tubes (PMTs): each scintillation flash hits a photocathodeA surface that emits electrons when light falls on it, via the photoelectric effect., which emits electrons by the photoelectric effect. These electrons are accelerated through a series of dynodesElectrodes at progressively higher voltages inside a photomultiplier tube. Each electron hitting a dynode releases several more electrons, producing an amplification cascade. at progressively higher voltages, with each dynode releasing several secondary electrons. This cascade produces a measurable electrical pulse from a single gamma photon.
- 4. Computer: analyses the arrival position (from which PMTs fire) and timing of pulses to build a 2D image showing the distribution and concentration of the tracer in the body.