Gamma Camera (OCR A Level Physics)

• A medical tracer's progress around the body can be detected using a gamma camera
• A gamma camera is comprised of four major components:
  • Collimator
  • Scintillator
  • Photomultiplier tubes
  • Computer and display

Collimator

• Images of slices of the body can be taken to show the position of the gamma-emitting radioactive tracers
• Once injected with a tracer, the patient lays stationary in a tube surrounded by a ring of detectors
• When gamma rays are emitted, they are absorbed by thin lead tubes known as collimators
• Collimators are the key to producing the sharpest and highest resolution images 
  
  • Only photons moving parallel to the collimator will be absorbed, this improves the sharpness of the image as scattered photons are excluded
  • The narrower and longer the collimators, the more gamma rays that will be absorbed and hence, the more electrons that will be produced, this improves the image quality as more electrons contributing to the electrical pulse output will increase the resolution of the image

Scintillator

• When the gamma-ray (γ-ray) photon is incident on a crystal scintillator, an electron in the crystal is excited to a higher energy state
  • As the excited electron travels through the crystal, it excites more electrons
  • When the excited electrons move back down to their original state, the lost energy is transmitted as visible light photons

Photomultiplier Tubes

• The photons produced by the scintillator are very faint
• Hence, they need to be amplified and converted to an electrical signal by a photomultiplier tube

Image Formation on a Computer

• The signals produced by the photomultiplier tubes are used to produce an image using the electrical signals from the detectors
• The tracers will emit lots of γ rays simultaneously, and the computers will use this information to create an image
• The more photons from a particular point, the more tracer that is present in the tissue being studied, and this will appear as a bright point on the image
• An image of the tracer concentration in the tissue can be created by processing the arrival times of the gamma-ray photons

• Gamma camera imaging can be used for diagnosing issues in multiple organs
• When imaging a patient using a gamma camera, a gamma emitter, usually technetium-99m, is used as the radioactive tracer
  • The 'm' stands for metastable which means its nucleus stays in a high-energy state for extended periods
• Tc-99m loses energy by the emission of a gamma photon with an energy of exactly 140 keV
• When Tc-99m decays through gamma emission (with a half-life of approximately 6 hours), it becomes Tc-99, which is stable and has a half-life of 210,000 years
• Technetium-99m is used because:
  • Its short half-life means it stays around long enough to be imaged but reduces harm to the patient.
  • Its chemical properties enable a small quantity to be incorporated into several tissues, so it can be used to image several organs at once