Gamma Camera: Working Principle

A gamma camera detects gamma photons emitted by a radiopharmaceutical inside the patient and produces a two-dimensional distribution image.

Schematic

Patient / Radiopharmaceutical
          ↓ Gamma photons
      Collimator
          ↓
  NaI(Tl) Scintillation Crystal
          ↓ Light
 Photomultiplier Tubes
          ↓
 Position + Energy Circuits
          ↓
 Pulse Height Analyser
          ↓
 Computer / Image Processor
          ↓
       Gamma Image

Construction

Main components are:

1. Collimator – accepts photons traveling in selected directions and determines spatial localization.
2. Scintillation crystal – commonly sodium iodide activated with thallium, NaI(Tl), converts gamma photons into visible light.
3. Photodetectors – traditionally photomultiplier tubes convert light into electrical signals.
4. Positioning electronics – estimate the interaction position.
5. Pulse Height Analyser – selects photons in an appropriate energy window and rejects many scattered events.
6. Computer – converts accepted events into an image.

Working

1. A radiopharmaceutical accumulates in the organ of interest.
2. The radionuclide emits gamma photons.
3. The collimator allows selected photon trajectories to reach the detector.
4. The crystal produces flashes of light when gamma photons interact.
5. Photodetectors convert the light into electrical pulses.
6. The system estimates the position and energy of each event.
7. The PHA accepts events within the selected energy window.
8. Accepted events are accumulated to form the image.

Applications

• Bone scans.
• Thyroid imaging.
• Renal imaging.
• Cardiac perfusion studies.
• Hepatobiliary studies.

Conclusion

A gamma camera converts the spatial distribution of emitted gamma radiation into an image using collimation, scintillation, photodetection, energy selection and computer processing.