Pulse Height Analyser in a Gamma Camera

Definition

A Pulse Height Analyser (PHA) is an electronic energy-discrimination system used in nuclear medicine to select detector pulses corresponding to a desired range of gamma-photon energies.

Need for PHA

A gamma camera receives useful photons as well as scattered photons. Compton-scattered photons have lower energies than the original photopeak. If these scattered events are accepted, image contrast and spatial accuracy decrease.

Working

Gamma Photon
     ↓
Collimator
     ↓
Scintillation Crystal
     ↓
Photomultiplier Tubes
     ↓
Position + Energy Signals
     ↓
Pulse Height Analyser
     ↓
Accept / Reject
     ↓
Image Computer

1. A gamma photon enters through the collimator.
2. The scintillation crystal converts the gamma photon into visible light.
3. Photomultiplier tubes or solid-state photodetectors convert the light into electrical pulses.
4. Pulse amplitude is approximately related to deposited photon energy.
5. The PHA compares the pulse height with a selected energy window around the photopeak.
6. Pulses inside the window are accepted for image formation; others are rejected.

Energy Window

For a radionuclide with a known photopeak energy $E_0$, a window may be selected around $E_0$, for example:

$$E_0 \pm \Delta E$$

The exact window is chosen according to the radionuclide and camera protocol.

Importance

• Rejects a large fraction of scattered photons.
• Improves image contrast.
• Improves quantitative accuracy.
• Helps isolate the desired photopeak.

Conclusion

The PHA acts as an energy filter in a gamma-camera system, accepting pulses within the selected energy range and rejecting unwanted radiation.