When a gamma photon interacts with the scintillation crystal, it deposits energy that produces a flash of light. The intensity of this light is proportional to the photon’s energy. The gamma camera electronics measure this signal and assign it to an energy spectrum.

For a radionuclide such as Tc-99m, most useful imaging photons have a characteristic energy (140 keV). On an energy spectrum, this appears as a photopeak.

However, not all detected photons are primary photons. Some undergo Compton scatter within the patient before reaching the detector. These scattered photons have lower energy and reduce image contrast if accepted.

To minimise this, the gamma camera uses an energy window, typically set around ±10% of the photopeak energy. Only photons within this window are counted for image formation.

Energy window calibration ensures that:

• The window is centred exactly on the true photopeak.

• Photomultiplier gain drift has not shifted the energy spectrum.

• The correct radionuclide is being detected.

If the energy window is set too low, excess scatter photons will be included, reducing contrast. If set too high or misaligned, true primary photons may be excluded, reducing sensitivity and count statistics.

Because photomultiplier tubes can drift over time, daily energy peaking ensures that the system remains correctly tuned.

Accurate energy calibration is also critical for multi-isotope imaging and quantitative SPECT.