What is intrinsic and extrinsic uniformity in gamma camera imaging?
Energy windowing is the process of selecting only detected photons that fall within a specified energy range around the photopeak of a radionuclide. It is used to reject scattered photons and improve image contrast.
Energy windowing selectively accepts photons near the radionuclide’s photopeak energy, reducing scatter and improving image contrast.
When gamma photons interact within the patient, some undergo Compton scatter and lose energy. If these lower-energy photons are accepted by the detector, they degrade spatial accuracy and reduce contrast. By applying an energy window centred on the known photopeak energy (for example, 140 keV for Tc-99m), the system preferentially records primary photons.
Energy windowing therefore improves image quality by reducing scatter contamination.
Understanding the physics
When a gamma photon emitted from a radionuclide reaches the detector, it ideally deposits its full energy within the crystal. These events form the photopeak in the energy spectrum.
However, many photons scatter within the patient before reaching the detector. During Compton scatter, the photon loses part of its energy and changes direction. These scattered photons may still reach the detector but deposit less than the original energy.
If all detected photons were accepted regardless of energy, scattered photons would be mispositioned and degrade contrast. To reduce this effect, the gamma camera sets an energy window, typically ±10% around the photopeak.
For example, for Tc-99m at 140 keV, a 20% window would accept photons roughly between 126 keV and 154 keV.
The width of the energy window depends on:
Detector energy resolution
Photon energy
Clinical protocol
A narrower window rejects more scatter but also reduces sensitivity. A wider window increases sensitivity but allows more scatter to be recorded. Therefore, energy window selection also reflects a resolution–sensitivity type trade-off.
Where this matters clinically
Proper energy windowing improves lesion contrast and reduces artefacts. Poor energy window selection can lead to excessive scatter contamination or unnecessary loss of counts. Energy peaking is routinely performed as part of gamma camera quality control to ensure accurate window placement.
Related questions
What is quality control in nuclear medicine imaging?