KAHIBARO
18.4. Energy Windows
Table of Contents
Lower energy threshold
In a digitized PET or SPECT simulation, a single event represents a processed detector signal that has already passed through the digitizer chain. One of the most important selections applied to singles is the energy window. The lower energy threshold is the minimum deposited energy that a single must have in order to be accepted and written to the singles output.
In GATE, the digitizer usually first sums the energy deposited in a crystal or channel, then optionally applies energy blurring to simulate the detector energy resolution, and finally applies thresholds. Unless you specify otherwise, the threshold is usually applied after blurring, which means the simulated measurement, not the true deposited energy, is compared with the threshold.
The purpose of a lower energy threshold is to reject events that are unlikely to represent useful primary photons. These include low energy deposits from scattered photons, from partial energy deposition, and from background radiation or noise. In PET, for example, 511 keV photons that undergo Compton scattering in the patient before reaching the crystal usually arrive with reduced energy. By choosing an appropriate lower threshold, many of these scattered events are removed, which improves image contrast and reduces bias in quantitative measurements.
In SPECT, where the photopeak is often around 140 keV for Tc 99m or 159 keV for I 123, the lower threshold plays a similar role. Events with energy far below the photopeak are dominated by scatter and are not helpful for reconstructing the primary photon distribution. For this reason, SPECT energy windows are typically defined as a symmetric or asymmetric interval around the photopeak energy, and the lower threshold is the lower bound of that interval.
In a typical PET simulation, you might see a lower energy threshold around 350 to 400 keV. For example, a 350 to 650 keV energy window is common in non TOF PET. In SPECT with Tc 99m at 140 keV, a 15 percent symmetric window corresponds to an approximate range from 119 keV to 161 keV, so the lower threshold is near 119 keV. These values are not universal, but they illustrate how the energy window relates to the isotope photopeak.
In GATE you configure the lower threshold directly in the digitizer settings, usually in units of energy such as keV. It is important to be explicit with units and to keep your threshold consistent with any energy blurring that you apply. If you make the lower threshold too low, you will record many scattered and noisy events, which increases storage requirements and can degrade image quality. If you make it too high, you will start to lose valid photopeak events, which reduces sensitivity.
The lower threshold does not work alone. It is part of the full energy window, which always has both a lower and an upper bound. Together, these define the range of singles that pass to later stages such as coincidence sorting in PET or projection formation in SPECT.
The lower energy threshold defines the minimum accepted energy for a single after digitization. It is a key tool for rejecting low energy scatter and noise while preserving most photopeak events.
Upper energy threshold
The upper energy threshold is the maximum deposited energy that a single may have in order to be accepted. Together with the lower threshold, it defines the complete energy window that selects valid detector events.
In many medical imaging systems, events with energy much higher than the photopeak are rare, but when they occur they often correspond to pile up, multiple photon interactions in the same detector element during the acquisition window, or events with strong electronic noise. For example, in PET, if two 511 keV photons deposit energy in the same crystal within the integration time, the measured energy can be close to or above 1 MeV. These piled up events do not correspond to a single physical photon and can distort energy spectra and downstream analyses if not removed.
The upper threshold eliminates such events. In PET, an energy window such as 350 to 650 keV is an example where 650 keV is the upper threshold. Any single with blurred energy above 650 keV is rejected. This helps to suppress pile up and some forms of scatter. In SPECT, the energy window is usually narrower around the photopeak, and the upper threshold is the high end of that window, such as 161 keV for a 140 keV Tc 99m photopeak with a 15 percent window.
In GATE, you configure the upper threshold in the same digitizer component where you set the lower threshold. The digitizer compares the processed energy value of each single with both thresholds. A single is accepted only if its energy satisfies
$$
E_{\text{low}} \leq E_{\text{single}} \leq E_{\text{high}} ,
$$
where $E_{\text{low}}$ is the lower threshold and $E_{\text{high}}$ is the upper threshold.
It is often convenient to think of the energy window in terms of a percentage around the photopeak. For a photon energy $E_0$ and a symmetric window of width $w$ (for example 20 percent), the thresholds are
$$
E_{\text{low}} = E_0 \left(1 - \frac{w}{2}\right),
\qquad
E_{\text{high}} = E_0 \left(1 + \frac{w}{2}\right).
$$
For a 511 keV PET photon and a 20 percent window, this gives approximately 409 keV and 613 keV. In practice, systems may use slightly different asymmetric windows, especially in SPECT, to better suppress scatter, so the formula is a guide rather than a strict rule.
Choosing the upper threshold too low will clip the upper side of the photopeak, which reduces the number of valid events and lowers sensitivity. Choosing it too high can let through more pile up and higher energy background events, which may bias spectra and images. In simulation, you can explore the effect of different thresholds by examining the singles energy spectrum and visually checking how many events lie above the photopeak region.
In the full PET or SPECT workflow, only singles that pass both the lower and upper thresholds proceed to the next steps. In PET, these events are given to the coincidence sorter to form coincidences. In SPECT, they are accumulated into projection images. Correctly configured upper and lower energy thresholds are therefore essential to simulate realistic system performance and to match experimental acquisition protocols.
The upper energy threshold defines the maximum accepted energy for a single. Together with the lower threshold, it creates an energy window that selects photopeak events and suppresses pile up, high energy background, and out of window scatter.
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