KAHIBARO
SPECT Radionuclides
Table of Contents
Tc-99m
Technetium 99m (Tc 99m) is the workhorse radionuclide for SPECT. It is a metastable nuclear state that decays to Tc 99 by emitting a single gamma photon with an energy of about 140 keV. This gamma energy is very well suited to typical gamma cameras because it is high enough to exit the patient with reasonable probability but low enough to be efficiently stopped in standard scintillation crystals such as NaI(Tl).
In GATE simulations that involve Tc 99m, you usually model a monoenergetic gamma source at 140 keV, possibly with a small energy spread if you want to account for the physical line width. The physical half life of Tc 99m is about 6 hours, which is long enough to acquire clinical images but short enough to keep patient radiation doses moderate. When you include time dependent activity in GATE, you can model the decay of Tc 99m activity during an acquisition using the exponential law $A(t) = A_0 e^{ (\ln 2) t / T_{1/2} }$ with $T_{1/2}$ equal to 6 hours.
From a simulation perspective, Tc 99m is often the first choice when you set up basic SPECT examples. It matches many standard clinical protocols such as bone scans or myocardial perfusion, and the nearly monoenergetic gamma simplifies the definition of energy windows. In a Tc 99m SPECT simulation, the primary energy window is typically centered on 140 keV, for example 20% wide. The choice of this window is not part of radionuclide physics itself but the energy of Tc 99m defines where this window must be placed.
In SPECT simulations with collimators, the gamma energy affects septal penetration and scatter in the collimator, which in turn influence image resolution and contrast. For Tc 99m the 140 keV photons interact strongly with lead, so parallel hole collimators are effective and septal penetration is relatively low compared with higher energy radionuclides. When you simulate Tc 99m in GATE, you therefore usually pair it with collimator designs optimized for low energy photons.
I-123
Iodine 123 (I 123) is another important SPECT radionuclide, often used for thyroid imaging and certain brain and cardiac studies. Its main gamma emission is around 159 keV. This energy is slightly higher than the Tc 99m line, which leads to somewhat different interactions in the patient and in the collimator. In GATE, you model I 123 by defining a gamma source with the appropriate emission energies and branching ratios. Compared with Tc 99m, I 123 has a more complex emission spectrum with additional photons at lower and higher energies, though the 159 keV line is dominant for imaging.
The physical half life of I 123 is about 13 hours. This longer half life compared with Tc 99m affects time dependent simulations where activity may change more slowly during the acquisition. When you simulate dynamic studies or long SPECT protocols, the slower decay of I 123 means that activity remains relatively stable over typical acquisition times, which you can represent in GATE by keeping the source activity approximately constant or by applying the appropriate decay law over the acquisition window.
Because I 123 photons are more energetic than Tc 99m photons, they are less attenuated in the patient but they also penetrate collimator septa more easily. In a simulation of an I 123 SPECT system you must therefore use a collimator model that is appropriate for medium energy photons. This has a direct effect on spatial resolution and background due to septal penetration. In GATE, the radionuclide choice does not change the geometry, but the photon energy that comes from the radionuclide definition determines how the gamma camera and collimator interact with the radiation.
In terms of energy windows, an I 123 SPECT simulation typically uses a primary window centered on 159 keV. The window width may differ from Tc 99m because of the higher energy and the presence of additional emissions. When studying scatter and penetration in GATE, I 123 is often used as an example of a radionuclide that requires medium energy collimators and may benefit from more sophisticated scatter correction schemes.
In-111
Indium 111 (In 111) is characteristic for having multiple gamma emissions, which makes it a good example of a multi peak SPECT radionuclide. It is widely used in imaging procedures such as labeled leukocyte scans or certain targeted therapies. The two main gamma energies are about 171 keV and 245 keV, both with significant branching ratios. In GATE, modeling In 111 means defining both of these gamma lines in the source description so that the simulation can produce realistic energy spectra at the detector.
The half life of In 111 is about 2.8 days, much longer than Tc 99m or I 123. For typical acquisition times, physical decay is modest, but for longitudinal or multi day simulations you need to account for the slowly decreasing activity. In a time dependent GATE simulation, In 111 therefore allows you to explore scenarios where the source distribution is essentially stable over each scan but changes over days.
Because In 111 photons are more energetic, they interact differently with both tissue and the collimator. Attenuation in the patient is reduced compared with 140 keV photons, which can improve penetration to deep organs but can also increase scatter at higher energies. In the collimator, septal penetration and scatter are more pronounced at 171 and especially at 245 keV. For this reason, In 111 SPECT systems usually employ medium or high energy collimators. When you simulate In 111 in GATE, you must pair the radionuclide with an appropriate collimator model to avoid unrealistic overestimation of image quality or underestimation of background counts.
A key feature of In 111 simulations is the presence of two photopeaks. In practical SPECT acquisitions, you often define two separate energy windows, one around 171 keV and one around 245 keV, and may also include scatter windows. In GATE, these windows are configured in the detector or digitizer part of the simulation, but they are determined by the In 111 emission spectrum. This makes In 111 useful for learning how multiple energy windows influence detection efficiency, scatter fractions, and energy spectra in gamma cameras.
For SPECT radionuclides, always use the correct gamma energies, main emission lines, and half lives in your GATE simulations. These parameters directly control attenuation, collimator performance, and energy window placement, and they are essential for realistic SPECT modeling.
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