KAHIBARO
9.5. Visualizing Particle Tracks
Table of Contents
Gamma tracks
Visualizing particle tracks is one of the most intuitive ways to understand what happens inside your GATE simulation. For beginners, track visualization is mainly a debugging and learning tool, not a way to obtain quantitative results. When you enable track display, the visualization engine draws a line following each particle’s path through the geometry, using the tracking information from Geant4.
In GATE, gamma tracks usually appear as straight or piecewise straight lines that connect the interaction points of the photon as it moves through different materials. Between two interaction points, a gamma travels without deflection, then it may undergo photoelectric absorption, Compton scattering, or pair production depending on its energy and the material. In the viewer you often see:
- A gamma entering a volume and stopping abruptly. This usually indicates a photoelectric interaction where the photon is fully absorbed. The track segment ends at the absorption point.
- A gamma that changes direction at discrete points. This is typical of Compton scattering. The photon loses some energy, and the visualization shows a “kink” in the track at each scattering point. The scattered photon segment continues with a new direction and lower energy.
- A gamma that appears to terminate where two new charged tracks start. At high enough energies, this may represent pair production, where the gamma is converted into an electron and a positron. The primary gamma track ends, and the secondary electron and positron tracks begin from the same location.
The visual appearance of gamma tracks depends on the visualization settings you choose. You can typically control whether you see all gammas, only primary gammas from your source, or only those that interact in a specific volume. You can also restrict the energy range of displayed tracks, which is useful to avoid clutter when many low energy photons are present.
It is helpful to associate visual styles with particle types. For example, gamma tracks may be drawn in one color, such as green or yellow, and with a distinct line style. When you run a test simulation with only a small number of events, you can follow individual gamma trajectories through your geometry by rotating and zooming the view. This is especially useful to verify that:
You have correctly placed sources inside or outside your detector or phantom.
Gammas are entering the expected volumes and not being blocked by unintended shielding or geometry mistakes.
The physics configuration allows the processes you expect, for instance Compton scattering in a detector crystal or absorption in a collimator.
In emission imaging examples such as PET or SPECT, gamma track visualization reveals typical behaviors, like photons escaping the patient without detection, photons interacting in collimators, or being fully absorbed in detector crystals. In transmission setups like CT, you can see beams of gammas or x rays traversing the patient or phantom and being attenuated according to material thickness and composition.
Because gamma tracks are often numerous, interactive visualization is easier if you simulate only a small number of events while you debug. Later, you turn off track visualization when you run large production simulations, since drawing tracks slows the simulation and does not change the underlying physics results.
Track visualization shows how particles move and interact, but it is not a substitute for quantitative scoring. Always rely on actors and output data for dose, energy deposition, or count statistics, and use tracks mainly to check geometry, sources, and physics behavior qualitatively.
Electron tracks
Electron tracks look quite different from gamma tracks. While gammas travel in straight segments between discrete interactions, electrons are charged particles and undergo continuous interactions with matter. In visualization, this appears as a wiggly or highly scattered path that changes direction frequently. The electron gradually loses energy along its path until it stops.
When you enable electron track display, you will typically see:
Short, tortuous tracks inside dense materials such as detector crystals or bone, where the electron range is limited.
Longer, curved paths in low density materials such as air or lung equivalent, where electrons can travel farther before stopping.
Multiple scattering that creates a zigzag or spiral-like trajectory, especially for higher energy electrons.
Electrons are responsible for much of the local energy deposition that leads to dose. Even when the primary particle is a gamma, it often transfers energy to an electron, which then deposits that energy along its track. In visualization, this often appears as a gamma track ending at a point where an electron track begins. The gamma interaction creates a secondary electron, also called a delta electron, which then travels a short distance and loses energy.
In medical detector simulations, electron tracks help you understand how energy is deposited in scintillation crystals or semiconductor sensors. You can see whether secondary electrons stay within a single crystal or cross into neighboring volumes, which affects spatial resolution and the interpretation of hits. In dose calculations, visualizing electron tracks inside a phantom or patient geometry helps you develop an intuition of how dose spreads beyond the initial gamma or proton interactions.
Because electrons can create many small steps, visualizing every electron track for many events can create a dense and confusing image. It is common to restrict track display to:
Primary electrons only, when the source itself is an electron beam.
Electrons above a certain kinetic energy threshold, so that very low energy, short-range electrons are not shown.
Electrons within a specific region, such as a detector layer or a volume of interest, to keep the scene readable.
The apparent thickness or segmentation of an electron track is a visualization artifact that reflects how Geant4 splits the continuous motion into discrete steps. It does not mean that the electron is jumping from point to point, but rather that the trajectory is sampled at many intermediate locations to model energy loss and scattering.
Electron tracks show where charged particles travel and interact, but the visual density of tracks does not directly represent dose or energy density. Always use dose or energy deposition actors for quantitative assessment, and view electron tracks only as a qualitative guide to particle transport.
Positron tracks
Positron tracks combine characteristics of electron tracks with a distinctive final annihilation signature. A positron is the antiparticle of the electron, with the same mass but opposite charge. It travels through matter, loses energy, and scatters in a manner similar to an electron, but eventually it annihilates with an electron in the medium, producing one or more gamma photons.
In visualization, a typical positron track from a medical radionuclide source, such as F 18, appears as:
A relatively short, curved trajectory originating from the decay position, usually within a patient or phantom volume. The path length depends on the initial positron energy and the material density, and is often on the order of a few millimeters for PET radionuclides in water or soft tissue.
Frequent small deflections along the way due to multiple scattering, similar to electron tracks. The higher the initial positron energy, the longer and more tortuous the track before it slows down.
A terminal point where the track ends and two gamma tracks are emitted in nearly opposite directions. This marks the annihilation event. In PET simulations, these are the 511 keV annihilation photons that the scanner detects.
By displaying positron tracks along with gamma tracks, you can see the full chain of events in PET simulations, from radioactive decay through positron transport to annihilation and detection. This helps you develop intuition about:
Positron range, which is the distance between the decay site and the annihilation point. This range contributes to the intrinsic spatial resolution limit of PET.
The relation between the original activity distribution and the annihilation distribution. Due to positron motion, they are not identical.
The directions in which annihilation photons leave the annihilation point and how they eventually reach detector crystals.
In simple test setups, you can place a small positron emitter in a water phantom and visualize a few events, watching each positron spiral and then annihilate. In a full PET scanner geometry, the scene can become quickly crowded, so it is helpful to restrict track display to a small number of events or to a region that contains the source.
You can also display only primary positrons from the source, without all secondary electrons and gammas, if you want a clear picture of positron motion alone. Alternatively, showing both positron and gamma tracks together allows you to see how annihilation gammas leave the phantom and interact in the detector, but you may need to reduce the number of events to avoid clutter.
Because positrons and electrons are visually similar, using distinct colors in the visualization helps. For example, you might display positrons in magenta and electrons in blue, while keeping gammas in another color. This color coding helps you quickly identify annihilation points where a positron track terminates at the origin of two gamma tracks.
The visible positron range in track visualization helps you understand resolution limits in PET, but you should not estimate quantitative range or resolution directly from the picture. For quantitative studies, use actors to record annihilation positions, detector responses, and reconstructed images, and use tracks only to confirm that the physics behavior matches your expectations.
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