KAHIBARO
14.1. What Is an Actor?
Table of Contents
Recording simulation information
In GATE, an actor is a tool that watches what happens during the simulation and records specific information that you care about. Instead of saving everything about every particle at every step, which would be huge and slow, you attach actors to selected volumes or to the whole simulation. Each actor knows what to record and how to summarize it.
You will meet many different actors in later chapters, such as statistics actors, energy deposition actors, dose actors, phase space actors, and fluence actors. All of them follow the same idea. They observe the passage of particles, apply some internal rules, and then fill counters, images, or data tables.
Conceptually, an actor has three main aspects. First, a scope, which is usually a volume or a set of volumes. For example, you might attach a dose actor to a water phantom so that it only records dose in that phantom, not in other parts of the geometry. Second, a trigger, meaning when it collects information. Some actors record data every time a particle takes a step, others only when an event ends, or at predefined time intervals. Third, an output, which is the concrete file or result it produces, such as an image file containing a 3D dose map or a ROOT file with a list of particle states.
In a typical GATE simulation, you configure actors in Python before starting the run. You specify the type of actor, where it should be attached, how fine the scoring grid should be if it is image based, and what quantities to store. During the simulation, you usually do not interact with actors directly. They run automatically while Geant4 transports the particles. At the end, you read the actor output files for analysis.
Actors are essential for controlling the size and usefulness of your simulation results. Without actors, you would either save too little information to answer your physics or imaging question, or you would save far too much data and overwhelm memory and disk. By choosing appropriate actors, you tell GATE which quantities are important for your study.
Actors are the primary mechanism in GATE to record simulation results. You must configure them before running the simulation. Their scope, trigger conditions, and output format determine what information is available for your later analysis.
Actors vs detector digitization
Actors and digitizers both relate to simulation output, but they describe two very different concepts. Actors are general scoring tools. They record physical quantities in volumes or at boundaries, such as dose, energy deposition, fluence, or phase space information. They represent what happens in the physics world, not what a real detector electronics system would output.
Digitization focuses on modeling the electronic response of imaging detectors, in particular for PET and SPECT. It takes raw particle interactions in detector materials, often called hits, and processes them step by step into signals that resemble what the real scanner would measure. This includes operations such as combining multiple hits into a single detector event, applying energy and time blurring, adding thresholds, and forming coincidences. The output of digitization is usually singles and coincidence lists, which have a similar structure to clinical acquisition data.
The key distinction is what question you are answering. If you want to know how much dose a patient receives, or how many particles cross a certain plane, you use actors. The details of detector electronics are not important in that case. If you want to study how an imaging system behaves, such as energy spectra, count rates, or coincidence timing, you use a digitizer chain. The digitizer transforms physical interactions into something that looks like measured detector data.
Internally, actors and digitizers also sit at different places in the workflow. Actors can be attached to almost any volume, including patient phantoms, water phantoms, or simple boxes, even if there is no detector. Digitizers operate on detector hits that occur in specific sensitive volumes that represent detector crystals or similar components.
In many simulations, you will use both. For example, in a PET study you might use a dose actor to score dose in a phantom and at the same time use a digitizer to obtain realistic singles and coincidence files from the detector. The dose actor will produce an image of absorbed dose in the phantom. The digitizer will produce data that you can use to reconstruct an image as a scanner would.
Actors record physical quantities in volumes, such as dose, energy deposition, or fluence. Digitizers model detector electronics and convert hits into singles and coincidences. Use actors when you care about physical scoring, and digitizers when you care about realistic detector signals.
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