KAHIBARO
25.6. Dynamic Imaging
Table of Contents
Time-dependent acquisitions
Dynamic imaging in GATE focuses on how the measured signal or dose changes as a function of time. Instead of treating your simulation as a single static snapshot, you divide it into time intervals and let sources, detectors or geometry change between these intervals. For absolute beginners it is useful to think of a dynamic simulation as a sequence of small, separate simulations, each one representing a time frame, but all managed inside one GATE run.
In GATE, time is measured in physical units such as $s$ or $ns$, and acquisition time is simply how long you let your sources emit particles and your detectors collect them. A dynamic acquisition is created by splitting this total acquisition time into time windows. Each window can represent, for example, an individual PET frame in a dynamic uptake study, a SPECT projection interval during a rotation, or a time bin during respiratory motion. You can then associate different source activities, positions or motions with each time window.
A central concept in dynamic imaging is that GATE progresses the simulation clock automatically as it simulates radioactive decays, beam particles or other events. You configure when sources are active and how they evolve, but you do not manually advance time step by step. Instead, you define start and stop times for your acquisition and for any time-dependent elements, and GATE ensures that events are generated only when the corresponding components are active.
For any dynamic simulation, always define:
- A clear total acquisition duration.
- Explicit time windows for each frame or projection.
- Time-dependent parameters such as activity or geometry motion in consistent units.
Mixing time units or forgetting to limit source activity to the intended time window is a common source of hidden errors.
In medical imaging, dynamic acquisitions are used in several typical scenarios. In PET, you may simulate a tracer injection and record a sequence of frames to study the time activity curve in a region of interest. In SPECT, you may simulate a rotating gamma camera that acquires counts over many angular positions, each for a fixed acquisition time, to form a sinogram. In CT or cone‑beam CT, dynamic imaging often means simulating a continuous rotation and binning projections into time slots. In all these cases, GATE provides mechanisms for time‑tagging events and grouping them by their simulation time, so that you can reconstruct separate images for each interval.
From a practical point of view, dynamic imaging affects how you set up sources and actors. Time‑dependent sources may change their activity according to radioactive decay or a user‑defined curve. Moving sources or detectors follow trajectories that are parameterized by time, such as periodic respiratory motion or stepwise rotation. Actors that record dose, hits or images can be configured to produce different outputs per time frame, or to include the time coordinate so you can bin data afterward during analysis.
For beginners, it is helpful to start with a simple static acquisition, then introduce only one dynamic aspect at a time, such as a rotating detector with fixed source and geometry. Once you are comfortable with this, you can add more complex time dependence, like changing source activity or deforming phantoms, to build realistic dynamic imaging simulations that match real acquisition protocols.
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