Free flight angular acceptance (FFAA) variance reduction technique for SPECT Monte Carlo simulations

Area of Science:

• Medical Imaging
• Nuclear Medicine
• Computational Science

Background:

• Current variance reduction techniques (VRTs) for SPECT Monte Carlo simulations often require system-specific pre-calculations or patient-specific training.
• This limits their flexibility for exploring novel detector designs and conducting research.

Purpose of the Study:

• Introduce Free Flight Angular Acceptance (FFAA), a novel VRT designed for on-the-fly application without pre-calculations.
• To enhance the flexibility and applicability of Monte Carlo simulations in SPECT research and development.

Main Methods:

• FFAA employs a two-step approach, handling primary and scattered photons separately.
• Primary photons are directed towards the detector using Free Flight (FF) biasing, with weights adjusted for attenuation.
• Scattered photons are tracked only if they meet an Angular Acceptance (AA) criterion after Compton or Rayleigh interactions, then transported using FF.

Main Results:

• FFAA achieves significant speedups, with mean voxel speed increases of approximately 200x in high-count regions and 100x in low-count regions compared to analog simulations.
• The method maintains high fidelity, accurately modeling critical effects like collimator scatter and septal penetration.

Conclusions:

• FFAA offers a flexible, pre-calculation-free VRT suitable for optimizing novel SPECT collimator designs.
• The technique is integrated into the open-source GATE 10 software, promoting wider adoption in the research community.