OpenTOPAS extensions for SPECT imaging simulation.

OpenTOPAS is a powerful, well-validated Monte Carlo platform, but it ships without the geometry and scoring components needed to model a clinical SPECT system: a physical parallel-hole collimator, an energy-resolved detector response, and efficient projection scoring. OpenTOPAS-SPECT adds those pieces, bringing GATE-style SPECT modeling (explicit collimator transport, crystal pulse-height response, forced-detection projections) into the OpenTOPAS parameter-file workflow.

The extensions are self-contained OpenTOPAS classes; no changes to the OpenTOPAS core are required. They are built through the standard TOPAS_EXTENSIONS_DIR mechanism.

Contents

Category	Class	Purpose
Geometry	TsParallelHoleCollimator	Parallel-hole collimator: round/hex/square holes on a square/hex lattice; a focal length turns it into a converging fan/cone-beam (or diverging) collimator.
Geometry	TsPinholeCollimator	Pinhole and multi-pinhole collimator: absorber plate with knife-edge aperture(s).
Geometry	TsSlitSlatCollimator	Slit-slat collimator: slit aperture crossed with parallel slats.
Geometry	TsStarGuideDetector	GE StarGuide-style pixelated CZT ring detector.
Scoring	TsEDepSpectrum	Energy-resolved detector response: per-event deposited-energy spectrum R(E_true -> E_meas), plus energy-weighted interaction position.
Scoring	TsForcedDetectionProjection	Forced-detection (variance-reduction) projection scorer using an analytic Metz-Frey collimator transmission.
Scoring	TsScoreStarGuideProjection	Pixel-binned projection scorer with Gaussian energy smearing for the StarGuide geometry.
Source	SpectDecaySource	Radioactive decay source: emits a parent radionuclide uniformly in a component and scales histories per Tf time bin by the true activity decay; g4radioactivedecay produces the full chain.
Physics	TsPenelopeNoRayleigh	G4EmPenelopePhysics with Rayleigh scattering removed for gamma (clean single-Compton bookkeeping).

Requirements

• OpenTOPAS 4.2.p2
• Geant4 11.2.2 (geant4-11-02-patch-02)

Building

Point TOPAS_EXTENSIONS_DIR at this repository (subdirectories are scanned recursively) when configuring the OpenTOPAS build:

cmake -S /path/to/OpenTOPAS \
      -B /path/to/OpenTOPAS-build \
      -DTOPAS_EXTENSIONS_DIR="/path/to/topas-spect" \
      -DGeant4_DIR=/path/to/geant4/lib/cmake/Geant4 \
      -DCMAKE_BUILD_TYPE=Release

cmake --build /path/to/OpenTOPAS-build --parallel 8

To combine with other extension sets, separate paths with ;: -DTOPAS_EXTENSIONS_DIR="/path/to/topas-spect;/path/to/other-extensions".

Quick start

Worked parameter files are in examples/, split into single-component characterisations and full-system runs:

• component/energy_response.txt, mono-energetic beam into a bare crystal slab; produces the crystal pulse-height spectrum R(E_true -> E_meas) (TsEDepSpectrum).
• component/collimator_detector_response.txt, point source in front of a collimator + crystal; produces the distance-dependent point-spread function and penetration tail.
• system/symbia_lehr_sensitivity.txt, isotropic point source + collimator + crystal; the ratio of photopeak-window counts to emitted decays gives the absolute system sensitivity. Selects the scanner geometry with a single IncludeFile = ../../systems/symbia_lehr.txt.

Run each example from its own folder (OpenTOPAS resolves IncludeFile relative to the launch directory, and the examples' includes point to ../../systems, ../../phantoms):

cd examples/component && topas energy_response.txt
cd examples/system   && topas symbia_lehr_sensitivity.txt

System presets

systems/ holds ready-to-include geometry presets for real scanners (Siemens Symbia LEHR/LPHR/ME/HE and GE StarGuide). A deck selects a system in one line, e.g. IncludeFile = systems/symbia_lehr.txt, and adds its own source, scorer and placements. See systems/README.md for the preset/deck contract.

Designing your own geometry

Beyond the presets, you can vary the collimator and detector design entirely from the parameter file: parallel-hole (round/hex/square), converging fan/cone-beam (focal length), pinhole and multi-pinhole, and slit-slat collimators; a general pixelated detector (TsPixelatedBox); and importing CAD parts (STL/PLY) for novel geometries. See docs/design.md and the worked decks in examples/design/.

Patient-like phantoms

Emit from realistic activity distributions, physiological organ uptake plus lesions (bone/nodal metastases), to test acquisition parameters on clinically relevant tasks. Composable analytic phantoms (native shapes + weighted volumetric sources) with literature-sourced uptake ratios live in phantoms/ (a Lu-177 PSMA scenario is included); voxelized XCAT anatomy is also supported. See docs/phantoms.md and examples/phantom/.

Variance reduction

SPECT detects ~1 in 5000 photons, so full-MC is slow. Use forced detection (TsForcedDetectionProjection, a fast analytic-collimator projection) or the native OpenTOPAS Vr/ toolkit (directional Russian roulette toward the detector, forced interaction, splitting). See docs/variance_reduction.md and examples/vr/.

Extension reference

TsParallelHoleCollimator (geometry)

A septa slab (Ge/<name>/Material, e.g. Lead or Tungsten) perforated by a grid of channels (Ge/<name>/Hole/Material, e.g. Vacuum). Hole shape and lattice are selectable.

Parameter	Type	Meaning
HoleDiameter	d (Length)	Channel size; for Hex holes this is the across-flats dimension
SeptalThickness	d (Length)	Wall thickness between channels (pitch = diameter + septum)
CollimatorLength	d (Length)	Slab thickness along the bore axis (z)
NHolesX, NHolesY	i	Number of channels in each transverse direction
HoleShape	s	Round (default), Hex, or Square (HoleDiameter = side, for CZT square-hole collimators)
Lattice	s	Square (default) or Hex (offset rows, spacing = pitch·√3/2)

The default Round holes on a Square lattice reproduce the distance-dependent geometric resolution and septal-penetration tail, and are the configuration used for the results in docs/validation.md. Real clinical collimators use hexagonal holes on a hexagonal lattice: set HoleShape = "Hex" and Lattice = "Hex". The hexagonal packing fits more open area (packing factor ~0.907 relative to a square lattice of the same pitch) and so gives higher absolute sensitivity.

TsEDepSpectrum (scoring)

Attach to a crystal component (Sc/<name>/Component). Accumulates the total energy deposited per incident primary and writes one ntuple row per event:

Column	Unit	Meaning
Edep	keV	Total deposited energy for the event (photopeak, Compton continuum, K-escape, ...)
X, Y	mm	Energy-weighted interaction centroid (global coordinates)

With a mono-energetic source this directly yields the detector energy-response function; with a collimator upstream and an energy-window cut in post-processing it yields counts-in-window system sensitivity.

The remaining scorers and the TsStarGuideDetector geometry are documented in their source headers.

Validation

See docs/validation.md. Summary against published figures for the Siemens Symbia and GE StarGuide systems:

• Crystal energy response reproduces the expected photopeak, Compton edge and characteristic K-escape peaks.
• Distance-dependent geometric resolution matches the analytic parallel-hole expression at the 10 cm reference distance.
• Absolute Symbia LEHR system sensitivity: 8.6e-5 counts/decay vs. the published 9.1e-5 counts/decay (5460 cpm/MBq), within 5%.

Acknowledgements

Built on OpenTOPAS and Geant4. Physics uses the Geant4 Penelope electromagnetic models.

Documentation

The full user guide, installation, quickstart, camera systems, phantoms, CT-driven detector motion, variance reduction, and validation, is in docs/. It builds into a hosted site with mkdocs build, or on Read the Docs via the included .readthedocs.yaml.

Citation

If you use OpenTOPAS-SPECT in published work, please cite it via CITATION.cff, alongside OpenTOPAS and Geant4.

License

OpenTOPAS-SPECT is released under the MIT License, the same permissive license as OpenTOPAS, so the extensions can be built and distributed alongside a OpenTOPAS installation without additional restrictions. Copyright (c) 2026 Bertolet Lab.