Kirov A S, MSKCCKirov A S, MSKCC

Overview of Geant4 Use and Issues in Imaging:

Emission Tomography (PET and SPECT)

Assen S. KirovDepartment of Medical Physics

Memorial Sloan-Kettering Cancer Center New York

Assen S. KirovDepartment of Medical Physics

Memorial Sloan-Kettering Cancer Center New York

Kirov A S, MSKCCKirov A S, MSKCC

Why simulate imaging systems and scans ?

• Better understand and model phenomena affecting image quality and accuracy

• Optimize scan imaging and processing parameters

• Calculate parameters needed for improved image reconstruction

Kirov A S, MSKCCKirov A S, MSKCC

Basis of this talk

Membership in the

OpenGATE collaboration

( www.opengatecollaboration.org )

Whose goal is

- the creation of an elaborate and flexible Monte Carlo program that allows realistic simulation of the physical processes, detector geometry and all factors affecting emission tomography applications

- realistic modeling of patient and small animal anatomy

- based on GEANT4

Kirov A S, MSKCCKirov A S, MSKCC

Basis of this talk

• Product of OpenGATE : GATE (GEANT4 Application for Tomographic Emission)

• Widely used for

– PET

– SPECT

• Potential for CT (why not ?)

• Potential for Radioimmunotherapy and Nuclide Therapy dosimetry

Jan et al, PMB 49 (2004) 4543Jan et al, PMB 49 (2004) 4543

Kirov A S, MSKCCKirov A S, MSKCC

Use of GEANT4 prior to GATE for imaging

• GEANT3 and GEANT4 based codes

– example Jan S (Thesis) : GePETos

• Summary of other codes:

– Buvat and Castiglioni (Q J Nuc Med, 46, p 48-61, 2002)

Kirov A S, MSKCCKirov A S, MSKCC

GATE architecture

GEANT 4GEANT 4

Core LayerCore Layer

Application LayerApplication Layer

User LayerUser Layer

Jan et al, PMB 49 (2004) 4543Jan et al, PMB 49 (2004) 4543

Kirov A S, MSKCCKirov A S, MSKCC

User Layer

…/gate/block/daughters/name crystal/gate/block/daughters/insert box/gate/crystal/placement/setTranslation 0.0 0.0 0.0 mm/gate/crystal/geometry/setXLength 30.0 mm/gate/crystal/geometry/setYLength 4.4 mm/gate/crystal/geometry/setZLength 4.75 mm/gate/crystal/setMaterial BGO…

…/gate/block/daughters/name crystal/gate/block/daughters/insert box/gate/crystal/placement/setTranslation 0.0 0.0 0.0 mm/gate/crystal/geometry/setXLength 30.0 mm/gate/crystal/geometry/setYLength 4.4 mm/gate/crystal/geometry/setZLength 4.75 mm/gate/crystal/setMaterial BGO…

-Verbosity and visualization-Geometry-Digitizer-Physics-Sources-Outputs-Experiment

-Verbosity and visualization-Geometry-Digitizer-Physics-Sources-Outputs-Experiment

Kirov A S, MSKCCKirov A S, MSKCC

GATE systems

• Geometry organization: Hierarchical structures

– For realistic signal processing organization

– Simplifies geometry input

GATE system:

{Hierarchically organized geometry + corresponding LMF output}

5 GATE systems

SPECT - 1PET – 3 (2 block based, 1 pixelated)

Generic -1

GATE system:

{Hierarchically organized geometry + corresponding LMF output}

5 GATE systems

SPECT - 1PET – 3 (2 block based, 1 pixelated)

Generic -1

Jan et al, PMB 49 (2004) 4543Jan et al, PMB 49 (2004) 4543

Kirov A S, MSKCCKirov A S, MSKCC

GATE time and movements

Needed for: - Patient and organ motion - Scanner rotation - Activity distribution changes

GEANT4 limitations: - geometry: static during simulation - no source movements

Needed for: - Patient and organ motion - Scanner rotation - Activity distribution changes

GEANT4 limitations: - geometry: static during simulation - no source movements

Solutions in GATE: - Geometry updated between simulation time steps - The source is confined to a smaller volume moving inside a larger emission volume

Santin et al, 2003 Strul et al, 2003

Solutions in GATE: - Geometry updated between simulation time steps - The source is confined to a smaller volume moving inside a larger emission volume

Santin et al, 2003 Strul et al, 2003

Jan et al, PMB 49 (2004) 4543Jan et al, PMB 49 (2004) 4543

Kirov A S, MSKCCKirov A S, MSKCC

GATE physics

• Sources

– Particle type, position (volume), direction (solid angle), energy (spectrum), activity

– Modified GEANT4’s RDM – virtual clock defines absolute time used to initialize GEANT4’s internal tracking time

– Multiple, voxelized sources

– e+ emission (Jan 2002):

» + spectra parametrization - faster

» annihilation photon noncolinearity

• GEANT4’s interaction processes and cuts

Jan et al, PMB 49 (2004) 4543Jan et al, PMB 49 (2004) 4543

Kirov A S, MSKCCKirov A S, MSKCC

GATE digitization

- Resolution blurring, spatial blurring, crosstalk

- Parallelizable and non-parallelizable dead times

- Coincidence processing

- Post simulation processing of output files stored before digitization

- Resolution blurring, spatial blurring, crosstalk

- Parallelizable and non-parallelizable dead times

- Coincidence processing

- Post simulation processing of output files stored before digitization

Kirov A S, MSKCCKirov A S, MSKCC

GATE validation

• PET at least 5 patient and microPET scanners

energy spectra, sensitivity, resolution, scatter faction, count rates, image quality,

contrast recovery

• SPECT at least 5 systems

energy spectra, sensitivity, resolution,…

• Prototype at least 4 systems

energy spectra, sensitivity, resolution…

More than 15 references in Jan et al, PMB 49 (2004) 4543 and at www.opengatecollaboration.org

Kirov A S, MSKCCKirov A S, MSKCC

GE Advance/ DLS PET scanner

Kirov A S, MSKCCKirov A S, MSKCC

MC based coincidence count rates for IQ phantom

See Schmidtlein et al, SU-EE-A4-03 Mod. Poster at this meeting See Schmidtlein et al, SU-EE-A4-03 Mod. Poster at this meeting

0

50

100

150

200

0 5 10 15 20 25 30 35 40Activity (kBq/cc)

Co

un

t R

ate

s (

kc

ps

)SF Trues SF Scatters

SF Randoms SF NECR

IQ Trues IQ Scatters

IQ Randoms IQ NECR

NECRNECR

ScatteredScattered

TruesTrues

RandomsRandoms

Kirov A S, MSKCCKirov A S, MSKCC

Other issues

• CT simulations – GATE could handle the geometry, not tested yet

• Optical imaging – very promising field for GEANT4 : small animal; 3D dosimetry (see Kirov et al, SU-FF-T-229 at this meeting), etc.

• Worst GATE/GEANT4 problem for imaging

» Very time consuming simulations

35 CPU days on 2 GHz !!

Kirov A S, MSKCCKirov A S, MSKCC

Conclusions

• GEANT4 with GATE - powerful, flexible, convenient for use and accurate tool for nuclear medicine imaging applications

• GEANT4 without GATE: worth pursuing only if a new approach can drastically improve performance

• Efficiency: work needed