1. Kirov A S, MSKCC
Overview of Geant4 Use
and Issues in Imaging:
Emission Tomography (PET and SPECT)
Assen S. Kirov
Department of Medical Physics
Memorial Sloan-Kettering Cancer Center
New York

2. Kirov 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

3. Kirov 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

4. Kirov 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) 4543

5. Kirov 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)

6. Kirov A S, MSKCC
GATE architecture
GEANT 4GEANT 4
Core LayerCore Layer
Application LayerApplication Layer
User LayerUser Layer
Jan et al, PMB 49 (2004) 4543

7. Kirov 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
…
-Verbosity and visualization
-Geometry
-Digitizer
-Physics
-Sources
-Outputs
-Experiment

8. Kirov 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 - 1
PET – 3 (2 block based, 1 pixelated)
Generic -1
Jan et al, PMB 49 (2004) 4543

9. Kirov 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
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) 4543

10. Kirov 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):
» β+ spectra parametrization - faster
» annihilation photon noncolinearity
• GEANT4’s interaction processes and cuts
Jan et al, PMB 49 (2004) 4543

11. Kirov 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

12. Kirov 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

13. Kirov A S, MSKCC
GE Advance/ DLS PET scanner

14. Kirov A S, MSKCC
MC based coincidence count rates for IQ phantom
See Schmidtlein et al, SU-EE-A4-03 Mod. Poster at this meetingSee 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 40
Activity (kBq/cc)
CountRates(kcps)
SF Trues SF Scatters
SF Randoms SF NECR
IQ Trues IQ Scatters
IQ Randoms IQ NECR
NECR
Scattered
Trues
Randoms

15. Kirov 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 !!

16. Kirov 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