Geant 4 Application for Tomographic Emission GATE a

Geant 4 Application for Tomographic Emission GATE a simulation platform for nuclear medicine based on GEANT 4 D. Lazaro, V. Breton For the Open. GATE Collaboration 1

Imaging in nuclear medicine l Two imaging modalities: § morphological imaging (MRI, scanner, X-rays…) detailed informations about the patient anatomy § Functional imaging (scintigraphic imaging: SPECT, PET) non-invasive method: a radioactive drug is injected to the patient and naturally evacuated from the organism spatial distribution of the radiopharmaceutical in the body study of the functional activity of an organ or a specific tissue (e. g tumor) PET ü Positron emitters (18 FDG 2 h) ü Resolution down to 2 mm SPECT ü Single photon emitters (99 m. Tc 6 h) ü Resolution down to 7 mm ü Poor sensitivity Multimodality ü Combined imaging modalities (e. g PET/CT) ü Resolution improved 2

Why simulations in Nuclear Medicine? Image Reconstruction Scanner design Protocol optimization Simulation Algorithm testing Data analysis Scatter correction Quantification recovery 3

Two approaches… General purpose simulation codes (GEANT 4, EGS 4, MCNP…) wide range of physics wide community of developers and users documentation, maintenance and support complexity speed Dedicated simulation codes (PETsim, Sim. SET, Eidolon, …) optimized for nuclear medical imaging applications (geometry, physics. . . ) ease of use and fast development maintenance, upgrades 4

Simulation requirements PET/SPECT dedicated tools GATE GEANT 4 core potentialities Realistic modelisation of PET/SPECT experiments l l l modelisation of detectors, sources, patient movement (detector, patient) time-dependent processes (radioactive decay, movement management, biological kinetics) Ease-of-use Fast Long-term availability, support and training 5

GATE A generic simulation platform for PET/SPECT applications Based on GEANT 4 l l object Oriented Analysis & Design wide range of physics models long term availability upgrades, documentation & support Specific developments regarding to Nuclear medical imaging needs l material database, sources, readout l time and movement management Ease-of-use for non C++ programmers l scripting commands to define all paramaters of the simulation (construction of the geometry, specification of the physical processes involved, of the sources. . . ) 6

GATE structure 3 different levels GEANT 4 core Developer level framework and application classes C++ programming User level sequence of scripting commands ü ü ü geometry construction physical processes involved sources (geometry, activity) movement (type, speed…) duration of the acquisition User interface Application classes Framework Geant 4 7

Geometry construction by scripting commands l A specific mechanism has been developed to help the user construct easily a geometry scripting commands geometry = combination of geometric volumes, like world Scanner Sourc e ‘russian dolls’ Body Head Rsector Crystal LSO D. Strul Uni Lausanne BGO 8

Geometry examples of GATE applications Multi-ring PET Triple-head gamma camera D. Strul IPHE Lausanne S. Staelens Uni Ghent 9

Source management Multiple sources § controlled by source manager § inserted via scripting § complex geometries: customized GPS Optimized decay § customized G 4 Radioactive Decay Module (RDM) § PET-specific sources 15 O 11 C 10

Timing Simulation time – a clock models the passing of time during experiments – the user defines the experiment timing Time-dependant objects – updated when time changes – allows programming of movement, tracer kinetics. . . 11

Physical processes Choices of processes via scripting commands: low energy, standard or inactive Cut settings LE photoelectric effect Standard photoelectric effect LE Compton scattering Standard Gamma conversion LE Rayleigh scattering PHOTONS ELECTRONS Standard Ionisation Standard Bremsstrahlung LE Gamma conversion LE Ionisation LE Bremsstrahlung 12

Photons: Standard or low energy processes ? crystal thickness (mm) Relative difference (%) sensitivity (%) Na. I(Tl) crystal sensitivity : comparison between values computed with different versions of GEANT 4 and NIST values crystal thickness (mm) 13

Electrons: Standard or low energy processes ? Relative difference (%) Stopping power in Na. I(Tl) in GEANT 4 and NIST values energy (ke. V) Relative difference (%) Stopping power in water in GEANT 4 and NIST values energy (ke. V) 14

Sensitive detector / Digitizer Hits Digis Energy response l Spatial response Centroid readout Threshold Electronics Pre-programmed components – Sensitive detectors – Trajectory analyser l Digitizer – Linear signal processing chain – Modular: set-up via scripting 15

Data output l Multiple parallel output channels: ROOT (real-time display, storage in ROOT files for further analysis) ASCII files è Binary files è Specific scanner formats (e. g Crystal Clear LMF…) GATE simulation Sinogram Reconstructed image 16

The Open. GATE Collaboration GATE under development since Autumn 2001 Objectives of the collaboration Develop a reliable generic simulation platform for nuclear medicine l l l shared development share results and know-how allow multiple work axes: development, validation… Current status: 10 groups Fields: planar scintigraphy, SPECT, PET, micro. PET Organization § Steering committee (one representative / group) § Technical meeting every two months 17

Current efforts of development Improvement of the detection models l Use of voxelised geometries (sources): under development l Validation work, against experimental data l § Small animal imaging gamma camera (LPC – IASA Athens) § Dual-headed gamma camera (University of Ghent) l Strategies to improve computation speed § Parallel computation § Use of computing grids 18

Validation of GATE against experiment l Simulation of a small animal imaging gamma camera – Cs. I(Tl) crystal array coupled to a PSPMT – Small animal imaging (study of new radiopharmaceuticals) PSPMT Lead shielding Cs. I(Tl) crystal array LEHR collimator source crystal array 19

GATE deployment on Data. GRID Computation speed becomes (too!) large in case of § voxelised geometry § huge number of events Potential solution under study at LPC: deployment of GATE on Data. GRID (first tests successful in July 2001) Input file Database file Input file exploding Database file GATE on the GRID Output file merging GATE output result file GRID-GATE output file 20

For more informations… – GATE web site http: //www-iphe. unil. ch/~PET/research/gate/ – Technical Coordinator: Daniel Strul, IPHE Lausanne daniel. strul@iphe. unil. ch – Spokesman: Christian Morel, IPHE Lausanne christian. morel@iphe. unil. ch 21