The GEANT4 simulation toolkit, and how it could be used for SPECT and PET
simulations
Giovanni Santin
INFN, Trieste & CERN, [email protected]
on behalf of the Geant4 Collaboration
MonteCarlo Simulations in Nuclear Medicine
16 - 17 july 2001 - Paris
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
2
Summary
• Introduction to GEANT4
• Medical applications: DNA, brachytherapy, ...
• PET & SPECT: some ideas and conclusions
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
3
The Geant4 Collaboration
• An international Collaboration of ~100 scientists from >40 institutes
– wide expertise in a variety of physics and software domains
• Manages Geant4 distribution, development and User Support
– CERN, KEK, SLAC, TRIUMF, JNL (Common)
– ESA, INFN +TERA, Lebedev, IN2P3, Frankfurt Univ.
– Atlas, BaBar, CMS, LHCB
– COMMON (Serpukov, Novosibirsk, US universities etc.)
– possible new memberships under discussion
• Based on a Memorandum of Understanding among the parties
Budker Inst. of PhysicsIHEP ProtvinoMEPHI Moscow Pittsburg University
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
4
The role of Geant
• Geant is a simulation tool, that provides a general infrastructure for– the description of geometry and materials
– particle transport and interaction with matter
– the description of detector response
– visualisation of geometries, tracks and hits
• The user develops the specific code for – the primary event generator
– the geometrical description of the set-up
– the digitisation of the detector response
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
5
The transparency of physics Advanced functionalities
in geometry, physics, visualisation etc.
Extensibility to satisfy new user
requirements thanks to the OO technology
Adopts standards wherever available (de jure or de facto)
Use of evaluated data libraries
Quality Assurance based on sound
software engineering
Subject to independent validation by a large
user community worldwide User support
organization by a large international
Collaboration of experts
Features relevant for medical applicationsFeatures relevant for medical applications
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
6
A look at the past
Physics simulation was handled through “packages”
– monolithic: either take all of a package or nothing
– difficult to understand the physics approach – hard to disentangle the data, their use and
the physics modeling
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
7
Transparency of Geant4 physics• No “hard coded” numbers
• Explicit use of units throughout the code
• Separation between the calculation of cross sections and the generation of the final state
• Calculation of cross-sections independent from the way they are accessed (data files, analytical formulae etc.)
• Distinction between processes and models
• Cuts in range (rather than in energy, as usual)– consistent treatment of interactions near boundaries between
materials
• Modular design, at a fine granularity, to expose the physics
– physics independent from tracking
• Public distribution of the code, from one reference repository worldwide
• The transparency of the physics implementation contributes to thevalidation of experimental physics results
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
8
Physics processes relevant for medical applications
• Hadronic interactions– ample variety of complementary and alternative models
• Multiple scattering – new improved model, taking into account also lateral
displacement
• Low Energy extensions of electromagnetic interactions– 250 eV electrons, photons– ~ 1 keV positive hadrons, ions– ICRU-compliant and ICRU-consistent – Barkas effect taken into account for antiprotons, negative ions– further extensions and refinements in progress
• Radioactive Decay Module– simulation of radioactive sources, including all the secondary emissions
• Neutrons– exploiting all the evaluated n data libraries worldwide
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
9
Low Energy Electromagnetic Physics
Geant4
Low Energy Electromagneticpackage extends the coverageof physics interactions
Needed for space and medical applications, physics, antimatter searches
• down to 250 eV250 eV for electrons and • based on the LLNL data libraries• shell effects
• down to ~ ~ 100 eV in the near future• based on Penelope Electron Photon Transport
down to ~ 1 keV~ 1 keV for hadrons and ions• Bethe-Bloch above 2 MeV• Ziegler and ICRU parameterisations
(with material dependence)
• free electron gas model• quantal harmonic oscillator model• charge dependence (Barkas effect)
http://www.ge.infn.it/geant4/lowE/
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
10
Low Energy Electromagnetic Physics
0.01 0.1 1 10
0.1
1
10
Geant4 LowEn NIST
/ (
cm 2
/g) i
n w
ater
Photon Energy (MeV)
Photon attenuation coefficient in water
Ion ionisation
Shell effects
Barkas effect
Protons, Ziegler
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
11
Other features relevant for medical applications
• Powerful tools relevant for complex geometries (CT)– CAD tool front-end
– fast algorithms for volume navigation performance
– volumes can be parameterised by material
• Fast and full simulation in the same environment – detailed handling of physics processes or
– possibility of parameterisations for faster processing
• Visualisation tools– wide variety functionalities available for all the most common drivers
• UI and GUI– user-friendly environement– can be easily tailored according to the user’s needs– GGE and GPE for automatic code generation
Ample documentation available from the web
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
12
Code, Examples and Documentation
• Code– ~1M lines of code, ~2000 classes (continuously growing)– publicly available from the web
• Documentation– 6 manuals
– Getting started & installation guide– User guide for application & toolkit developer– Software & physics reference manuals
• Examples– distributed with the code
– navigation between documentation and examples code – simple detectors– different experiment types– demonstrate essential capabilities
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
13
Quality Assurance
• Extensive use of Quality Assurance systems – fundamental for a toolkit of
wide public use
• Commercial tools– Insure++, Logiscope etc.
• C++ coding guidelines– scripts to verify their applications
automatically
• Code inspections– within working groups and across
groups
• Testing
– Unit testing• in most cases down to class
level granularity
– Integration testing• sets of logically connected
classes
– Test-bench for each category• eg.: test-suite of 375 tests for
hadronic physics parameterised models
– System testing• exercising all Geant4
functionalities in realistic set-ups
– Physics testing• comparisons with
experimental data
– Performance Benchmarks
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
14
Geometry
Multiple representations
• CGS (Constructed Solid Geometries)– simple solids
• STEP extensions– polyhedra, spheres, cylinders,
cones, toroids, etc.• BREPS (Boundary REPresented
Solids)– volumes defined by boundary
surfaces– include solids defined by NURBS
(Non-Uniform Rational B-Splines) • External tool for g3tog4 geometry
conversion
• CAD exchange– interface through ISO STEP
(Standard for the Exchange of Product Model Data)
• Fields– of variable non-uniformity
and differentiability
– use of various integrators, beyond Runge-Kutta
– time of flight correction along particle transport
Role: detailed detector description and efficient navigation
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
15
Things one can do with Geant4 geometry
One can do operations with
solids
These figures were visualised with
Geant4 Ray Tracing tool
...and one can describe complex geometries, like
Atlas silicon detectors
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
16
Geant4 geometry examples
Borexino at Gran Sasso Lab.
Chandra (NASA)
CMS (LHC, CERN)
GLAST (NASA)
ATLAS at LHC, CERN
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
17
Visualization and UI
• Visualisation– Various drivers– OpenGL, OpenInventor, X11,
Postscript, DAWN, OPACS, VRML
• User Interfaces– Command-line, Tcl/Tk,
Tcl/Java, batch+macros, OPACS, GAG, MOMO
• Also choice of User Interfaces:
– Terminal (text) or– GUI: Momo (G4), OPACS, Xmotif
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
18
User support
• Wide international user community, in a variety of fields of application– HEP and nuclear physics, astrophysics, space sciences, shielding and
radioprotection, medical physics, theoretical physics, fine arts etc.
• Effective model of user support– granular organisation– provided by a wide network of experts, each one in its domain of expertise– automatic tools for bug notifications– consultancy, requests of enhancements and new developments etc.– priority given to member parties
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
19
- DNAMulti-disciplinary Collaboration of
astrophysicists and space scientists particle physicists medical physicists biologists physicians
Study of radiation damage at the cellular and DNA level in the space radiation environment(and other applications, not only in the space domain: radiotherapy, radiobiology, ...)
http://www.ge.infn.it/geant4/dna/
• capability to model DNA as a “geometry”• capability to handle biochemical processesGeant4
C, Fe, ...
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
20
Geant4 allows a complete flexible description of the
real geometry
BrachytherapyRadioactive sources are used to deposit therapeutic doses near tumors while preserving surrounding healthy tissues
3 mm steel cable
5.0 mm
0.6 mm
3.5 mm
1.1 mmActive Ir-192 Core <E> = 356 keV
Courtesy of National Inst. For Cancer Research, Genova, Italy
Montecarlo topics:
• Dose calculation• Computation of dose deposition kernels for treatment planning dose
calculation algorithms based on convolution/superposition methods
• Separation of primary, first scatter and multiple scatter components
for complex dose deposition models
Computation of other model-dependent parameters, e.g. anisotropy
function
Accurate computation of dose deposition in high gradient regions (i.
e. near sources)
• Verification of experimental calibrations
Naso-pharynx endocavitary treatment
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
21
0
30
60
90
120
150
180
• source geometry• auto-absorption• encapsulation• shielding effects
Anisotropy
Geant4 Radioactive Decay Module
is capable of handling the generation of the whole radioactive chain of the 192Ir source
Isodose curvesThe simulated source is placed in a 30 cm water box
10.000.000 photons, 1 mm3 voxels12 h CPU time on Intel Pentium 300 MHz
Courtesy of National Inst. For Cancer Research, Genova, Italy
Courtesy of National Inst. For Cancer Research
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
22
Pixel Ionisation Chamber
Relative dose with 6 MV photons beam
Dosimetric Studies
G4vs
experimental dataDeposited energy vs
Depth in water
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
23
Bragg Peak of Protons in Water
Magic CubeRelative dose with
270 MeV protons beam in water
Deposited energy vs Depth in water
Courtesy of INFN & ASP, Torino, Italy
and experimental data
• Sandwich of 12 parallel plate (25x25) cm2 ionization chambers• Each chamber:
• passive material (N2,G10,Mylar) • anode (0.035 mm Cu) • active material (3 mm N2)
• passive material • air gap (2 cm, tissue equivalent of adjustable thickness)
• Thickness of a chamber as water equivalent ~1.1 mm
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
24
Geant4 for scatter compensation in Megavoltage 3D CT
• Use GEANT4 to obtain digitally reconstructed radiographs (DRRs), including full scatter simulation
This represents a great improvement over approaches based on ray-casting.
• The study of DRRs synthesized by Geant4 allows users to produce a model for scatter compensation of megavoltage radiographs
• This will help to produce a more accurate megavoltage 3D CT reconstruction and therefore a more reliable tool for patient positioning and treatment verification
• Activity in progress at the Italian National Institute for Cancer Research, Genova
• Other possible areas of application of Geant4:– LINAC head simulation– Scatter analysis in total body irradiation
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
25
Work in progress
In vivo TLD dosimetry
• Simulation of the energy deposition of low energy photons in TLD LiF100 nanodosimeters
• Used to calculate dose to patient in radiodiagnostic examinations: mammography virtual colonscopy
CT image interface
Interface between Geant4 and DICOM3 CT scan images format in order to perform in tissue simulation
Courtesy of IST, Genova and IRCC Institute for Cancer Research and Treatment, Italy
CT slice of a head with the dose deposition of a proton beam obtained with the GEANT code
Medical Dept., University of Piemonte Orientale and INFN Torino
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
26
PET & SPECT simulations with Geant4.Why not?
• Detailed description of both– human tissues and properties– detector geometry and response (non-linear resolution function of the PET
scanner, etc.)
• Energy range of Physics processes involved – covered by the G4 standard or
– LowEnergy extension of EM processes
• Past experience in the medical physics community shows reliability and innovation in G4 simulations
• Injected radioactive tracer described by the Radioactive Decay Module• Simulation of patient motion with geometry modification inside the same run
MC
in N
ucl
ea
r M
ed
icin
e -
Pa
ris 1
6 J
uly
20
01
G
ea
nt4
fo
r P
ET
& S
PE
CT
G.S
an
tin
27
• Geant4 is a simulation Toolkit, providing advanced tools for all the domains of detector simulation
• Geant4 is characterized by a rigorous approach to softwareengineering
• Its areas of application span diverse fields: HEP and nuclearphysics, astrophysics and space sciences, medical physics,radiation studies etc.
• Geant4 is open to extension and evolution Geant4 physics keeps evolving
– with attention to User Requirements
– facilitated by the OO approach
• An abundant set of physics processes is available, often with a variety of complementary and alternative physics models. Continuos physics validation test.
• Geometry description: powerful, accurate and rich
• Wide and growing medical user community
• User Support granted by the Geant4 Collaboration
• G4 URL: http://wwwinfo.cern.ch/asd/geant/geant4.html
Summary