The GEANT4 simulation toolkit, and how it could be used for SPECT and PET

simulations

Giovanni Santin

INFN, Trieste & CERN, Genevagiovanni.santin@ts.infn.it

on behalf of the Geant4 Collaboration

MonteCarlo Simulations in Nuclear Medicine

16 - 17 july 2001 - Paris

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Summary

• Introduction to GEANT4

• Medical applications: DNA, brachytherapy, ...

• PET & SPECT: some ideas and conclusions

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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

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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

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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

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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

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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

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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

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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/

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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

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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

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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

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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

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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

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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

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Geant4 geometry examples

Borexino at Gran Sasso Lab.

Chandra (NASA)

CMS (LHC, CERN)

GLAST (NASA)

ATLAS at LHC, CERN

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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

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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

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- 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, ...

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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

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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

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Pixel Ionisation Chamber

Relative dose with 6 MV photons beam

Dosimetric Studies

G4vs

experimental dataDeposited energy vs

Depth in water

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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

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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

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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

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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

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• 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