Geant4 examples: Example B3

This example simulates schematically a Positron Emitted Tomography system.

GEOMETRY DEFINITION

The support of gamma detection are scintillating crystals. A small number of such crystals are optically grouped in a matrix of crystals. In this example, individual crystals are not described; only the matrix of crystals is and it is still called 'Crystal' hereafter.

Crystals are circularly arranged to form a ring. Few rings make up the full detector (gamma camera). This is done by positionning Crystals in Ring with an appropriate rotation matrix. Several copies of Ring are then placed in the full detector.

The head of a patient is schematised as a homogeneous cylinder of brain tissue, placed at the center of full detector.

The Crystal material, Lu2SiO5, is not included in the G4Nist database. Therefore, it is explicitly built in DefineMaterials().

PHYSICS LIST

The physics list contains standard electromagnetic processes and the radioactiveDecay module for GenericIon. It is defined in the B3PhysicsList class as a Geant4 modular physics list with registered physics builders provided in Geant4:

G4DecayPhysics - defines all particles and their decay processes
G4RadioactiveDecayPhysics - defines radioactiveDecay for GenericIon
G4EmStandardPhysics - defines all EM standard processes

This physics list requires data files for:

low energy electromagnetic processes which path is defined via the G4LEDATA envirnoment variable
radioactive decay hadronic processes which path is defined via the G4RADIOACTIVEDATA envirnoment variable.

PRIMARY GENERATOR

The default particle beam is an ion (F18), at rest, randomly distributed within a zone inside a patient and is defined in B3PrimaryGeneratorAction::GeneratePrimaries(). The type of a primary particle can be changed with G4ParticleGun commands (see run2.mac).

DETECTOR RESPONSE : scorers

A 'good' event is an event in which an identical energy of 511 keV is deposited in two separate Crystals. A count of the number of such events corresponds to a measure of the efficiency of the PET system. The total dose deposited in a patient during a run is also computed.

Scorers are defined in DetectorConstruction::CreateScorers(). There are two G4MultiFunctionalDetector objects: one for the Crystal (EnergyDeposit), and one for the Patient (DoseDeposit)

EventAction::EndOfEventAction() collects informations event per event from the hits collections, and accumulates statistic for RunAction::EndOfRunAction().

STACKING ACTION

Beta decay of Fluor generates a neutrino. One wishes do not track this neutrino. Therefore one kills it immediately, before created particles will put in a stack. The function B3StackingAction::ClassifyNewTrack() is invoked by G4 kernel each time a new particle is created.

The following paragraphs are common to all basic examples

VISUALISATION

The visualization manager is set via the G4VisExecutive class in the main () function in exampleB3.cc. The initialisation of the drawing is done via a set of /vis/ commands in the macro vis.mac. This macro is automatically read from the main () function when the example is used in interactive running mode.

By default, vis.mac opens an OpenGL viewer. The user can switch to other graphics systems by commenting out this line and instead uncommenting one of the other /vis/open statements, such as HepRepFile or DAWNFILE (which produce files that can be viewed with the HepRApp and DAWN viewers, respectively).

The DAWNFILE, HepRepFile drivers are always available (since they require no external libraries), but the OGL driver requires:

the visualisation & interfaces categories have been compiled with the environment variable G4VIS_BUILD_OPENGLX_DRIVER.
exampleB3.cc has been compiled with G4VIS_USE_OPENGLX. (This is best done through Configure or CMake.)

For more information on visualization, including information on how to install and run DAWN, OpenGL and HepRApp, see the visualization tutorials, for example,

The tracks are automatically drawn at the end of each event, accumulated for all events and erased at the beginning of the next run.

USER INTERFACES

The user command interface is set via the G4UIExecutive class in the the main () function in exampleB3.cc The selection of the user command interface is then done automatically according to the Geant4 configuration. The default command interface, called G4UIterminal, is done via a standard G4cin/G4cout. On Linux and Sun-cc one can use a smarter command interface G4UItcsh. It is enough to set the environment variable G4UI_USE_TCSH before compiling exampleB3.cc

HOW TO RUN

compile and link to generate an executable
```
% cd B3
% make
```

execute exampleB3 in the 'interactive mode' with visualization

% exampleB3
and type in the commands from run1.mac line by line:  
Idle> /control/verbose 2
Idle> /tracking/verbose 2
Idle> /run/beamOn 1 
Idle> ...
Idle> exit

Idle> /control/execute run1.mac
....
Idle> exit

execute exampleB3 in the 'batch' mode from macro files (without visualization)
```
% exampleB3 run2.mac
% exampleB3 exampleB3.in > exampleB3.out
```