GAMOS 5.1.0 User's Guide | ||
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The class PETEventClassifierUA in the directory NuclearMedicine/PET classifies the events as PET by looking at the reconstructed hits. It is a GAMOS user action, so you can activate it with the command
/gamos/userAction PETEventClassifierUA
First it counts how many reconstructed hits have 511 keV [1] within a precision given by the parameter
/gamos/setParam PET:EvtClass:511EPrec ENERGY_FRACTION
That is the energies accepted will be those between 511*(1-ENERGY_FRACTION) and 511*(1+ENERGY_FRACTION). Alternatively you may use the following two parameters
/gamos/setParam PET:EvtClass:511EPrecMin ENERGY_MIN
/gamos/setParam PET:EvtClass:511EPrecMax ENERGY_MAX
where ENERGY_MIN is the minimum energy, that by default takes a value of 0.7*511 keV, and ENERGY_MAX is the maximum energy, that by default takes a value of 1.3*511 keV.
To recover hits when one of several Compton interactions have occurred you may switch the merging of hits that are close into a single one. You may set the distance to merge hits with the parameter
/gamos/setParam PET:EvtClass:ComptonRecHitDist DIST
DIST takes by default a value of 0, what means that no Compton hits merging will be done. The merging of hits first identifies the hit with biggest energy and merges with it those hits that are closer than the given distance. The same algorithm is repeated with the hits not associated until no hit is left. If hits merging is chosen you may choose among several algorithms to try to identify the hit that corresponds to the first gamma interaction (and this will be the position of the whole set). See section Identifying Compton interactions to learn how to do it. If no algorithm is selectedm the position will be the one of biggest energy, or second biggest, or n-th biggest, where the order is given by the parameter
/gamos/setParam PET1stHitByEnergy:Order ORDER
ORDER takes by default a value of 1, that is, the position is that of the hit with biggest energy (see section on Identifying Compton interactions).
If two hits are found it will be checked that their relative time difference is less than the value given by the parameter
/gamos/setParam PET:EvtClass:CoincidenceTime COINCIDENCE_TIME
which by default takes a value almost infinite (it is assumed that one of the two started the trigger and the other must be in the coincidence time opened at that moment).
If two 511-keV hits are finally found, the event is classified as a good PET event. Then the sub-classification code enters in the game:
More than two 511-keV hits: If more than two hits are found, the two that are closer to 511 keV will be taken and the event will receive a subclassification type of 3->2.
Random coincidence: It is checked that each of the two 511-keV hits is built only from tracks from the same 'original' gamma[2], and that the two hits come from the same event
Scattered: The event is classified as scattered if any of the 511-keV gammas has suffered an interaction in the list of volumes defined by the parameter
/gamos/setParam DetCountScatteringUA:VolumeNames VOLUME_1 VOLUME_2 ...
and this interaction is of one of the process types defined by the parameter
/gamos/setParam DetCountScatteringUA:ProcessNames PROCESS_1 PROCESS_2 ...
(by default Compton are Rayleigh interactions are counted); and it has lost in the volumes more energy than the parameter
/gamos/setParam DetCountScatteringUA:EnergyMin ENER_MIN
(by default the minimal energy is 0.)
Check PET line distance: a line joining the position of the two reconstructed hits is built and the distance of closest approach (DCA) to the origin of the positron is calculated; the events are classified as 'near' of 'far' if the DCA is smaller or bigger than the parameter
/gamos/setParam PET:EvtClass:LineDistToVtx DISTANCE
where DISTANCE has a default value of 10*mm.
The ClassifyPET method returns an integer with several digits containing the event classification:
0 if it is not PET, 1 if it is PET and PET line is close to the event vertex, 2 if it is PET and PET line is far from event vertex
10*1 if the search for 511-keV reconstructed hits found more than 2
100*1 if the event is a random coincidence event
1000*1 if the event is a scattered event
At the end of the run a table is printed with the number of events in each of the combinations of the sub-classification types.
[1] | Indeed the exact mass of the electron is always used |
[2] | 'original' gammas are gammas that are primary particles or that are directly created by the annihilation of positron that is a primary particle |
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