PET event classification

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:

The ClassifyPET method returns an integer with several digits containing the event classification:

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.

Notes

[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