## Title: Statistical reconstruction algorithm based on the method of moments in positronium imaging
## Presenter: Roman Y. Shopa
## Date: 17.02 2024
## Participants:
Wojciech Krzemień (WK)
Konrad Klimaszewski (KK)
Roman Shopa (RS)
Lech Raczynski (LR)
Mateusz Bała (MB)
Maciej Zajkowski (MZ)
Dominik Strzelecki (DS)
Aleksander Ogonowski (AO)
## Questions/Remarks:
WK: At what stage are the wj weights calculated?
RS: The answer is given on p.9, the weights are calculated with the algorithm analogical to MLEM algorithm. As input the estimate time per event is provided.
KK: Final result should be given by m?
RS: m is the positronium mean lifetime estimated for all decay mechanisms combined, m = w/activity.
The ortho-positronium lifetime is extracted using the method of moments.
WK:Do the obtained values of the lifetimes are comparable to the introduced ones in the MC?
RS: There is a bias. The results are systematically lower. It might be due to low statistics and/or median post-filter applied.
LR: Does the bias occur both in the background and signal regions?
RS: Yes, some negative bias observed, yet less pronounced as for the hot phantom spheres. Maybe there is still some problem in the MC scripts
KK: The last slide represent already orto-lifetime, after applying the method of moments?
RS: Yes.
MB: What is MPF?
RS: Median Post-Filter - is a convolution filter similar to Gaussian, applied to the final reconstructed image. One takes a voxel and its neighbors and calculates the median from them. Median filter works well for gradient and edge preservation, while taking the mean blurs them.
MB: What shape of the function E(\nu) is assumed for the methods of moments in the article given at p.13?
RS: Apparently, no shape is defined in the article, but maybe I had missed it. In the original article, moments are calculated numerically from the measured E(\nu).
MB: The assumption of intensity ratio between ortho and para =3 is for a vacuum?
RS: Good question. They assume it as a constant, but maybe the ratio may be violated inside the matter.
WK: For vacuum it is just a combinatorial relation: PS has one possible spin state while OPS has 3 possible spin states.