ABSTRACT
Objective To propose a method for high-energy photon 3D decoding of PET detector to decode reactive crystal position and calculate γ ray depth in the scintillating crystal. Methods Light transmitting windows were set between the crystal arrays, visible photon groups were guided to distribute in the photoelectric sensor array during a single event, and sensor signal intensity was regulated to realize position and depth decoding of the high-energy photon.Monte Carlo method was used to compare the designs of different light transmitting windows to determine the effects of the crystal surface treatment mode on the results. Experiments were executed for further verification. Results The experimental results had consistency with simulation ones;the signal received through the reaction crystal was always stronger than that through the adjacent, which could decode the reaction crystals; the relations between the depth of interaction (DOI) and energy differences of the photo sensors were monotonic with the changes of DOI,and thus could be used to calculate the DOI;when using single window,the DOI vs.Diff.Energy curves had two segments which were not desired for DOI measurement;crystals with rough surfaces were more desired since the qualities of their DOI vs.Diff.Energy curves were better;the dual window method had the best DOI vs.Diff.Energy curves in term of linearity.Conclusion The method can realize 3D decoding of the discrete crystal array,which improves the energy resolution of the PET system. [Chinese Medical Equipment Journal, 2018,39(5):22-28,63]
ABSTRACT
Objective To study positron yield quantitatively in different phantoms by means of PET imaging after high energy photon irradiation,and to investigate the feasibility of bio-dose verification by PET in high energy photon radiotherapy.Methods The phantoms (hydrogel,HDPE,inhomogeneous phantom) were scanned with PET 1 min later after exposure to 2,4,6,8 Gy dose with/using 25 and 50 MV photon irradiation.The positron account rates were recorded every minute in PET scanning process and then fitted to get the half-lives of yielded nuclides.Positron distribution in each phantom was compared with dose distribution to investigate the relationship between positron activity and dose delivered.Results The half-lives of nuclides yielded in hydrogel and HDPE were supposed to be 2.23 and 19.47 min,respectively by fitting results,which had negligible difference with half-lives of 11C (20.2 min) and 15O (2.08 min).The positron amounts induced by 50 MV photon in hydrogel and HDPE were 3.88 and 3.86 times that of 25 MV photon,and increased in proportion to dose delivered.Except for build-up region and cavity,activity distributions in depth and off-axis were similar to dose distribution.Conclusions The amount and distribution of positron induced by high energy photon are related to dose delivered,and it is feasible to do dose verification with PET imaging in high energy photon radiotherapy.