THE EFFECT OF RADIATION TRANSPORT ON SPATIAL RESOLUTION

In recent years gamma camera prototypes based on multianode or crossed wire anode photomultiplier are developing to obtain very high spatial resolution values in SPECT,PET or planar gamma camera. If spatial resolution increases up to a value less than 1 mm, it is worthwhile to know what is the theoretical limit related to the radiation transport inside the scintillation crystal. Such limit depends on crystal composition and from the energy of gamma ray. To this aim a Monte Carlo method was developed in combination with a scintillation crystal geometry consisting in solid of 10xl0x30 mm volume limiting a 3D-array of elements each of one with 0.1x0.1x0.1 mm dimension. The Monte Carlo code is based on the variance reduction technique and considers all interactions of photons and electrons inside the crystal and records, step by step, the deposited energy in each element of the array. The compositions of four different scintillation crystals were simulated: NaI, CsI, BGO and YAP (Yttrium Aluminum Perovskit). They were irradiated with 140 keY and 511 keV photon energy under pencil beam geometry. The results show the transport of X-ray fluorescence as the most relevant effect in spreading the energy deposition. In particular 0.5 mm, 1 mm, and 2 ram were the maximum values of spot diameter obtained for CsI, NaI and BGO respectively. The smallest value of energy deposition spread was 0.3 mm for Yttrium Aluminum Perovskit (YAP).