Investigation of the effect of source distance and scattering medium on spatial resolution and contrast of Gamma camera images [Persian]

By identifying the effect of any parameter such as distance, attenuation and scattering on the Line Spread Function (LSF), one can compensate the quantitative and qualitative destructive effect of such parameters by deconvolution method. Using a 99mTC line source, this study was performed on a single head ADAC SPECT system operating in planar mode. Variation of FWTM and FWHM and LSFs as a function of source to collimator distance in air, source depth in scattering medium (water) and the combined effects of distance and depth of the source in scattering medium were investigated. The equations of spatial resolution (FWHM) with the above mentioned conditions were found to R1=0.07481+4.0862mm for I≥100mm for and Rd=0.0102d+10.962mm for d≥0mm and Rh=0.2512h–14.609mm for h≥100mm respectively. Where R1, Rd and Rh are spatial resolution (FWHM). I and h are the source to collimator distances in air and scattering medium respectively and d is the source to collimator distance in the scattering medium eliminating distance effect on FWHM. These variations were found to be linear having different slopes. The results showed that increasing source to collimator distance in air degrades the spatial resolution considerably, i.e. an increase of 10 cm degrades the spatial resolution by 54%. In this case the variation in FWTM is almost similar to FWHM. By increasing the source depth only in scattering medium (Eliminating the effect of source distance) no noticeable change on spatial resolution is seen, but due to accumulation of scattered photons under both wings of the LSF, the increase in FWTM is more intense than FWHM. This phenomena causes the broadening of sharp edges which in turn leads to loss of contrast and errors in determining the size of lesions and the uptake of activity in ROI.