Optimizing septal spacing for gamma camera PET imaging

Summary form only given. PET imaging on hybrid gamma cameras can benefit from the use of some septa, which restrict detection of radiation from outside the field of view, limit scatter, and keep the data more 2D for simplicity of reconstruction. We have built a system with variably-spaced septa to investigate optimal spacing. Measurements were made with a whole-body phantom, 80 cm long, 37 cm wide, and 22 cm deep. Acquisitions were done with septa (3.2 mm/spl times/5.7 cm/spl times/56 cm) spaced at 1.4 cm, 2.8 cm, 4.2 cm, 5.6 cm, 8.4 cm, and with no septa (graded absorber only.) Singles and coincidence rates were normalized to phantom radioactivity for each configuration. Coincidence/singles ratios were determined. As expected, singles and coincidence efficiencies increased substantially with decreased collimation. The coincidence/singles ratio improved 20% from 1.4 cm to 2.8 cm, but only modestly for increasing septal spacing. Adjusting for the effects of scatter, there was no increase in coincidence/singles ratio beyond 2.8 cm. The 2.8 cm spacing would be appropriate, therefore, if dose is not limited. If dose is limited, higher sensitivity configurations would be favored.