Effect of intrinsic activity of Lu-176 in small animal SPECT with L_0.9GSO crystal

Objectives L_0.9GSO crystal has desirable properties for SPECT scanner with high resolution and sensitivity: short attenuation length, high light output and short decay time. Despite these advantages, L_0.9GSO crystal contains natural radioactivity from Lu-176. In this study, we estimated background count rate of Lu-176 and evaluate its effects on the image contrast using GATE simulation code.

Methods The crystal for animal SPECT system has surface area of 50×50 mm². Two different thicknesses, 1 and 3 mm, were assumed. The intrinsic activities calculated from the density (7.3 g/cc) and natural abundance (2.59%) of Lu-176 were 655 and 1965 cps/sec for 1 and 3 mm crystals. The intrinsic activities were counted using two different energy windows, 40% for I-125 (21.9~42.6 keV) and 20% for Tc-99m. We also scanned I-125 and Tc-99m flood phantoms, which consist of 4 square segments with 0, 10, 20, and 30 MBq activities, using a micro parallel-hole collimator with ~10^-5 sensitivity, and compared the contrast between the segments.

Results The intrinsic activities were 1.2 and 2.8 cps/cm² for 1 and 3 mm in I-125 energy window, and 1.3 and 2.3 cps/cm² for 1 and 3 mm in Tc-99m energy window. The image contrasts between the 10 and 30 MBq segments were 2.97 in I-125 simulation and 2.76 and 2.82 for 1 and 3 mm crystals in Tc-99m simulation. The contrast degradation compared with the non-intrinsic activity condition was less than 2.5%.

Conclusions The amount and effect of intrinsic activity from Lu-176 were minor in our SPECT design mainly due to the thin thickness of LGSO crystal with high stopping power.