Collimation of a 137Cs point source for transmission scanning in PET

Transmission scanning with ¹³⁷Cs allows high photon flux and is cost effective due to the long half-life of the source. However, it suffers from a high scatter fraction and from emission contamination (EC) in the transmission (TX) sinogram. Both effects lead to an underestimation of the attenuation values, which is about 10-15 % each. On our systems, EC is measured in the transmission energy window and subtracted, while scatter is compensated by scaling the μ-values. In this paper we have investigated possibilities to improve the transmission signal rather than applying extensive corrections. The ratio between EC and TX can be reduced by increasing the source strength, however, the current source activities are already at the upper limit regarding dead-time and detector pile-up. Scatter can be suppressed by axial collimation, which has been shown by other authors before. To address both issues, scatter and EC, we investigated transaxial and axial collimation for our brain NaI-system, the HEAD PENN-PET and our GSO-brain scanner (GPET). Simulations were performed to investigate object and collimator scatter for both approaches. First results showed that transaxial collimation allows to increase the source strength and suppresses scatter in the center of the sinogram, however this scatter suppression leads to an artifact in the transmission image. Axial collimation improves the accuracy of the attenuation values.