Effect of 176Lu background on singles transmission for LSO-based PET cameras.

We explore how the radioactive background from naturally occurring 176Lu affects single photon transmission imaging for lutetium orthosilicate (LSO) scintillator-based PET cameras by estimating the transmission noise equivalent count rate (NECR) including this background. Assuming a typical PET camera geometry (80 cm detector ring diameter), we use a combination of measurement and analytic computation to estimate the counting rates due to transmission, scatter and background events as a function of singles transmission source strength. We then compute a NECR for singles transmission. We find that the presence of radiation from the naturally occurring 176Lu reduces the NECR by 60% or higher for source strengths less than 10 mCi, and that a 25% reduction of the NECR can occur even with a source strength of 40 mCi.