Measuring PET scanner sensitivity: relating countrates to image signal-to-noise ratios using noise equivalents counts

True coincidence count (TCC) and noise equivalent count (NEC) curves were measured with a standardized 20-cm-diameter nylon cylinder for five different CTI/Siemens PET (positron emission tomography) scanners with several scanner-collimator combinations: (1) 831/08-12 with 1-mm collimator septa; (2) 933/08-12 and 933/08-16 with 3→1-mm tapered collimator septa; and (3) 931/08-12 with 3→1-mm tapered and a 1-mm collimator septa and the 931/08-16 with 3→1-mm tapered collimator septa. In addition, TCC and NEC curves on the 933/08-12 were compared with those from an Alderson brain phantom. In general, it is found that the TCC curves indicated peak count rates and activity levels that were as much as 50% higher than the corresponding values from NEC curves. The primary factor causing this difference is the noise effect of the randoms component. It is demonstrated that, compared to the Alderson brain phantom, the standard 20-cm cylinder is a poor predictor of count rate performance for PET brain imaging