Study of the relationship between NECR and image SNR for PET systems

Objectives The concept of noise equivalent count rate (NECR) [1] for PET system is not entirely straightforward. One vital issue is whether a higher NECR with standard NEMA measurement corresponds to better imaging performance. The study includes the following tasks: 1) revisiting the original NEMA analytical derivation; 2) validating three methods for NECR calculation based on the NEMA standard; 3) studying the spatial dependence of NECR and relationship between NECR and image SNR.

Methods The NEMA measurement for NECR (Method 1) was conducted for a Biograph PET/CT system with the initial activity of 30.8 mCi. The simulation was performed using the GATE[2]. Two NECR methods were compared to Method 1: 1.the count rates and NECR were determined from the sinograms by using the spatial information of each line-of-response (LOR), as in Method 1; 2. the event id in GATE can be used to identify the coincidence events (Method 3) [3]. To elucidate the spatial dependence of the NECR-SNR² relationship, a cylindrical phantom with a rod phantom of 3cm diameter was located at three radial offsets from the scanner's center (Fig.1a).

Results For Method 1, the NECR result is in good agreement with published values [4]. And both Method 2 & 3 provide a reliable derivation of NECR. SNR² is plotted against the NECR for three positions (Fig.1b). Under a constant NECR, a higher SNR is observed at the FOV center compared to the edge of FOV. In addition, the linear relationship between SNR² and NECR does not hold when the activity levels increase beyond the condition associated with peak NECR.

Conclusions Our results demonstrate that linear correlation between the image SNR² and NECR is spatially dependent due to the dead time effect, and the NECR does not reflect local SNR behavior. The NECR-SNR² correlation could be quite different for those PET systems of novel geometries when compared to a whole body system. As a result, it may be necessary to measure NECR and image SNR at multiple positions to make a comprehensive comparison between two different systems.

Research Support We would like to thank Canadian Breast Cancer Foundation Grant (2011R00356) for financial support, and Dr. Troy Farncombe at Hamilton Health Science for providing NEMA measurement data.