Evaluation of image noise in prospectively respiratory gated PET

The aim of this study was to quantify image noise and signal recovery in respiratory gated PET. A Jaszczak phantom filled with ¹⁸F was placed on a platform moving sinusoidally. Two source to background activity ratios were used: 5:1 and 3:1 (sphere:background). An Anzai belt, a surface tension monitoring device, was strapped around the phantom to track the motion and to trigger the gated PET cycle. Data were acquired into 12 bins throughout one gating cycle. The binned data were also summed to produce image sets representing acquisitions with different numbers of gates, including a non-gated image set. The image noise was estimated using the bootstrap method. Images were generated from 100 sinogram replicates, reconstructed using OSEM, and filtered to a final image resolution of 6 mm. From the reconstructed image replicates, mean and standard deviation images were created, from which the average image signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of each sphere were calculated. The SNR and CNR were evaluated as a function of the number of gates. The SNR and CNR result in the expected Poisson limited correlation with the number of gates for the larger lesions. Because of the motion, the CNR calculated from the images produced with no or few gates is nearly a factor of 2 less than the expected value for the 3 smallest spheres. As the number of gates increases, the CNR correlates with the expected values. Beyond 4 gates, image noise dominates over any signal improvement, which is reflected in the low CNR values of the smallest spheres. The results of this study show that gating can provide significant improvement in signal recovery with minimal loss of CNR for small, moving lesions.