[Comparison of noise equivalent count rate and image quality for two-dimensional and three-dimensional PET scans]

The aim of this study was to investigate the correlation between noise equivalent count (NEC) rates and the signal-to-noise ratio (S/N) in reconstructed images. The NEC rates were determined using uniform 20 cm and 70 cm tall, 20 cm diameter cylinders filled with 11C. The phantoms were scanned in both two-dimensional and three-dimensional modes. The reconstructed image noise was evaluated using FBP and OSEM algorithms (4 iterations and 8 subsets). The images were filtered to a final image resolution of 6.5 mm. From the reconstructed image sets, averages and standard deviations of images were generated, from which the average image S/N (=average/standard deviation) was calculated within an 18 cm central ROI. The S/N of a central slice and an end slice was compared with the NEC. The NEC was found to have a linear relationship to the image S/N of all slices, depending on differences in noise properties specific to the reconstruction algorithm. In two-dimensional mode, although the image S/N of the central slice and the edge slice showed a linear relationship with the NEC, in three-dimensional mode, the S/N of the central slice did not show a relationship with the NEC. The linear relationship was also found in both two- and three-dimensional acquisition modes, as well as for the different activity distributions. These results indicate that the NEC is not only a measure for comparing the count rate performance of imaging systems. However, an absolute evaluation is impossible to depend on reconstruction algorithm, slice number, and phantom type.