Evaluation of fractional errors in pixels and regions in a time of flight PET scanner

The coefficient of variation (standard deviation over mean: s/m) was computed in regions of interest (ROIs) and compared to the value obtained in pixels in a time of flight PET scanner. Using a homogeneous cylindrical phantom, the authors collected events in list mode and reconstructed a dynamic series of 256*256 images for 0.5, 1, 2, 5, 10 and 50 (*10⁶) events per image. Circular ROIs with the same and increasing sizes were drawn at different positions in the field of view. Using Hanning (cosine) filtered images, the s/m varied between 76 and 8.3% (103 and 14%) for pixels while it was at most 7.5% (9.2%) for ROIs. The s/m values were higher in the direct plane (DP) than in the cross plane (CP) due to its higher sensitivity. Two-way analysis of variance showed no ROI position effect in DP or CP separately but a significant count/pixel effect. s/m for pixels and for ROIs were linearly correlated to the expected noise to signal ratio calculated according to a published model. For ROI studies, cosine filtered images demonstrated a 22% higher s/m than Hanning filtered images.