TOF-PET Small-Lesion Image Quality Measured Over a Range of Phantom Sizes

The purpose of this study was to parameterize time-of-flight (TOF) image quality improvement over non-TOF PET using small lesions in a tapering phantom that represents a range of body sizes. A set of 46 1.0-cm spherical inserts was divided into six groups and positioned in a fillable, tapered phantom that represents a range of body dimensions (an oval cross section from 38.5 × 49.5 cm to 6.8 × 17.8 cm with a length of 51.1 cm). These hot spheres were positioned in a uniform, warm background (8:1 radioactivity concentration) and were distributed in six regions of different cross-sectional areas. Two PET/CT studies were acquired on a Discovery 690 PET/CT (GE Healthcare), which has a reported timing resolution of 650 ps. The total exam consisted of six frames (bed positions), with each frame centered on one of the sphere sets. Images were reconstructed with and without TOF information using 3D OSEM with 1-20 iterations of eight subsets, a 50-cm field of view, and a 128 × 128 × 47 image matrix with all corrections modeled in the iterative loop, and z-axis smoothing, but no post-filter was applied. An automated algorithm was used to segment and place regions of interest on the hot spheres in the CT, and then these regions were applied to the PET images to measure contrast recovery and background variability (image noise). As the number of iterations increases for TOF and non-TOF iterative reconstruction, the ratio of contrast for TOF to non-TOF was ~1.0 for small body sizes, but the TOF contrast improvement ranges from 10%-25% for body sizes with diameters >30.0 cm. When comparing the small lesion signal-to-noise ratios between TOF and non-TOF images, the square of the SNR ratios for TOF to non-TOF was a linear function of body size and timing resolution that was parallel to the predicted improvement of TOF. The squares of the SNR ratios for TOF to non-TOF were linearly proportional to D_eff/σ_{d, but with a negative offset. TOF image quality was equal to non-TOF for a diameter of ~17.5 cm.}