Comparison of list-mode and DIRECT approaches for time-of-flight PET reconstruction

Early clinical results with time-of-flight (TOF) PET systems have demonstrated the advantages of TOF information in PET reconstruction. Reconstruction approaches in TOF-PET systems include list-mode and binned iterative algorithms as well as confidence-weighted analytic methods. List-mode iterative TOF reconstruction retains the full accuracy of the data in the spatial and temporal domains but is computationally intensive. We have developed an approach [DIRECT (direct image reconstruction for TOF)] to speed up TOF reconstruction that takes advantage of the reduced angular sampling requirement of TOF data by grouping list-mode data into a small number of azimuthal views and co-polar tilts and depositing the grouped events into histo-images, arrays with the sampling and geometry of the final image. All physical effects are included in the system model and deposited in the same histo-image structure. Using histo-images allows efficient computation during reconstruction without ray-tracing or interpolation operations. The DIRECT approach was compared with the 3D list-mode TOF ordered subsets expectation maximization (OSEM) reconstruction for phantom and patient data taken on the UPenn prototype LaBr₃ TOF-PET scanner. The total reconstruction time with DIRECT for data with 430-ps timing resolution was 1/3 that of list-mode TOF-OSEM to achieve comparable image quality. While the reconstruction time for list-mode TOF-OSEM depends on the number of events and sizes of the spatial and TOF kernels, the time for DIRECT is independent of these factors. The DIRECT approach is able to reproduce the image quality of the list-mode iterative TOF reconstruction both qualitatively and quantitatively in measured data with a significantly reduced time.