Systematic and Distributed Time-of-Flight List Mode PET Reconstruction

Philips has recently released the time-of-flight (TOF) PET/CT GEMINI-TF scanner. It uses 4 times 4 times 22 mm³ LYSO crystals, which has 600 ps timing resolution, 12% energy resolution and 4.8 mm spatial resolution. This paper describes the system design and general approach of TOF reconstruction in Philips´ GEMINI-TF scanner. A relaxed list mode ordered subset expectation maximization (LMOSEM) algorithm is used in the reconstruction, with chronologically ordered subsets. The sensitivity and emission object in LMOSEM are computed in the spherically symmetric basis function (blob) space. The multiplicative correction factors of detector normalization, isotope decay, system dead-time and crystal timing are pre-corrected for each list mode event. Attenuation, scatter and randoms are corrected in the reconstruction system matrix. A TOF-dependent single scatter simulation (SSS) is implemented for TOF scatter estimation. To accelerate reconstruction, the sensitivity calculation and list mode reconstruction are distributed to multiple processors, using a dynamic load balancing scheme. For this paper, a uniform cylinder phantom with cold and hot cylinder inserts, a NEMA body IEC phantom and a patient study are reconstructed with both TOF and non-TOF reconstructions. We have demonstrated that TOF reconstruction converges faster than non-TOF, and controls noise well than non-TOF. It has better contrast-noise trade-offs than non-TOF for cold regions and small hot lesions.