Accelerated time-of-flight scatter correction for whole-body PET

Utilization of Time-Of-Flight (TOF) information allows to improve image quality and convergence rate in PET image reconstruction. In order to obtain quantitatively correct images accurate scatter correction (SC) is required that accounts for the study-specific scatter time distribution.
The currently accepted TOF-SC gold standard - the TOF extension of the single scatter simulation approach (TOF-SSS), see [1] - is computationally demanding and can substantially slow down the reconstruction which becomes a substantial problem in whole-body investigations. On the other hand, the accuracy of existing simplified TOF-SC algorithms is limited in many cases. We have developed a new TOF-SC algorithm in order to improve this situation.

Our TOF-SC method is based on a separate estimation of scatter spatial distribution (via SSS) and scatter time distribution via a dedicated fast algorithm. The key idea of the algorithm is the use of an immediate scatter approximation (ISA) for scatter time distributions calculation. The underlying assumption is that for this calculation a distinction between the emission point from which the annihilation event originates and actual scatter point is not crucial.
The proposed approach was evaluated in phantom measurements providing challenging high activity contrast conditions as well as in representative clinical patient data sets.

The reconstructed images are in excellent quantitative agreement with those obtained with TOF-SSS while scatter estimation time was reduced by a factor of four and overall reconstruction time by a factor of two in whole-body studies, even when using a listmode reconstruction not optimized for speed.

Our results show that ISA is a viable alternative to TOF-SSS offering a factor of four TOF scatter estimation acceleration without compromising the image quality.

[1] C. C. Watson, Extension of Single Scatter Simulation to Scatter Correction of Time-of-Flight PET, IEEE Transations Nucl. Sci., vol. 54(5), pp. 1679–1686, 2007