  In the calculation of Positron Emission Tomography (PET) image
reconstruction, System Response Matrix (SRM) characterizes the numerical
relationship between measurement space and image space. Due to a significant
amount of calculation and low efficiency of the current SRM algorithm, the
application of iterative reconstruction in the medical and industrial fields is
limited. The time and space of computing SRM are the most important factors
that researchers are concerned about. In this paper, the Straight Line
Truncation (hereinafter referred to as SLT) method is applied to calculate the
SRM. We verified that it improves the speed of calculating the SRM and reduces
its spatial complexity. The experimental results show that the forward
projection speed of SLT algorithm is six times more than that of the
traditional Ray-Tracing algorithm. Besides, the forward projection speed of the
small angle of SLT is about 23 times that of the traditional Ray-Tracing
algorithm, and the storage space of SLT is about one sixty-fourth of the
storage space of traditional Ray-Tracing algorithm.
