Comparison of performance of preconditioners investigated in this
study. Objective function vs. elapsed CPU time in reconstructions performed with
SDP-BSREM algorithm with four preconditioners: M1, M2, P1, and P2, and with 12
subsets (left) and 24 subsets (right), respectively, for the brain phantom with
high count data. Preconditioners P1 and P2 were generalized from M1 and M2,
respectively.