TABLE I.

Comparison between the proposed algorithm and the previous algorithm for direct reconstruction.

	Prevous Algorithms [13]	Proposed Algorithm
Surrogate Function	paraboloidal surrgoate	EM surrogate
Monotonic Convergence	yes	yes
Reconstruction Step	paraboloidal update	EM update
Smoothing Step	spatial filtering	spatial filtering
Fitting Step	least squares fitting by the Levenberg-Marquardt (LM) algorithm	penalized likelihood fitting by the modified LM algorithm
Compartment type	any types	any types
Background Events	must be positive	no requirement