Medical image reconstruction remains a topic of interest to medical researchers. Although the algorithms have become more mature, one particular challenge has persisted over the decades: finding an economic and efficient computing system that can quickly perform image reconstructions. This paper presents a TCP/IP-based distributed computing system using P2P technology for iterative EM medical image reconstruction. It describes system architecture and implementation. It reports and discusses computational results from the system. This study explores the feasibility and applicability of large-scale computation of iterative medical image reconstruction in a P2P and/or Grid environment.