As the algorithms that are used to reconstruct medical images from measurable projection data continue to become mature, medical image reconstruction has remained an interesting and important topic to medical researcher. The main challenge in medical image reconstruction is how to establish an economical and efficient computing system that can be used to perform fast image reconstructions. In this paper, we present a distributed computing system that is based on P2P technologies; and demonstrate how the design and implementation of this system addresses the challenge through a case study of an iterative EM medical image reconstruction algorithm. Computational experiments are designed to study the performance of the EM algorithm using the system. In the aggregate, this study provides an insights into the large-scale computation of iterative medical image reconstruction in a Grid environment.