Quantitative analysis of myocardial perfusion requires detection of myocardial boundaries in many short-axis MR images. Manual tracing of myocardial boundaries is a time-consuming and tedious task, which may limit the clinical use of quantitative analysis. In this paper, we propose an automatic detection algorithm based on active contours. For initialization, starting contours need to be defined. Instead of manually tracing the initial contours, our approach presents an automatic method for finding initial contours of epicardium and endocardium. Once the initial contours are defined, we apply the active contours called stochastic active contour scheme (STACS) for image segmentation