A BUL file is a data file saved in the I4 Bullseye data format. It contains static and dynamic cardiac data that is represented in 2D and 3D representations. BUL files can be opened by the I4 and JSim programs developed by the National Simulation Resource (NSR) Physiome Project.

BUL files are typically created by I4, a biomedical image processing program used in researching heart-related activity. However, BUL files may be created by JSim, a program used to build and analyze numeric models related to biomedicine and physiology. NOTE: The National Simulation Resource (NSR) Physiome Project is a research effort that began at the University of Washington Department of Bioengineering.