Computer simulations allow for predicting and interpreting the outcome of NMR and MRI measurements. A survey of existing solutions prompted us to develop an easy-to-use software for more realistic simulations of diffusion tensor imaging (DTI), that could be used to support MR based fibertracking. The aim was to be able to model microscopic structures that result in deviations from the single tensor model, e.g. observed in HARDI measurements. Validation of diffusion tensor imaging (DTI) results and new methods of diffusion modeling in different geometries remains challenging. Suitable and parametrizable phantom datasets are required but currently rarely available. Therefore, the University of Mannheim MRI Simulator (UMMRIS), a flexible particle-based Bloch equation simulation framework has been developed using an object-oriented approach (C++).