MR Simulation with Phase Distribution Graphs: Off-Resonant Pulse Response and Slice-Selection
Purpose: Phase Distribution Graphs were introduced as a powerful tool for on-resonant MRI simulations. Herein, we extend the concept beyond the assumptions of hard and on-resonant RF pulses in Phase Distribution Graph simulations, while retaining differentiability across all parameters. The extension into the frequency domain further generalizes the Phase Distribution Graph simulation framework for sequence simulation and optimization. Theory and Methods: Using an effective axis rotation model, the RF operator is generalized to off-resonant rotations, originating from field inhomogeneities, chemical shift, and field gradients. Discretization of the RF pulse shape enables the modeling of the magnetization response to shaped RF pulses and thus slice selection. Results: Including off-resonant pulse response into a PDG based MRI simulation enables the simulation of key MRI features, such as fat suppression, binomial pulses, or Bloch-Siegert shifts. The discretization of RF shapes enables the simulation of their respective frequency response and thus slice selective excitation, refocusing or saturation pulses, including multi-band excitation. In combination with flow, typical inflow artifacts in slice-selective MRI can be simulated. Discussion and Conclusion: Extension for off-resonance removes a major limitation of phase distribution graphs and makes the simulation framework more realistic. As the method is not restricted to specific RF shapes, optimization of RF pulse shapes in amplitude, phase and frequency is now possible in the context of or jointly with specific MR sequences.