Particle‐based MR modeling with diffusion, microstructure, and enzymatic reactions

Purpose To develop a novel particle‐based in silico MR model and demonstrate applications of this model to signal mechanisms which are affected by the spatial organization of particles, including metabolic reaction kinetics, microstructural effects on diffusion, and radiofrequency (RF) refocusing effects in gradient‐echo sequences. Methods The model was developed by integrating a forward solution of the Bloch equations with a Brownian dynamics simulator. Simulation configurations were then designed to model MR signal dynamics of interest, with a primary focus on hyperpolarized 13 C MRI methods. Phantom scans and spectrophotometric assays were conducted to validate model results in vitro. Results The model accurately reproduced the reaction kinetics of enzyme‐mediated conversion of pyruvate to lactate. When varying proportions of restrictive structure were added to the reaction volume, nonlinear changes in the reaction rate measured in vitro were replicated in silico. Modeling of RF refocusing effects characterized the degree of diffusion‐weighted contribution from preserved residual magnetization in nonspoiled gradient‐echo sequences. Conclusions These results show accurate reproduction of a range of MR signal mechanisms, establishing the model's capability to investigate the multifactorial signal dynamics such as those underlying hyperpolarized 13 C MRI data.