Purpose Biophysical models of diffusion MRI have been developed to characterize microstructure in
various tissues, but existing models are not suitable for tissue composed of permeable spherical cells.
In this study we introduce Cellular Exchange Imaging (CEXI), a model tailored for permeable spherical
cells, and compares its performance to a related Ball & Sphere (BS) model that neglects permeability.
Methods We generated DW-MRI signals using Monte-Carlo simulations with a PGSE sequence in
numerical substrates made of spherical cells and their extracellular space for a range of membrane
permeability. From these signals, the properties of the substrates were inferred using both BS and CEXI
Results CEXI outperformed the impermeable model by providing more stable estimates cell size and
intracellular volume fraction that were diffusion time-independent. Notably, CEXI accurately estimated the
exchange time for low to moderate permeability levels previously reported in other studies ( < 25μm/s).
However, in highly permeable substrates ( = 50μm/s), the estimated parameters were less stable,
particularly the diffusion coefficients.
Conclusion This study highlights the importance of modeling the exchange time to accurately quantify
microstructure properties in permeable cellular substrates. Future studies should evaluate CEXI in clinical
applications such as lymph nodes, investigate exchange time as a potential biomarker of tumor severity,
and develop more appropriate tissue models that account for anisotropic diffusion and highly permeable