Characterisation of restricted diffusion and exchange using the velocity autocorrelation function
Arthur Chakwizira1, Samo Lasič2, Alexis Reymbaut3, Carl-Fredrik Westin4, Filip Szczepankiewicz5, and Markus Nilsson5
1Medical Radiation Physics, Lund, Lund University, Lund, Sweden, 2Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital, Amager and Hvidovre, Copenhagen, Denmark, 3Random Walk Imaging, AB, Lund, Sweden, 4Department of Radiology, Brigham and Women's hospital, Harvard Medical School, Boston, MA, United States, 5Clinical Sciences Lund, Lund University, Lund, Sweden
Temporal velocity correlations of diffusing particles carry information about the structure of the diffusion environment. Studying high-order velocity autocorrelation functions can aid in understanding how signal is influenced by restricted diffusion and exchange, and in turn how to design experiments that are sensitive/independent to these phenomena. In this work, we employ numerical simulations on a variety of substrates to demonstrate this notion. We find that the fourth order velocity autocorrelation bears a distinctive signature of exchanging systems. In addition, we highlight that the effect of exchange on the second order velocity autocorrelation is negligible when exchange is barrier-limited.
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