Estimating the axon diameter from intra-axonal water diffusion with arbitrary gradient waveforms: Resolution limit in parallel and dispersed fibers
Markus Nilsson1, Samo Lasic2, Daniel Topgaard3, and Carl-Fredrik Westin4
1Lund University Bioimaging Center, Lund University, Lund, Sweden, 2CR Development AB, Lund, Sweden, 3Physical Chemistry, Lund University, Lund, Sweden, 4Brigham and Women's Hospital, Harvard Medical School, Boston, MA, United States
The use of non-conventional gradient waveforms has brought renewed interest in non-invasive estimation of the axon diameter from diffusion MRI. Using conventional single diffusion encoding (SDE) at clinical MRI scanners, axons smaller than the resolution limit (4-5 microns) were indistinguishable from each other. We here predict the resolution limit for arbitrary gradient waveforms in systems with (i) parallel axons and (ii) orientation dispersion. Results show that SDE is optimal for parallel fibers, but that multiple diffusion encodings (MDE) are preferred where there is orientation dispersion. With 300 mT/m and MDE, the resolution limit was 2.8 microns for dispersed fibers.
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