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Description
An axon is a component of a neuron found in the central nervous system (CNS). Axon geometry is thought to change with diseases and disorders. The gold-standard for measuring axon diameters and axon densities within fibres is electron microscopy (EM). EM requires postmortem samples making the method unsuitable to understand the time course of any change in geometries of axons in the CNS. Recently, magnetic resonance (MR) imaging has been used to measure these geometric properties of axons. The goal is to create in vivo measurements for longitudinal studies to understand the time course of changes in diseases and disorders.
MR measurements typically overestimate the diameters of axons. One theory for this overestimation is based on MR measurements being conducted in a single direction with geometric models which assume axons to be long, parallel, impermeable cylinders. This work examines axon diameter measurements from EM images by emulating MR methodology. We investigate the impact of the direction and number of measurements, as well as the type of cells on diameter and density measurements.
A 1 mm image slice perpendicular to the genu substructure was imaged at 15.2T in 3 male and 3 female ex vivo 12-week-old CDI mice. Six diffusion-weighted oscillating gradient spin echo MR images were acquired per frequency (50-450 Hz) using apodised cosine waveforms (TE=50 ms, TR=1250 ms) with bmax ≤ 1500 s/mm² and 2 averages using a 15T Bruker NMR system. Ten 9.4 μm x 5.5 μm electron microscopy (EM) images each from each mouse were analyzed.
The differences between direction of analysis, number of measurements, and axon diameter sizes were not statistically significant. Cells other than myelinated axons have smaller diameters, on average, than myelinated axons. These other cells might contribute to densities measurements in MRI.
The authors acknowledge funding from NSERC, and Vanderbilt Institute of Imaging Science for use of their 15 T magnet in the experiments.
| Keyword-1 | MRI |
|---|---|
| Keyword-2 | axon diameter |
| Keyword-3 | electron microscopy |