The microcapillaries array plates (as shown in Fig. 1a) are thin square plates (each of dimensions 20 mm x 20 mm x 1 mm) made up of borosilicate glass (Incom, inc). The microcapillaries array plates will simply be referred to as ‘plates’ from here onwards. Each plate consists of many microcapillaries. This study uses three pairs of plates with nominal microcapillary diameters of 5, 10 or 20 p,m, and an open area fraction between 60 and 65 % (Fig. 1b-d). The ground truth diameters of the microcapillaries are provided by the manufacturer and these are the only available sizes which broadly mimic the possible in vivo axon diameters that are encountered in the central nervous system .
The three pairs of plates are slotted into a phantom holder containing distilled water such that the microcapillaries are aligned parallel to the main magnetic field in order to reduce susceptibility artefacts. They are soaked in the distilled water for 1 week to ensure that the microcapillaries are filled and to remove air bubbles at the plate surface.
Fig. 1 (a) Photograph of an example microcapillanes array plate with microcapillary diameters of 5 p,m (plate 1). Each plate is vertically slotted into a phantom holder containing distilled water. Magnified light microscopy images of (b) 5 p,m (plate 1), (c) 10 p,m (plate 1) and (d) 20 p,m (plate 1), at the approximate point indicated by the red arrow, to show the cross-section of the microcapillaries array plate