Measure	Micro/macro?	Interpretation
Fibre Density (FD)	Microscopic density	A microscopic estimate of the density of axons* within a particular fibre population in a given voxel. An increase in FD could result from developmental processes, such as growth in axon diameter, or increase in the number of axons occupying a given space, whereas a decrease in fibre density could be due to a loss of axons, such as in neurodegeneration.
Fibre Cross-section (FC)	Macroscopic morphology	A morphological measure of the macroscopic change in cross-sectional area perpendicular to a fibre bundle experienced during registration to a template image. Increases in FC could reflect expansion of a fibre bundle via increased extra-cellular space, myelination, or both.
Fibre Density and Cross-section (FDC)	Microscopic and macroscopic effects	A combined measure that incorporates both the microscopic and macroscopic effects described above, thus providing sensitivity to any differences related to the capacity of the white matter to transmit information. Increases in FDC highlight this increased capacity to transmit information.