  Axons in white matter have been shown to have varying geometries within a
bundle using ex vivo imaging techniques, but what does this mean for diffusion
MRI (dMRI) based spherical deconvolution (SD)? SD attempts to estimate the
fibre orientation distribution function (fODF) by assuming a single dMRI fibre
response function (FRF) for all white matter populations and deconvolving this
FRF from the dMRI signal at each voxel to estimate the fODF. Variable fibre
geometry within a bundle however suggests the FRF might not be constant even
within a single voxel. We test what impact realistic fibre geometry has on SD
by simulating the dMRI signal in a range of realistic white matter numerical
phantoms, including synthetic phantoms and real axons segmented from electron
microscopy. We demonstrate that variable fibre geometry leads to a variable FRF
across axons and that in general no single FRF is effective to recover the
underlying fibre orientation distribution function (fODF). This finding
suggests that assuming a single FRF can lead to misestimation of the fODF,
causing further downstream errors in techniques such as tractography.
