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

Impact of within-voxel heterogeneity in fibre geometry on spherical deconvolution