Distributed corpus callosum involvement in amyotrophic lateral sclerosis: a deterministic tractography study using q-ball imaging

Diffusion tensor imaging (DTI) has become a useful tool for investigating early white matter (WM) abnormalities in motor neuron disease. Furthermore, fiber tracking packages that apply multi-tensorial algorithms, such as q-ball imaging (QBI), have been proposed as alternative approaches to overcome DTI limitations in depicting fiber tracts with different orientations within the same voxel. We explored motor and extra-motor WM tract abnormalities in phenotypically heterogeneous amyotrophic lateral sclerosis (ALS) cases aiming to establish a consistent QBI-based WM signature of disease. We performed a whole-brain, QBI tract-based spatial statistics analysis with deterministic tractography of genu, body and splenium of corpus callosum (CC) and corticospinal tracts (CST) in 20 ALS patients (12 classical and 8 lower motor neuron variants) compared to 20 healthy controls. Mean tract length, fiber volume and density, and generalized fractional anisotropy were extracted and related to clinical indices of pyramidal impairment (upper motor neuron score), disease disability (ALS functional rating scale-revised) and progression. ALS patients showed significantly decreased fiber density and volume, and increased tract length in all regions of CC and left CST (p < 0.05, corrected). In CC body, pyramidal impairment was inversely correlated to fiber density (p = 0.01), while in CC splenium, clinical disability (p = 0.01) and progression (p = 0.02) were inversely correlated to tract length. Our findings further suggest that QBI tractography might represent a promising approach for investigating structural alterations in neurodegenerative diseases and confirm that callosal involvement is a consistent feature of most ALS variants, significantly related to both pyramidal dysfunction and disease disability.