FUNCTIONAL NEURORADIOLOGY Axial diffusivity is increased in the degenerating superior cerebellar peduncles of Friedreich's ataxia Riccardo Della Nave & Andrea Ginestroni & Stefano Diciotti & Elena Salvatore & Andrea Soricelli & Mario Mascalchi Received: 10 September 2010 / Accepted: 16 November 2010 / Published online: 3 December 2010 # Springer-Verlag 2010 Abstract Introduction Decreased fractional anisotropy (FA) demon- strated by diffusion tensor MR imaging (DTI) in areas of white matter (WM) damage is generally associated with increase of radial diffusivity, while axial diffusivity is reported to be decreased, unchanged, or increased. Aiming to better define the type of axial diffusivity change occurring in a typical human neurodegenerative disease, we investigated axial and radial diffusivity in Friedreich's ataxia (FRDA) which is characterized by selective neuronal loss of the dentate nuclei and atrophy and decreased FA of the superior cerebellar peduncles (SCPs). Methods Axial and radial diffusivity of the whole-brain WM were evaluated in 14 patients with FRDA and 14 healthy volunteers using DTI at 1.5 T and the tract-based spatial statistics (TBSS) method, part of FSL software. Results TBSS analysis showed a single area in the central midbrain corresponding to the decussation of the SCPs which exhibited lower FA in patients than in controls. In this area, a significant increase of both axial and radial diffusivity was observed. No clusters of significantly decreased axial diffusivity were observed, while additional clusters of increase of radial diffusivity were present throughout the brain. Conclusions The selective decrease of FA in SCPs of FRDA patients reflecting chronic WM tract damage is associated with increase of both the axial and radial diffusivity, the latter more pronounced than the former. The ultrastructural and biophysical bases of the increased axial diffusivity in chronically degenerating WM tracts deserve further studies. Keywords Axial diffusivity . WM degeneration . Diffusion tensor imaging Introduction Diffusion tensor imaging (DTI) enables in vivo evaluation with magnetic resonance (MR) of the microstructure of the brain white matter (WM) [1]. Several parameters can be calculated from the diffusion tensor model describing diffusive motion of water protons including fractional anisotropy (FA), mean diffusivity (MD), and axial (λ||) and radial (λ⊥) diffusivity. Quantitatively, λ|| is equal to the magnitude of the primary eigenvalue or parallel diffusion, while λ⊥ is the mean of the two tertiary eigenvalues or diffusion perpendicular to the primary axis. The variable combination of axial and radial diffusivity accounts for the degree of asymmetry of the ellipsoid reflected in the FA value. Electronic supplementary material The online version of this article (doi:10.1007/s00234-010-0807-1) contains supplementary material, which is available to authorized users. R. Della Nave S. Giuseppe Hospital, Radiodiagnostic Section, Empoli, Italy A. Ginestroni : S. Diciotti : M. Mascalchi (*) Radiodiagnostic Section, Department of Clinical Physiopathology, University of Florence, Florence, Italy e-mail: m.mascalchi@dfc.unifi.it E. Salvatore Department of Neurological Sciences, University of Naples Federico II, Naples, Italy A. Soricelli IRCCS Fondazione SDN and University of Naples Parthenope, Naples, Italy Neuroradiology (2011) 53:367–372 DOI 10.1007/s00234-010-0807-1