Jerome Hodel
The main topic of our research group is to develop advanced MR techniques in the field of neurodegenerative and inflammatory diseases. Because clinical disability in patients with multiple sclerosis (MS) is directly dependent on spinal cord lesions, we thought that diffusion tensor imaging could be a useful tool for the evaluation of white matter integrity at the early stage. However, DTI of the spinal cord remains challenging, as single-shot echo-planar imaging (SS-EPI) is prone to susceptibility artifacts related to the long echo-planar readout train used.
With my colleagues, we attempted to optimize the SS-EPI DTI sequence at 3T for the tractography-based measurements of DTI indices (fractional anisotropy, axial and radial diffusivities) over the cervical spinal cord in patients with myelitis. We used several techniques to improve diffusion-tensor image quality, including pulse-triggering, coronal acquisition, and reduced field-of-view (rFOV). Reducing the FOV in the phase-encoding direction leads to a drastic shortening of the echo-planar readout train and is thus useful to attenuate susceptibility-related artifacts. The rFOV acquisition was established by a non-coplanar application of the excitation and the refocusing pulses in conjunction with outer volume suppression. Such an approach was particularly suitable for the imaging of the spinal cord, as an ample truncation of the FOV in at least one dimension could be easily achieved. Indeed, in our study, the EPI readout train was shortened to 35 phase encode lines with a TE of 38.7 ms.
The rFOV SS-EPI DTI sequence appeared reliable for the routine assessment of DTI indices over the cervical spinal cord at 3T. In our cohort of patients with MS or neuromyelitis optica (NMO), DTI metrics were significantly correlated with disability scores, suggesting the potential clinical value of this approach. Moreover, using DTI, different patterns of myelitis, including spinal cord atrophy and active inflammatory lesions, were recognized.