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Diffusion tensor magnetic resonance imaging (DT-MRI) is a powerful technique which provides quantitative information about structural and orientational features of the central nervous system. Development of DT-MRI based technology for identification for individual fibre tracts is important; this will allow detailed assessment of the damage to the intrinsic nerve tract, which could be helpful in understanding how tissue damage causes clinical deficits in various neurological conditions. In this context, the assessment of cord damage with DT-MRI is particularly appealing. The cord contains uniformly orientated fibres, thus obviating some caveats of anisotropy measures in the brain—for example, those related to the presence of crossing fibres on a voxel scale. Unfortunately, because of the small size of the cord and its sensitivity to artefacts related to the cerebrospinal fluid, cardiac and respiratory motions, spinal cord DT-MRI presents some technical difficulties and has been used in only very small and preliminary studies.1,2
Against this background, we carried out DT-MRI of the cervical cord in a 62 year old man with syringomyelia and primary progressive multiple sclerosis. In 1989, he had complained of bilateral arm weakness and sensory loss and cervical syringomyelia was diagnosed. In 1995, he complained of progressive gait disturbances, followed by subacute onset of bilateral visual loss. Primary progressive MS was diagnosed based upon brain MRI and cerebrospinal fluid findings.3 Although he now requires bilateral assistance for walking, following surgical intervention the upper limb signs and symptoms related to syringomyelia did not worsen over the past five years.
We acquired sensitivity encoding (SENSE) single shot echo planar imaging (EPI) sequence4 of the cervical cord and the brainstem of the patient. This sequence collected 16 images per section, including two images with no diffusion weighting (b∼0 s/mm2) and 14 images with the same b factor of 900 s/mm2 but with gradients applied in different directions. The diffusion unweighted images were needed to compute the DT, and the gradient orientation was chosen according to the algorithm proposed by Jones et al,5 designed to optimise DT-MRI acquisition. The measurement was repeated four times to improve the signal to noise ratio. Three saturation bands were used, positioned in the anterior part of the neck and transversely at the edges of the field of view in the vertical direction. From the SENSE single shot EPI images, a colour encoded sagittal image was obtained. The blue colour indicates preferential fibre direction along the z axis, the green colour preferential water molecular motion along the x axis, and the red colour preferential water molecular motion along the y axis. A sagittal T2-weighted sequence of the cervical cord was also acquired.
The sagittal T2-weighted image (fig 1A) shows a syrinx extending for the whole length of the cervical cord. The colour encoded SENSE single shot EPI image (fig 1B) demonstrates the presence of preserved white matter fibre tracts around and beyond the syrinx. This latter finding is consistent with the relatively preserved motor and sensory functions of the patient, despite the extensive cervical syrinx visible on the T2-weighted image.
In this patient, the preserved tissue geometry of the cervical cord around the syrinx underpins the potential of DT-MRI to provide a more complete picture of cord damage in different neurological disorders. Compared with conventional MRI, it enabled us to obtain quantitative information of the pathological characteristics of the tissue beyond the abnormalities visible on MRI. This shows promise in overcoming the well known discrepancy between aspects of conventional MRI and the clinical findings, reported in numerous neurological conditions. Moreover, clinical application of cord DT-MRI tractography may have prognostic value with regard to functional recovery after acute inflammatory or demyelinating pathologies, as it may enable us to investigate the residual integrity of clinically important pathways.
Competing interests: none declared
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