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The “Normal Appearing” Spinal Cord in Spinal Cord Injury: Diffusion Tensor Imaging Alterations Beyond the Visible Zone of Injury 2012

General Spine

Vahe, M, Zohrabian, M.D.
Benjamin, Zussman, B.S., Non ASSR Member
Andrea, Frangos, M.S., Non ASSR Member
Ralph, Marino, M.D., Non ASSR Member
Richard, Gorniak, M.D., Non ASSR Member
Adam, E, Flanders, M.D., Non ASSR Member

Paper/Non-Mentor

Purpose

Advanced MR imaging may improve our objective assessment of the extent of neurologic damage after spinal cord injury (SCI). Diffusion tensor imaging (DTI), in particular, has been shown to be a quantitative surrogate for myelin integrity. In cervical spondylotic myelopathy and multiple sclerosis, prior investigators have shown alterations in DTI metrics that extend well beyond the anatomic boundaries of macroscopic cord injury on conventional MRI. No work to date, however, has systematically reported similar DTI alterations in "normal appearing" spinal cord (NASC) after SCI. The purpose of this study was to prove the existence of a DTI "envelope" that demarcates a predictable zone that extends beyond the conventional zone of injury on MRI, and to determine if this process relates to the extent of neurologic injury.

Methods & Materials

A retrospective analysis of 45 patients (29 men, 16 women; mean age: 48 years, range 15-87) with blunt cervical spine trauma admitted to the Regional Spinal Cord Injury Center of the Delaware Valley and imaged within 24 hours of injury was performed. Patients were stratified by neurologic injury using the American Spinal Injury Association (ASIA) impairment scale (AIS) into motor complete (AIS A, n=12; AIS B, n=7), motor incomplete (AIS C, n=6; AIS D, n=6), and neurologically intact (AIS E, n=14). Neurologic level of injury (NLI) spanned C3-C8 and total upper extremity motor index score (MIS) ranged from 0-50 (mean: 17.8). Imaging consisted of standard MRI of the entire cervical spine at 1.5 T (Philips Intera), in conjunction with a single axial DTI sequence using a single-shot spin-echo EPI technique: TR/TE: 2500/75 msec; FOV: 20 cm; slice thickness 3-4 mm; 128 x 128 matrix; 0.95 mm3 voxels; 6 directions, b=800 s/mm2; 4 signal averages; scan time: 3 min 40 sec. DTI data was mapped to anatomic location of macroscopic injury on conventional sagittal MRI using table position and slice number. Manual regions of interest were drawn to incorporate the entire cross section of the cervical spinal cord, which included both gray and white matter. Weighted DTI averages based on number of voxels within NASC were calculated for DTI metrics fractional anisotropy (FA), apparent diffusion coefficient (ADC), primary eigenvector (λ1), and radial diffusivity (λ23). Statistical analyses consisted of non-parametric testing to identify differences in NASC DTI metrics between AIS classes. Correlations were also performed to identify associations between NASC DTI metrics and AIS as well as upper extremity MIS.

Results

There were no significant differences amongst the DTI parameters FA (p=.81), ADC (p=.19), λ1 (p=.12), and λ23 (p=.18) in edematous segment of cord and NASC within the same patient, suggesting that NASC in SCI patients may in fact not be "normal." Statistically significant differences for both FA (p=.01) and ADC (p=.02) of NASC between SCI patients (AIS A-D) and neurologically intact patients (AIS E) were identified. Additionally, there were statistically significant differences in DTI metrics FA (p=.01), ADC (p=.01), λ1 (p=.02) and λ23 (p=.04) of NASC between motor complete SCI and motor incomplete SCI. With the exception of a weak, yet statistically significant correlation between FA values in NASC and AIS (r=-.29, p=.02), no association was found between DTI values and extent of neurologic impairment as dictated by AIS and MIS.

Conclusion

There are subtle but measurable changes in DTI metrics within NASC in the setting of cervical SCI, which may provide additional information to characterize the extent of cord injury with MRI. A more accurate model of SCI using MRI may include conventional imaging, DTI of the injured segment, and DTI of the NASC.

References/Financial Disclosures

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