Filtered Diffusion-Weighted MRI of the Human Cervical Spinal Cord: Feasibility and Application to Traumatic Spinal Cord Injury

Abstract

Background and purpose: In traumatic spinal cord injury, DTI is sensitive to injury but is unable to differentiate multiple pathologies. Axonal damage is a central feature of the underlying cord injury, but prominent edema confounds its detection. The purpose of this study was to examine a filtered DWI technique in patients with acute spinal cord injury.

Materials and methods: The MR imaging protocol was first evaluated in a cohort of healthy subjects at 3T (n = 3). Subsequently, patients with acute cervical spinal cord injury (n = 8) underwent filtered DWI concurrent with their acute clinical MR imaging examination <24 hours postinjury at 1.5T. DTI was obtained with 25 directions at a b-value of 800 s/mm². Filtered DWI used spinal cord-optimized diffusion-weighting along 26 directions with a "filter" b-value of 2000 s/mm² and a "probe" maximum b-value of 1000 s/mm². Parallel diffusivity metrics obtained from DTI and filtered DWI were compared.

Results: The high-strength diffusion-weighting perpendicular to the cord suppressed signals from tissues outside of the spinal cord, including muscle and CSF. The parallel ADC acquired from filtered DWI at the level of injury relative to the most cranial region showed a greater decrease (38.71%) compared with the decrease in axial diffusivity acquired by DTI (17.68%).

Conclusions: The results demonstrated that filtered DWI is feasible in the acute setting of spinal cord injury and reveals spinal cord diffusion characteristics not evident with conventional DTI.

FIG 1.

Schematic representation of the fDWI/DDE MR imaging technique for spinal cord injury. Traumatic injury to the spinal cord (A) results in microscopic damage to axons, illustrated here as beading and end-bulbs that reflect the underlying acute pathology (B). A prominent edema response (light blue) is typical surrounding the injury site. Traditional DWI/DTI derives measures reflecting the bulk sum of all features. With fDWI/DDE, a high-strength diffusion-weighting perpendicular to the spinal cord suppresses extracellular edema (and CSF) to estimate tissue-specific diffusivity metrics less confounded by edema.

FIG 2.

fADC_‖ and AD maps for the healthy spinal cord. Single-subject fADC_‖ (A) and AD maps (B) at C4 for a healthy individual on a 3T system. Comparison of the mean white matter fADC_‖ or AD values (C) and SNR (D).

FIG 3.

fADC_‖ and AD maps for an individual (subject 3) with an acute spinal cord injury. A T2-weighted image for an individual with an acute spinal cord injury on a 1.5T system. Single slices above, at, and below the injury site (as labeled in the T2 image) for fADC_‖ and AD maps. Ax GRE indicates axial gradient recalled-echo.

FIG 4.

fADC_‖ and AD compared at each individual section for acute spinal cord injury (n = 8). There is a large, unidirectional decrease in fADC_‖ at the injury site compared with a lesser, multidirectional decrease in AD values. The asterisk indicates significance compared with the first section (P < .05).

FIG 5.

Correlations of fADC_‖ and AD at each individual section for the intact spinal cord and acute spinal cord injury. Correlations are significant for the intact spinal cord; however, a lower correlation and nonrandom residuals for the acute SCI setting indicate that fADC_‖ and AD do not have a simple linear relationship, suggesting that they provide differing information.