Diffusion-weighted MR imaging of the normal human spinal cord in vivo

Abstract

Background and purpose: Diffusion-weighted imaging is a robust technique for evaluation of a variety of neurologic diseases affecting the brain, and might also have applications in the spinal cord. The purpose of this study was to determine the feasibility of obtaining in vivo diffusion-weighted images of the human spinal cord, to calculate normal apparent diffusion coefficient (ADC) values, and to assess cord anisotropy.

Methods: Fifteen healthy volunteers were imaged using a multi-shot, navigator-corrected, spin-echo, echo-planar pulse sequence. Axial images of the cervical spinal cord were obtained with diffusion gradients applied along three orthogonal axes (6 b values each), and ADC values were calculated for white and gray matter.

Results: With the diffusion gradients perpendicular to the orientation of the white matter tracts, spinal cord white matter was hyperintense to central gray matter at all b values. This was also the case at low b values with the diffusion gradients parallel to the white matter tracts; however, at higher b values, the relative signal intensity of gray and white matter reversed. With the diffusion gradients perpendicular to spinal cord, mean ADC values ranged from 0.40 to 0.57 x 10(-3) mm2/s for white and gray matter. With the diffusion gradients parallel to the white matter tracts, calculated ADC values were significantly higher. There was a statistically significant difference between the ADCs of white versus gray matter with all three gradient directions. Strong diffusional anisotropy was observed in spinal cord white matter.

Conclusion: Small field-of-view diffusion-weighted images of the human spinal cord can be acquired in vivo with reasonable scan times. Diffusion within spinal cord white matter is highly anisotropic.

fig 1.

Navigator-corrected, multi-shot, spin-echo echo-planar pulse sequence with diffusion-sensitizing gradients of amplitude (G), duration (δ), and separation (Δ). The diffusion gradients can be applied along any imaging direction. The navigator echo (the first spin-echo of this dual echo sequence) is not spatially encoded, and is used for phase correction of the second echo

fig 2.

Axial diffusion-weighted images (2 R-R/106/4 [TR/TE/excitations]) showing the appearance of the cervical spinal cord with the diffusion-probing gradients in the three cardinal axes, with increasing b values from left to right. R-L, A-P, and C-C indicate the direction of the diffusion gradients. The R-L and A-P directions are perpendicular to the white matter tracts. The C-C direction is parallel to → the white matter tracts. Note the relatively greater signal attenuation with the diffusion gradients in the C-C direction, reflecting the underlying tissue anisotropy. Note also the reversal of the relative signal intensity of gray and white matter at the higher b values (640, 1000) in the C-C direction, due to the higher ADC of white matter along this axis

fig 3.

Sagittal diffusion-weighted images (2 R-R/106/4) showing the appearance of the cervical spinal cord with the diffusion-probing gradients in the three cardinal axes, with increasing b values from left to right. The R-L and A-P directions are perpendicular to the white matter tracts. The C-C direction is parallel to the white matter tracts. Again, there is relatively greater signal attenuation with the diffusion gradients in the C-C direction. Note the sensitivity of the diffusion-weighted sequence to pulsation at the highest b value in the A-P diffusion direction, resulting in artifact in the phase-encoding direction