4.7-T diffusion tensor imaging of acute traumatic peripheral nerve injury

Abstract

Diagnosis and management of peripheral nerve injury is complicated by the inability to assess microstructural features of injured nerve fibers via clinical examination and electrophysiology. Diffusion tensor imaging (DTI) has been shown to accurately detect nerve injury and regeneration in crush models of peripheral nerve injury, but no prior studies have been conducted on nerve transection, a surgical emergency that can lead to permanent weakness or paralysis. Acute sciatic nerve injuries were performed microsurgically to produce multiple grades of nerve transection in rats that were harvested 1 hour after surgery. High-resolution diffusion tensor images from ex vivo sciatic nerves were obtained using diffusion-weighted spin-echo acquisitions at 4.7 T. Fractional anisotropy was significantly reduced at the injury sites of transected rats compared with sham rats. Additionally, minor eigenvalues and radial diffusivity were profoundly elevated at all injury sites and were negatively correlated to the degree of injury. Diffusion tensor tractography showed discontinuities at all injury sites and significantly reduced continuous tract counts. These findings demonstrate that high-resolution DTI is a promising tool for acute diagnosis and grading of traumatic peripheral nerve injuries.

Keywords: AUC = area under the curve; DTI = diffusion tensor imaging; DTPA = diethylene triamine pentaacetic acid; FA = fractional anisotropy; MD = mean diffusivity; MRI; MRN = MR neurography; PBS = phosphate-buffered saline; PFA = paraformaldehyde; ROC = receiver-operator characteristic; ROI = region of interest; SNR = signal-to-noise ratio; diffusion tensor imaging; diffusion tensor tractography; nerve transection; neurography; neurotmesis; peripheral nerve injury; λ‖ = axial diffusivity; λ⊥ = radial diffusivity.

FIG. 1

Predicted nerve injury severity correlated with proximal axon caliber. A and B: Representative fluorescent microscopy images of CM-DiI membrane label in (A) sham and (B) completely transected sciatic nerves. Axoplasm is visible as holes in the center of red membrane rings. C: Axon caliber was measured 3 mm proximal to the injury site for all axons visible in the axial cross-sections of sham and completely transected nerves (mean ± SEM; n = 6). D: Mean axon caliber correlated moderately with the predicted injury severity (r² = 0.59; n = 15; p = 0.002). However, correlation strongly improved after removing the nerves predicted to be 75% transected (r² = 0.82; n = 12; p = 0.0001). Dashed lines indicate the 95% CI. **p < 0.01, unpaired t-test with Welch correction.

FIG. 2

FA enabled detection of complete nerve transection with high sensitivity and specificity. A: FA was reduced at and proximal to injury sites (mean ± SEM; n = 6). B: MD was reduced only within injury sites (mean ± SEM; n = 6). C: Independent eigenvalues (λ1, λ2, λ3) and λ⊥ were measured at proximal, injury, and distal regions. Axial diffusivity (λ∥) was reduced within injury sites and λ⊥ was increased at and proximal to injury sites (mean ± SEM; n = 6). D: ROC curve analysis of FA shows high sensitivity and specificity for detection of any nerve injury (AUC 1.00; n = 21; p < 0.001). Unpaired t-tests corrected for multiple comparisons using the Holm-Sidak method. *p < 0.05; **p < 0.01.

FIG. 3

Tract continuity diminished in completely transected nerves. A and B: Diffusion tensor tractography in sham (A) and completely transected (B) sciatic nerves. Blue lines indicate seed points for tractography located at 3 mm proximal and distal to the injury sites. C: Continuous tract counts through the injury region were significantly reduced in completely transected nerves (mean ± SEM; n = 6). Unpaired t-test with Welch correction. *p < 0.01.

FIG. 4

Principal diffusion vectors are heterogeneous at transection sites. Principal diffusion vector glyphs overlaying the axial FA map of the injury region in sham (upper) and completely transected (lower) nerves. Colors indicate vector direction. Lighter regions of the map indicate higher FA.

FIG. 5

FA and λ⊥ detected all partial injuries and λ⊥ correlated with injury severity. Quantitative analysis of DTI parameters in sham, partial, and complete sciatic nerve transection injuries. A: FA was similarly reduced in the injury region of all complete and partial nerve injuries (mean ± SEM; n = 21). C: MD showed a nonsignificant increase in all injury groups. E: Minor eigenvalues and λ⊥ were significantly increased in the injury region in all injury groups (mean ± SEM; n = 21). B, D, F, and H: There was no correlation of FA (B), MD (D), λ∥ (F), or λ⊥ (H) measured at injury sites with the number of axons in injured nerves. G: λ⊥ was negatively correlated to axon caliber in injured nerves and indirectly to injury severity (r² = 0.357). One-way ANOVA corrected for multiple comparisons with Tukey test. *p < 0.05; #p < 0.01; $p < 0.001; ***p < 0.0001.

FIG. 6

FA remained sensitive to nerve transection in ex vivo hind limbs. A and B: Ex vivo diffusion tensor tractography of rat hind limbs with sham (A) and completely transected (B) sciatic nerves. C: FA measured at injury and each region proximal and distal to the injury (mean ± SEM; n = 8). D: FA approached sham values with increasing distance from the injury site but remained significantly lower at all points (mean ± SEM; n = 8). Unpaired t-tests corrected for multiple comparisons using the Holm-Sidak method. *p < 0.05; **p < 0.01.