BACKGROUND. Diffusion tensor imaging (DTI) is a magnetic resonance imaging (MRI) technique that measures the anisotropy of water diffusion and the extent of anisotropy in biological tissue. The technique was primarily developed to depict the structural integrity of larger axonal bundles in the central nervous system; however, smaller structures like peripheral nerves can be visualized using high-field strength MRI with strong magnetic field gradients. DTI has attracted substantial attention for peripheral nerve depiction; nonetheless, its clinical applicability remains limited due to the scarce knowledge about diffusion tensors.
AIM. In this study, we aimed to acquire ex vivo human peripheral nerves using a high-resolution MRI system (i.e., magnetic resonance (MR) microscopy) and quantify diffusion tensor indices of peripheral nerves within different anatomical compartments (nerve fascicles, perineurium, and interfascicular epineurium). Moreover, we aimed to determine whether MR microscopy is comparable to the histological examination in the depiction of peripheral nerve.
METHODS. Nine-millimeter-long segments of the median nerve were obtained from five fresh cadavers. The nerves were scanned on a 9.4-T wide-bore vertical superconducting magnet using a tridimensional pulsed gradient spin-echo (PGSE) imaging sequence in 19 different gradient directions (b = 1150 s/mm$^2$) and once without diffusion gradient (b = 0 s/mm$^2$). Each sample was acquired in sixteen 0.625 mm thick slices and had the fascicles, perineurium, interfascicular epineurium, and nerve cross-sectional area manually delineated. The diffusion tensors were calculated from the region-average diffusion-weighted signals for all diffusion gradient directions. The correlations between diffusion tensors and parameters at the fascicular level (number of fascicles, fascicular ratio, cross-sectional area of fascicles and nerve) were assessed using linear regression and one-sample t-test; and diffusion tensors were compared between the donors using one-way or two-way ANOVA. The acquired diffusion tensor imaging data was employed for display with trajectories and diffusion ellipsoids. After the MRI acquisition, 0.01 mm thick histological slices were prepared from nerve samples, stained with hematoxylin and eosin, depicted with an optical microscope under 40× magnification, and had the area of fascicles and nerve measured. The measurements of MR microscopy and histological slices were compared using two-way repeated measures ANOVA.
RESULTS. The fascicles proved to be the most anisotropic nerve compartment. In the interfascicular epineurium, the diffusion was more prominent in orthogonal directions. Diffusion tensor indices differed significantly between the subjects within the fascicles (P<0.0001) and perineurium (P≤0.0001); however, there were no differences noted within the interfascicular epineurium. There were no correlations between diffusion tensor indices and nerve structure at the fascicular level. MR images and histological slices showed no statistically significant differences in the assessment of the number or area of fascicles, while there was a statistically significant difference in the assessment of a nerve cross-sectional area (P<0.001).
CONCLUSIONS. High-field strength MRI enables the depiction of peripheral nerves and shows the anisotropic motion of water molecules within the fascicles, namely in the direction of the nerve fibers. In the fascicles and perineurium, the diffusion tensors differ between the donors. High-resolution MR acquisition of peripheral nerve approaches the ability to display anatomical compartments seen in standard histological assessment of the peripheral nerve.
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