Visualization of peripheral nerve degeneration and regeneration: Monitoring with diffusion tensor tractography

Abstract

We applied diffusion tensor tractography (DTT), a recently developed MRI technique that reveals the microstructures of tissues based on its ability to monitor the random movements of water molecules, to the visualization of peripheral nerves after injury. The rat sciatic nerve was subjected to contusive injury, and the data obtained from diffusion tensor imaging (DTI) were used to determine the tracks of nerve fibers (DTT). The DTT images obtained using the fractional anisotropy (FA) threshold value of 0.4 clearly revealed the recovery process of the contused nerves. Immediately after the injury, fiber tracking from the designated proximal site could not be continued beyond the lesion epicenter, but the intensity improved thereafter, returning to its pre-injury level by 3 weeks later. We compared the FA value, a parameter computed from the DTT data, with the results of histological and functional examinations of the injured nerves, during recovery. The FA values of the peripheral nerves were more strongly correlated with axon-related (axon density and diameter) than with myelin-related (myelin density and thickness) parameters, supporting the theories that axonal membranes play a major role in anisotropic water diffusion and that myelination can modulate the degree of anisotropy. Moreover, restoration of the FA value at the lesion epicenter was strongly correlated with parameters of motor and sensory functional recovery. These correlations of the FA values with both the histological and functional changes demonstrate the potential usefulness of DTT for evaluating clinical events associated with Wallerian degeneration and the regeneration of peripheral nerves.

Introduction

MRI, an indispensable tool in the diagnosis of central nervous system disorders, has rarely been applied to diseases of the peripheral nervous system, because it is difficult to delineate peripheral nerves on account of their poor contrast with the surrounding tissues. The standard repertoire for diagnosing peripheral nerve disorders includes clinical and electrophysiological examinations, supplemented by more invasive procedures.

For the differential diagnosis of peripheral nerve lesions, the visualization of peripheral nerves using MRI has been attempted using special techniques such as MR neurography (Filler et al., 2004, Howe et al., 1992). However, the interpretation of the images obtained by MR neurography is based on visual inspection, and is therefore qualitative and subjective. Furthermore, since MR neurography cannot image continuous nerve fibers over their entire length, it is not considered useful for examining the growth of regenerating peripheral nerves. To visualize nerve fibers in MRI, a contrast agent such as superparamagnetic iron oxide (SPIO) (Bendszus and Stoll, 2003) or gadofluorine M (Bendszus et al., 2005, Wessig et al., 2008) can be injected, but this is invasive. The difficulty in visualizing axons makes these methods impractical for evaluating peripheral nerve injury in the present clinical scenario.

To overcome these shortcomings, here we applied diffusion tensor imaging (DTI), a non-invasive method that reveals the microstructure of tissues on the basis of its ability to monitor the random movements of water molecules (Basser et al., 1994). Diffusion tensor tractography (DTT) refers to the analysis and reconstruction of the data obtained by DTI, by which the orientation of nerve fibers can be followed to trace specific neural pathways, such as that of the corticospinal tract in the brain or the spinal cord (Conturo et al., 1999, Fujiyoshi et al., 2007, Mori and Zhang, 2006, Tuch et al., 2001). Mac Donald et al have obtained results indicating that DTI may be more sensitive than conventional MRI for evaluating traumatic brain injury (Mac Donald et al., 2007b).

Recent advances in MRI technology have made it possible to delineate peripheral nerve tracts in humans (Hiltunen et al., 2005, Meek et al., 2006, Skorpil et al., 2004). However, the reliability of DTT imaging has not yet been validated with detailed histological studies and quantitative analyses, so it has remained unclear whether the changes in DTT parameters actually correspond to the anatomical degeneration and regeneration of axonal fibers. Although the disintegration of axonal structures and demyelination occurring after peripheral nerve injury, known as Wallerian degeneration, is known to reduce the anisotropy of peripheral nerves (Beaulieu et al., 1996, Stanisz et al., 2001), and DTI has been shown to be useful for detecting axonal injury after traumatic brain injury (Mac Donald et al., 2007a,b) and ischemic injury of the optic nerve (Song et al., 2003, Sun et al., 2008), peripheral nerve tracking during the process of Wallerian degeneration has never been reported. We believe that since no proper tools are presently available for the visualization of peripheral nerves, it is important to evaluate the validity of applying DTT to assess peripheral nerve degeneration and regeneration. The objectives of the present study were to determine whether DTT is useful for tracking peripheral nerves, and to determine the relevance of the tracking parameters for evaluating fibers after peripheral nerve injury, by comparing them with histological and functional parameters of recovery.