Magnetic Resonance Neurography: Diffusion Tensor Imaging and Future Directions

Key points

Nerve anatomy and peripheral neuropathy

To understand the new MRN technologies, as well as related normal and abnormal appearances of the peripheral nervous system (PNS) using these techniques, an understanding of the peripheral nerve structure, composition of its different tissues, and knowledge of the widely used classification of peripheral neuropathy is important.

The axon is the functional unit of the peripheral nerve, supported by surrounding Schwann cells and myelin layers. A layer of loose connective tissue, the endoneurium,

Limitations of current diagnostic tests and imaging

In addition to the clinical examination, nerve conduction and electromyography (EMG) studies and quantitative neurosensory testing are most commonly used to assess peripheral neuropathies and nerve injuries. Although these techniques remain the reference standard, there are limitations. First, the information about the exact location, extent, and cause of nerve disorders is often limited.²¹ In addition, electrodiagnostic studies depend on operative and interpretative skills of the examiner and

High-resolution MRN and new three-dimensional sequences

The increasing use of 3-T MR scanners, new phased-array surface coils, and parallel imaging techniques allow the acquisition of high-resolution and high-contrast images in short imaging times. Current state-of-the-art MRN provides detailed anatomic depiction of peripheral nerves and improved characterization of pathologic states (Figs. 1 and 2).1, 27, 29, 30

Axial T1-weighted and fluid-sensitive fat-suppressed T2-weighted images serve as the mainstay in MRN interpretation for prudent assessment

Whole-body MRN

Multimodality scanners which are able to acquire coregistered structural and functional information, such as single-photon emission computed tomography (SPECT)/computed tomography (CT) and positron emission tomography (PET)/CT, play an increasingly important role in the evaluation of human disease. However, disadvantages of SPECT/CT and PET/CT are a long preparation time for the examination, exposure to ionizing radiation, and possible mismatch between anatomic and functional data sets caused

Diffusion tensor imaging/DWI

DWI and diffusion tensor imaging (DTI) in particular are MR imaging techniques based on the thermally driven random motion (diffusion) of water molecules within biologic tissues. Tissues have distinct structural properties which hinder diffusion in some directions and facilitate it in other directions.

DTI shows great potential as a noninvasive technology to detect axonal injury in the central nervous system (CNS), which has been shown by a large number of studies; data in CNS studies indicate

Magnetization transfer imaging

On conventional MR imaging, tissue contrast is generated from variations in proton density and relaxation times of water protons. Longitudinal and transverse components of the magnetization in homogeneous samples relax monoexponentially with characteristic decay times T1 and T2. In biologic tissues there are protons with free mobility (water protons) and protons with restricted mobility because of bonds to macromolecules or membranes. These restricted protons have a T2 relaxation time too short

Contrast agents and non-MR imaging techniques

Because the current mainstay of MRN is T1-weighted and T2-weighted imaging, use of extracellular contrast agents in peripheral neuropathy is clinically often limited to inflammatory or infectious and tumorous conditions. It may also be used postoperatively to assess scar tissue and in diffuse peripheral neuropathies. As a consequence, any new contrast agents to be introduced for peripheral nerve imaging must have a significant advantage compared with current extracellular contrast agents to

Summary

MRN has progressed greatly in the past 2 decades. Excellent depiction of 3D nerve anatomy and disorders is currently possible using state-of-the-art MR imaging techniques. Further developments in the years to come will include the use of high-resolution 3D and diffusion-based imaging and potentially nerve-specific MR contrast agents as well as molecular imaging.