Use of Advanced Magnetic Resonance Imaging Techniques in Neuromyelitis Optica Spectrum Disorder

Abstract

Brain parenchymal lesions are frequently observed on conventional magnetic resonance imaging (MRI) scans of patients with neuromyelitis optica (NMO) spectrum disorder, but the specific morphological and temporal patterns distinguishing them unequivocally from lesions caused by other disorders have not been identified. This literature review summarizes the literature on advanced quantitative imaging measures reported for patients with NMO spectrum disorder, including proton MR spectroscopy, diffusion tensor imaging, magnetization transfer imaging, quantitative MR volumetry, and ultrahigh-field strength MRI. It was undertaken to consider the advanced MRI techniques used for patients with NMO by different specialists in the field. Although quantitative measures such as proton MR spectroscopy or magnetization transfer imaging have not reproducibly revealed diffuse brain injury, preliminary data from diffusion-weighted imaging and brain tissue volumetry indicate greater white matter than gray matter degradation. These findings could be confirmed by ultrahigh-field MRI. The use of nonconventional MRI techniques may further our understanding of the pathogenic processes in NMO spectrum disorders and may help us identify the distinct radiographic features corresponding to specific phenotypic manifestations of this disease.

Figure 1. Diffusion Tensor Imaging

A, Fiber bundles are composed of axons with myelinated sheaths. B, The corresponding diffusion tensor is modeled by an ellipsoid. Parallel diffusivity (D_par) corresponds to the diffusivity in the main direction of the fiber bundle (reflecting axonal integrity), and perpendicular diffusivity (D_per) is related to the diffusivity orthogonal to this direction (reflecting the myelination).

Figure 2. Resting-State Functional Magnetic Resonance Imaging

A, Axial (left) and sagittal (right) views of the brain functional network. Nodes are located toward the coordinates of the regional centroids of the automated anatomical labeling template. Short-distance connections corresponding to the red edges are predominantly in the posterior cortex, whereas the long-distance connections shown in blue are between the frontal cortex and the regions of the parietal and temporal association cortex. B, Expanded Disability Status Scale (EDSS) as a function of the hub disruption index. A hub disruption index of 0 corresponds to a normal network. The farther the index deviates from 0, the more significant the reorganization of the network (in terms of topology). A correlation score highlights the fact that the reorganization of the brain network is a marker of the severity of the disease. The solid line represents the linear regression fit across all participants.

Figure 3. Optic Radiations

Optic radiation tractography was performed using a diffusion tensor imaging/magnetic resonance imaging (MRI) scan (Siemens Avanto 1.5-T MRI scanner, with 30 directions). Two seed points (the brightly colored fiber bundles) have been defined, the first one in the lateral geniculate body and the second one in the white matter at the posterior part of the occipital horn of the lateral ventricle. The fiber bundles are color coded according to their directions of impulse transmission.

Figure 4. Magnetic Resonance Imaging Scans of Neuromyelitis Optica (NMO) and Multiple Sclerosis (MS) Lesions at 7 T

Multiple sclerosis lesions are characteristically centered on a small vein in T2*-weighted sequences (blue arrowheads pointing to lesion surface and yellow arrowheads pointing to central intralesional vein) (A), a finding not present in 7-T magnetic resonance imaging scans of patients with NMO spectrum disorder who have brain parenchymal lesions (blue arrowheads) (B).