DTI & DIR in Optic Neuritis

Optic neuritis is an inflammatory demyelinating disorder of the optic nerve that commonly presents with acute visual loss , which can result from a range of causes, making the correct diagnosis and prognosis of distinct conditions important for proper treatment . Optic neuropathies can be either acute or chronic, with acute optic neuropathies having relatively abrupt onsets, while chronic optic neuropathies have a slow and prolonged development .

However, given that ON patients typically recover near-normal vision over time, they are most helpfully analyzed longitudinally, and by comparison between groups based on time of onset. For example, a basic distinction between acute ON representing subjects with recent onset (e.g., no greater than a month) and remote ON representing subjects with at least one episode of ON at least 1 year prior to the current investigation has been reported .

Traditionally, ophthalmologists rely on measurements of the patient's eye and retina, as well as direct tests of their vision to make a diagnosis. In recent years, magnetic resonance imaging (MRI) has become increasingly valuable in analyzing the impacts of optic neuropathies both on and beyond the optic nerve . Visualization and comparison of the downstream impact that optic neuropathies have on pre- and post-geniculate visual pathway regions as well as regions beyond the visual system form a typically untapped addition to standard neuro-ophthalmological assessments .

Nevertheless, the diagnosis of optic nerve affection and consideration of potential differential diagnoses are important in clinical routine. Especially the identification of its distinct pattern can help to differentiate neuromyelitis optica spectrum disease (NMO-SD) and MOG antibody disease (MOG-AD) from MS or other auto inflammatory illnesses. Further consideration of the optic nerves in follow up examinations of MS patients as one potential disease manifestation is reasonable.

Magnetic resonance imaging plays a pivotal role in diagnosis and exclusion of secondary causes. Conventional MRI sequences, including T1-weighted, T2-weighted, and post-contrast imaging, demonstrate optic nerve swelling and enhancement but provide limited quantitative assessment of axonal and myelin integrity.

Diffusion Tensor Imaging (DTI) is an advanced MRI technique that enables in-vivo assessment of white matter microstructure by measuring the directionality and magnitude of water diffusion.

Quantitative DTI parameters such as fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD), and radial diffusivity (RD) reflect axonal damage and demyelination. Traditional anatomical MRI has been extensively used in previous studies to investigate the morphological changes in various brain structures that might accompany disease. The appearance or size of particular grey and white matter regions can be detected and often correlates with important functional and/or clinical variables .

Each voxel in a DTI volume can be represented by an ellipsoid with three principal axes and eigenvectors.The measure along the longest axis which usually runs parallel to the measured axon is referred to as axial diffusivity (AD), while the mean magnitude of flow along the two perpendicular axes is referred to as radial diffusivity (RD). AD and RD measures have shown value in diagnosing axonal damage and myelin damage, respectively .

FA reflects the relative ratio between the anisotropic to isotropic nature of the measured diffusion, with perfectly isotropic diffusion having a value of zero and perfectly anisotropic diffusion having a value of one . An increased MD is commonly associated with damaged tissue because of the resulting increase in free diffusion that this elicits . A relatively high MD can characterize a high level of disorganization, while a low MD represents a highly organized structure .

In contrast, AD and RD values are tied to directional diffusivity and are increasingly representative of microstructural and anatomical details . Nevertheless, it is important to note that these four metrics are not entirely independent: a change in one metric would be expected to affect others .

Several studies have demonstrated alterations in DTI parameters of the optic nerve in optic neuritis; however, the role of DTI in idiopathic optic neuritis remains under-evaluated. This study aims to assess optic nerve microstructural changes using DTI and to correlate imaging findings with clinical and electrophysiological parameters.

In recent years, double inversion recovery (DIR) sequences have been increasingly used. This sequence type uses two different inversion pulses to suppress signals from cerebrospinal fluid (CSF) as well as signals from white matter. This results in a higher lesion-white matter contrast, which improves the detection of cortical lesions and T2-hyperintensities in the infratentorial region specially in MS . Interestingly, lesions of the optic nerves are apparent as well. They revealed the good clinical performance of the DIR in this acute setting. Consequently, we were wondering if DIR sequences are also capable of detecting optic nerve lesions in follow up examinations of optic neuritis patients .

Developing an understanding of how distinct optic neuropathies relate to MRI data metrics can improve the potential for identifying novel biomarkers. This will likely allow for a more integrated and all-encompassing understanding of these conditions with respect to severity, recovery, and treatment.

• Number of optic nerve lesions detected using Double Inversion Recovery MRI. [Time frame: Baseline At Time of MRI examination]