Advances in neuroimaging of the visual pathways and their use in glaucoma
Introduction
Recently developed neuroimaging techniques such as diffusion tensor (DT) magnetic resonance (MR) imaging and functional MR imaging (fMRI) can be used to evaluate the microstructural integrity of white-matter fibers and the functional activity of gray matter. Their introduction has allowed a reexploration of the normal anatomy of white-matter tracts in the living human brain and the elaboration of connectional models of brain function. During the last 10 years, these techniques have also been used for the in vivo study of a variety of brain pathologies. First used in the investigation of ischemic stroke, they are now becoming increasingly important in the evaluation of intracranial neoplasms, inflammatory disorders, developmental anomalies of the central nervous system (CNS), and neurodegenerative diseases.
Both techniques can also be utilized to investigate the integrity and functional connections of the visual pathways, including areas that play key roles in glaucoma: pre- and postgeniculate white-matter fibers, the lateral geniculate nuclei (LGNs), and various areas of the visual cortex. This article provides an overview of some of the newer MRI techniques being used in neuro-ophthalmology and their specific applications in the study of patients with glaucoma.
Section snippets
Conventional MR imaging and the visual pathways
The macroscopic anatomy of the visual system can be demonstrated and assessed in vivo with conventional MR imaging. The optic nerve is a white-matter tract that is sheathed by the leptomeninges and dura mater, and its subarachnoid space is continuous with the intracranial subarachnoid space. On oblique coronal MR images, the intraorbital segment of the optic nerve ranges in thickness from 3.1 (anteriorly) to 2.5 mm (posteriorly), whereas the mean dural diameter measures between 5.1 (anteriorly)
Diffusion MR imaging
Diffusion is a random process that results from the thermal translational motion of molecules. In biological tissues, water diffuses inside, outside, around, and through cellular structures. Cellular membranes hinder these movements and force the water to take more tortuous paths. Diffusion is restricted by the presence of cellular swelling or increased cellularity, whereas necrosis, which involves the breakdown of cellular membranes, decreases diffusion-path tortuosity and increases apparent
Functional MR imaging
fMRI involves the application of MR techniques for high-resolution (spatial and temporal) investigations of brain physiology. Local changes in cerebral hemodynamics are closely linked to local cerebral activity, and they can be assessed with the technique known as blood oxygen level–dependent (BOLD) fMRI. The BOLD contrast effect depends on changes in the deoxyhemoglobin content of blood during states of increased neuronal activity (Ogawa et al., 1990). Many studies have been performed to
Proton MR spectroscopy
MR spectroscopy is a noninvasive tool for investigating the chemical environment of the brain. The proton MR spectrum is characterized by at least three peaks, which represent (1) the compounds creatine and phosphocreatine (Cr), which are generally associated with cellular energy metabolism; (2) choline (Cho), which is associated with cell membrane synthesis; and (3) N-acetyl aspartate (NAA), a marker of neuronal integrity. Several studies have demonstrated the usefulness of MR spectroscopy in
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