The pathological cascade of tissue damage in mild traumatic brain injury is set forth by a perturbation in ionic homeostasis. However, whether this class of injury can be detected in vivo and serve as a surrogate marker of clinical outcome is unknown. We employ sodium MRI to test the hypotheses that regional and global total sodium concentrations: (i) are higher in patients than in controls; and (ii) correlate with clinical presentation and neuropsychological function. Given the novelty of sodium imaging in traumatic brain injury, effect sizes from (i), and correlation types and strength from (ii), were compared to those obtained using standard diffusion imaging metrics. 27 patients (20 female, age 35.9 ± 12.2 years) within two months after injury and 19 controls were scanned with proton and sodium MRI at 3 Tesla. Total sodium concentration, fractional anisotropy and apparent diffusion coefficient were obtained with voxel averaging across 12 grey and white matter regions. Linear regression was used to obtain global grey and white matter total sodium concentrations. Patient outcome was assessed with global functioning, symptom profiles, and neuropsychological function assessments. In the regional analysis, there were no statistically significant differences between patients and controls in apparent diffusion coefficient, while differences in sodium concentration and fractional anisotropy were found only in single regions. However, for each of the 12 regions, sodium concentration effect sizes were uni-directional, due to lower mean sodium concentration in patients compared to controls. Consequently, linear regression analysis found statistically significant lower global grey and white matter sodium concentrations in patients compared to controls. The strongest correlation with outcome was between global grey matter sodium concentration and the composite z-score from the neuropsychological testing. In conclusion, both sodium concentration and diffusion showed poor utility in differentiating patients from controls, and weak correlations with clinical presentation, when using a region-based approach. In contrast, sodium linear regression, capitalizing on partial volume correction and high sensitivity to global changes, revealed high effect sizes and associations with patient outcome. This suggests that well-recognized sodium imbalances in traumatic brain injury are: (i) detectable non-invasively, (ii) non-focal, (iii) occur even when the antecedent injury is clinically mild. Finally, in contrast to our principle hypothesis, patients’ sodium concentrations were lower than controls, indicating that the biological effect of traumatic brain injury on the sodium homeostasis may differ from that in other neurological disorders.

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T HE VISUAL SYSTEM includes both ocular and neuroanatomy. Recent research has suggested that many visual impairments affect the entire visual system. In addition, their severity cannot be predicted solely via ocular imaging or brain imaging alone. [1][2][3] This suggests the need for a more in-depth understanding of pathogenesis at a systematic level, rather than individual anatomic structures alone. Whereas other imaging modalities are often limited to specific uses in evaluating single aspects of visual impairment in an anatomical structure, magnetic resonance imaging (MRI) can be employed to assess structure, metabolism, and function throughout the visual pathways. Here, we briefly introduce the key anatomical components of the visual system, as well as relevant MRI techniques in examining the pathology and recovery of these tissues. We then detail specific imaging considerations and a selection of recent clinical applications for MRI in each of the major anatomic regions, along with new developments that are being actively explored. Specific MRI parameters for several studies in each anatomic region can be found in Table 1.

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