In vivo diffusion tensor imaging of the human optic nerve: Pilot study in normal controls

Abstract

Diffusion tensor imaging (DTI) of the optic nerve (ON) was acquired in normal controls using zonally oblique multislice (ZOOM) DTI, which excites a small field of view (FOV) using a fast sequence with a shortened EPI echo train. This combines the benefit of low sensitivity to motion (due to the single-shot acquisition used), with the additional advantage of reduced sensitivity to magnetic field susceptibility artifacts. Reducing the bright signal from the fat and cerebrospinal fluid (CSF) surrounding the nerve are key requirements for the success of the presented method. Quantitative MRI measurements are becoming essential in clinical research because they enable one to compare quantitative results obtained at different time points in the same patients/volunteers in longitudinal studies or in different patients/volunteers in cross-sectional studies. One of the measurements that is becoming more and more popular in research projects, especially in the brain, is the diffusion tensor (DT), because from it many parameters can be derived that are rotationally-invariant (i.e., unaffected by positioning in the scanner) and have high sensitivity to the structural properties of tissues and hence to structural changes (1).Initial observations of diffusion anisotropy in the optic nerve (ON) were reported a decade ago in freshly excised ON of the garfish (2), while fairly recently the effects of pathology on the diffusion properties of the ON were shown in a study on anesthetized mice with retinal ischemia (3). Quantitative MRI of the human ON in vivo is generally very challenging because of its small size (diameter ϭ ϳ3-7 mm depending on the position along the ON), its uncontrolled motion, the presence of magnetic field susceptibility artifacts (especially near the chiasm), and high signal from the orbital fat and cerebrospinal fluid (CSF) that surrounds the ON. ON diffusion measurements are particularly difficult to obtain because of the high sensitivity of the acquisition to motion. Previous publications have measured the apparent diffusion coefficient (ADC) in the ON (as opposed to measuring the mean diffusivity (MD) from the eigenvalues of the DT) (4 -6). Preliminary applications of ON ADC measurements to disease states have also been reported. In work by Wheeler-Kingshott and Hickman, diffusion-weighted (DW) zonal oblique multislice (ZOOM) echo-planar imaging (EPI) was used to acquire coronal ADC maps of the ON in vivo (6) and to determine ADC changes in a cohort of optic neuritis patients (7). Indications of the sensitivity of the ADC for detecting pathological changes were also reported in a study of chronic optic neuritis (4). Koch et al. (8) showed the intracranial portion of the ON in a qualitative tractography study using DT maps of healthy brains, and the results of a quantitative DT study of the ON in healthy volunteers were recently reported (9).Our aim was to develop a protocol that allowed us to further assess the structural properties of the ON by measuring the full diffusion tensor (DT) (10). Ultimately, ...

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Toward a practical protocol for human optic nerve DTI with EPI geometric distortion correction

“…These results imply that the FA and Ќ could be used to observe the demyelination of optic nerves. Figure 6 compares the DTI measures of the NC cohort after correction to three studies by Kim et al (SK,14), by Wheeler-Kingshott et al (CW,8), and by Xu et al (JX, 13). They are generally comparable to the measurements reported in these studies.…”

“…Bar graphs of the DTI measures averaged across normal subjects compared to the DTI measures from the literature. The first two letters in the legend indicate our results (OR) and other results (8,13,14), the letters after hyphens are the eye types (R, L, and B mean right, left, and both eyes, respectively), the following numbers are the numbers of subjects, and the letters inside the parentheses are method names. The error bars denote the standard deviations across subjects.…”

Combination of Mn2+‐enhanced and diffusion tensor MR imaging gives complementary information about injury and regeneration in the adult rat optic nerve

“…However, EPI is prone to artifacts, including eddy currents that cause ghosting, chemical shift artifacts, and image distortions due to field inhomogeneity and magnetic susceptibility variations (40). DTI-EPI of the ON is particularly challenging because of its small size, uncontrolled motion, air in the nasal cavities causing susceptibility effects, and high signal from fat and cerebrospinal fluid (CSF) surrounding the ON (41).…”