Short Term Reproducibility of a High Contrast 3-D Isotropic Optic Nerve Imaging Sequence in Healthy Controls

Abstract

The optic nerve (ON) plays a crucial role in human vision transporting all visual information from the retina to the brain for higher order processing. There are many diseases that affect the ON structure such as optic neuritis, anterior ischemic optic neuropathy and multiple sclerosis. Because the ON is the sole pathway for visual information from the retina to areas of higher level processing, measures of ON damage have been shown to correlate well with visual deficits. Increased intracranial pressure has been shown to correlate with the size of the cerebrospinal fluid (CSF) surrounding the ON. These measures are generally taken at an arbitrary point along the nerve and do not account for changes along the length of the ON. We propose a high contrast and high-resolution 3-D acquired isotropic imaging sequence optimized for ON imaging. We have acquired scan-rescan data using the optimized sequence and a current standard of care protocol for 10 subjects. We show that this sequence has superior contrast-to-noise ratio to the current standard of care while achieving a factor of 11 higher resolution. We apply a previously published automatic pipeline to segment the ON and CSF sheath and measure the size of each individually. We show that these measures of ON size have lower short-term reproducibility than the population variance and the variability along the length of the nerve. We find that the proposed imaging protocol is (1) useful in detecting population differences and local changes and (2) a promising tool for investigating biomarkers related to structural changes of the ON.

Keywords: Magnetic Resonance Imaging; Optic Nerve; Reproducibility.

Figure 1

Comparison of the clinical standard of care (A) T1W image, (B) T2W image and (C) our proposed high-resolution sequence for a single subject. Note that the resolution of the clinical standard of care yields one slice containing the ON, which is shown, while a medial slice was chosen for our proposed method (C)

Figure 2

Contrast-to-noise ratio (CNR) comparison for contrast between CSF-Fat (left) and ON-CSF (right) for the clinical standard of care T1W, T2W and the optimized VISTA sequence. ** indicates the results are significantly different by Wilcoxon rank-sum test at p<0.01.

Figure 3

Comparison of population variability for scan 1 (blue) and scan 2 (red) as well as the computed reproducibility as the standard deviation of the difference of each scan-rescan measurement divided by $\sqrt{2}$. The computed reproducibility error bars are shown on the overall population mean distribution for comparison.