• Marios C Yiannakas, Ahmed T Toosy, Rhian E Raftopoulos, Raj Kapoor, David H Miller, and Claudia A M Wheeler-Kingshott.
• Nuclear Magnetic Resonance Research Unit, University College London Institute of Neurology, London, United Kingdom. m.yiannakas@ucl.ac.uk
• Invest. Ophthalmol. Vis. Sci. 2013 Jun 21; 54 (6): 4235-40.

PurposeTo present a new acquisition and analysis protocol for reliable and reproducible segmentation of the entire intraorbital optic nerve (ION) mean cross-sectional area by means of magnetic resonance imaging (MRI) at 3 tesla (T).MethodsEight healthy volunteers (mean age 31, five were male) gave written informed consent and both of their IONs were imaged individually using a coronal-oblique T2-weighted fast multidynamic image acquisition scheme; the proposed acquisition scheme has its rationale in combining separately acquired volumes and registering them to account for motion-related artifacts commonly associated with longer acquisitions. Mean cross-sectional area of each ION was measured using a semiautomated image analysis protocol that was based on an active surface model previously described and used for spinal cord imaging. Reproducibility was assessed for repeated scans (scan-rescan) and repeated image analysis performance (intraobserver).ResultsMean and SD values of the left ION cross-sectional area for the eight healthy volunteers were 5.0 (±0.7) mm² and for the right ION were 5.3 (±0.8) mm². Mean scan-rescan coefficient of variation (COV) for the left ION was 4.3% and for the right was 4.4%. Mean intraobserver COV for the left ION was 2.1% and for the right was 1.8%.ConclusionsThis study presents a new MRI acquisition and analysis protocol for reliable and reproducible in vivo measurement of the entire ION mean cross-sectional area as demonstrated in a pilot study of healthy subjects. The protocol presented here can be used in future studies of the ION in disease state.

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