This study offers a new approach for the quantification of CSF dynamics. Non-invasive method to quantify the CSF dynamics in the subarachnoid space of the optic nerve is highly desirable. The aim of the study was to measure slow-flow CSF velocities in healthy controls and normal tension glaucoma patients between the intracranial cavity and the subarachnoid space of the optic nerve. Prospective observational study. Eleven age-matched healthy volunteers and 15 normal tension glaucoma patients. Using phase contrast images, the phase shift in MRI diffusion images can be used to determine the flow velocity. Flow-range ratio between the intracranial cavity and the subarachnoid space of the optic nerve was calculated. Flow-range ratio between the intracranial cavity and the subarachnoid space of the optic nerve was calculated. First, phantom measurements were provided to validate the slow-flow velocity calculations. Second, flow-range ratio was validated for the healthy controls (0.63 ± 0.05), with the range being similar for the right and left optic nerve (P = 0.1). Statistically significant results were obtained (P < 0.05) when comparing the flow-range ratio in the optic nerve of healthy controls (n = 22 eyes, 0.63 ± 0.05) with the flow-range ratio in pathological optic nerves (n = 23, 0.55 ± 0.08) of normal tension glaucoma patients. MANOVA revealed no dependency between flow-range ratio and patient dependent variables. Diffusion-weighted imaging provides a method to evaluate CSF flow within the subarachnoid space of the optic nerve in a non-invasive manner. Compared to healthy controls, patients with normal tension glaucoma measure a significantly lower flow-range ratio. This finding suggests a possible role of impaired CSF dynamics in the pathophysiology in normal tension glaucoma. © 2017 Royal Australian and New Zealand College of Ophthalmologists.