Figure (A) T1W scout image. (B) Basal BF (in color) overlaid on a scout image. (C) Averaged basal BF as a function of distance from the optic nerve head (ONH). (D) Activation maps from two repeated trials from the same subject. Color maps are overlaid on the BF images. (E) BF profiles under air (solid line) and hypercapnic inhalation (dotted line) across the retinal thickness. Profile was obtained from the posterior retina ROI. INTRODUCTION Blood flow (BF) is intricately coupled to basal metabolic function under normal physiological conditions, and it is often perturbed in disease states (1), such as glaucoma, diabetic retinopathy, and retinal ischemia. In vivo BF imaging of the retina has been reported using optical techniques such as fluorescein angiography, indocyanin-green angiography, laser Doppler flowmetry and laser speckle imaging. These methods are generally qualitative and depth ambiguous. Moreover, optical scattering and disease-induced opacity, such as vitreal hemorrhage and cataract, could hamper efficacy of optical imaging techniques. The goals of the present study were to explore the feasibility of imaging BF and BF responses of hypercapnic (5% CO 2 , 21% O 2 and balanced N 2) inhalation in the unanesthetized human retina using MRI. Pseudo-continuous arterial spin-labeling technique (pCASL) (2) with background suppression and single-shot turbo spin-echo (TSE) acquisition was implemented. pCASL was used to improve BF sensitivity, background suppression was used to enhance sensitivity and minimize eye movement artifacts, and TSE was used to achieve high spatial resolution free of susceptibility-induced signal drop off. Quantitative basal BF and hypercapnia-induced BF changes were analyzed.