This study reports a high-resolution MRI approach to image basal blood flow and hypercapnia-induced blood-flow changes in the unanesthetized human retina on a 3-Tesla MRI scanner. Pseudo-continuous arterial-spin-labeling technique with static-tissue suppression was implemented to minimize movement artifacts and improve blood-flow sensitivity. Turbo spin-echo acquisition was used to achieve high spatial resolution free of susceptibility artifacts. The size, shape and position of a custom-made receive radiofrequency coil were optimized for sensitivity in the posterior retina. Synchronized eye blink and respiration to the end of each data readout minimized eye movement and physiological fluctuation. Robust high-contrast blood-flow MRI of the unanesthetized human retina was obtained at 500×800μm2 in-plane resolution. Blood flow in the posterior retina was 93±31mL/100mL/min (mean±SD, N=5). Hypercapnic inhalation (5% CO2) increased blood flow by 12±2% relative to air (P<0.01, N=5). This study demonstrates the feasibility of blood-flow MRI of the retina of unanesthetized human. Because blood flow is tightly coupled to metabolic function under normal conditions and it is often perturbed in diseases, this approach could provide unique insights into retinal physiology and serve as an objective imaging biomarker for disease staging and testing of novel therapeutic strategies. This approach could open up new avenue of retinal research.

This study reports high-resolution MRI of lamina-specific structures in the human retina. These initial results are encouraging. Further improvement in spatiotemporal resolution is warranted.

PURPOSE.To investigate blood flow (BF) in the human retina/ choroid during rest and handgrip isometric exercise using magnetic resonance imaging (MRI).METHODS. Four healthy volunteers (25-36 years old) in multiple sessions (1-3) on different days. MRI studies were performed on a 3-Tesla scanner using a custom-made surface coil (7 3 5cm in diameter) at the spatial resolution of 0.5 3 0.8 3 6.0 mm. BF was measured using the pseudo-continuous arterial-spinlabeling technique with background suppression and turbospin-echo acquisition. During MRI, subjects rested for 1 minute followed by 1 minute of handgrip, repeating three times, while maintaining stable eye fixation on a target with cued eye blinks at the end of each data acquisition (every 4.6 seconds).RESULTS. Robust BF of the unanesthetized human retina/ choroid was detected. Basal BF in the posterior retina/choroid was 149 6 48 mL/100 mL/min with a mean heart rate of 60 6 5 beats per minute, mean arterial pressure of 78 6 5 mm Hg, ocular perfusion pressure of 67 6 4 mm Hg at rest (mean 6 SD, n ¼ 4 subjects). Handgrip significantly increased retina/ choroid BF by 25% 6 7%, heart rate by 19% 6 8%, mean arterial pressure by 22% 6 5% (measured at the middle of the handgrip task), and ocular perfusion pressure by 25% 6 6% (averaged across the entire handgrip task) (P < 0.01), but did not change intraocular pressure, arterial oxygen saturation, end-tidal CO 2 , and respiration rate (P > 0.05).CONCLUSIONS. This study demonstrates a novel MRI application to image quantitative BF of the human retina/choroid during rest and isometric exercise. Retina/choroid BF increases during brief handgrip exercise, paralleling increases in mean arterial pressure. Handgrip exercise changes ocular perfusion pressure free of potential drug side effect and can be done in the MRI scanner. MRI offers quantitative BF with large field of view without depth limitation, potentially providing insights into retinal pathophysiology. (Invest Ophthalmol Vis Sci. 2012; 53:4299-4305)

Osteoarthritis (OA) is a disease whose hallmark is the degeneration of articular cartilage. There is a worsening epidemic of OA in the United States today, with considerable economic costs. In order to develop more effective treatments for OA, noninvasive biomarkers that permit early diagnosis and treatment monitoring are necessary. T1rho and T2 mapping are two magnetic resonance imaging techniques that have shown great promise as noninvasive biomarkers of cartilage degeneration. Each of the two techniques is endowed with advantages and disadvantages: T1rho can discern earlier biochemical changes of OA than T2 mapping, while T2 mapping is more widely available and can be incorporated into existing imaging protocols in a more time-efficient manner than T1rho. Both techniques have been applied in numerous instances to study how cartilage is affected by OA risk factors, such as age and exercise. Additionally, both techniques have been repeatedly applied to the study of posttraumatic OA in patients with torn anterior cruciate ligaments.