Atrophy of the optic nerve caused by biomechanical changes and/or abnormalities in blood flow, deep inside the optic disc is believed to be the origin of glaucoma.1 Reduction in capillary density at the fovea is also linked with glaucoma.2 90% of blood flow for nourishment of the retina originates from the choroidal vessels and choriocapillaris. Therefore, non-invasive imaging of blood flow abnormalities at the lamina cribrosa, and the retinal and choroidal capillaries are important for early detection/understanding of ocular diseases. Present optical techniques face difficulties in providing fine details of deep tissue structures inside the eye.3 Vitreous opacity and cataract make imaging further challenging. Techniques like MRI and ultra-sound have low resolution for ocular imaging.
Imaging at high resolution deep inside the eye is difficult to achieve unless the interaction of light/US is tissue/fluid selective. For example, US imaging with microbubbles as a contrasting agent can achieve ~10 times higher resolution than the diffraction limit. Imaging of vessels as small as 20 mm at a depth > 8 mm in the rat's brain has been reported.4 Qian and coworkers applied this technique successfully to image the posterior pole of a rabbit eye at a depth of ~ 14-18mm. An 18 MHz linear array transducer with compounding plane wave imaging technique was used. Microvasculature structure was reconstructed by deconvoluting the centroid intensity detected from the resonating microbubbles. The authors detected an increase in vessel density from the retina to choroid, and fine choroid vessels branching from ciliary artery. They also successfully imaged the retrobulbar vessels beyond the sclera. However, the axial resolution (~ 100-120 mm) is not high enough to distinguish fine vessels between the retina and choroid. Nevertheless, the results are encouraging, and have room for further improvements (theoretical resolution limit ~ 1.7 mm) e.g., by using monodisperse microbubbles.5