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Editors Selection IGR 20-2
Basic Science: Beyond OCT-A: imaging the deep eye vasculature
Comment by Toru Nakazawa on:
86174 In Vivo Visualization of Eye Vasculature Using Super-Resolution Ultrasound Microvessel Imaging, Qian X; Kang H; Li R et al., IEEE Transactions on Bio-Medical Engineering, 2020; 67: 2870-2880
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
References
- Nakazawa T. Ocular bloodflow and influencing factors for glaucoma. Asia Pac J Ophthalmol. 2016;5:38-44.
- Liu K, Xu H, Jiang H, et al. Macular vessel density and foveal avascular zone parameters in patients after acute primary angle closure determined by OCT angiography. Sci Rep. 2020;10:18717.
- Upputuri PK, Sivasubramanian K, Mark CSK, et.al. Recent developments in vascular imaging techniques in tissue engineering and regenerative medicine. BioMed Res Int. 2015; 783983.
- Errico C, Pierre J, Pezet S, et al. Ultrafast ultrasound localization for deep superresolution vascular imaging. Nature. 2015;527:499.
- Segers T. Appl Phys Lett. 2020;116:173701.
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