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Editors Selection IGR 19-4

Anatomical Structures: Imaging ouflow pathways

Comment by Alex Huang & Ji-Hye Park on:

118535 3D imaging of aqueous veins and surrounding sclera using a dual-wavelength photoacoustic microscopy, Ni L; Zhang W; Kim W et al., Biomedical optics express, 2023; 14: 6291-6300

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This research by Ni et al. employed dual-wavelength photoacoustic microscopy to visualize the anatomy of aqueous veins and the surrounding scleral texture in ex-vivo porcine and human eyes. The system's unique optical absorption properties enabled the vasculature to be imaged while avoiding more cumbersome post-processing needed for anterior segment OCT (AS-OCT). Further, dual wavelengths were used to attempt evaluation of aqueous humor outflow (AHO) pathways independent of the surrounding sclera.

Several limitations of this study must be noted. First, this approach still requires perfusion of a tracer, similar to aqueous angiography, and unlike AS-OCT. For aqueous angiography, tracers are FDA-approved for intravenous human use. In contrast, the tracer used here was a red tattoo dye, and its unique properties may have created limitations. For example, imaging was performed only after the TM was 'scratched' with a needle. It is possible that, without creating some level of ablation to the TM, the authors were unable to visualize the tracer. Second, while the 1200 nm laser setting enabled detection of collagen to provide information on the sclera, it may be an overstatement to claim that the authors fully revealed scleral texture. This is because the sclera is more than collagen and contains numerous extracellular matrix components, including different collagen subtypes, elastin fibers, proteoglycans, and glycoproteins. Moreover, as the 1200 nm laser can detect melanin, it is unclear whether underlying uveal tissue was also captured. Lastly, this approach still requires coupling. Photoacoustic imaging requires the probe to touch the eye or at least touch the eye through a coupling agent/lubricant. Translated to patient, this can get messy when imaging the entire eye and is not non-invasive/non-contact like OCT.

Ultimately, many approaches now exist to evaluate AHO pathways in both patients and experimentally in post-mortem eyes. Each has its own advantages and disadvantages. Ideally, for humans, the best method would include characteristics such as no touch/ coupling agent required, no dye, rapid measurement, high resolution, and minimal image processing. No current method satisfies all these criteria, and further work is needed to achieve this goal.

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