Imaging Provides Cellular Views of Retinal Layers

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In a milestone for imaging the retina’s deepest layers, NEI researchers have successfully used adaptive optics (AO) combined with indocyanine green (ICG) angiography to visual­ize the entire photoreceptor/retinal pigment epithelium/choriocapillaris complex in living human eyes.¹

Simultaneous visualization. In the AO-ICG study, simultaneous imaging of the three retinal layers revealed that the dye localized not only to the choroi­dal vasculature but also to the retinal pigment epithelium (RPE) cells.

“This is a unique interaction that was unexpected,” said senior investi­gator Johnny Tam, PhD, at the NEI. “Typically, when people think of angi­ography they think of blood vessels, but this work is interesting because it shows a nonvascular structure, the RPE, that’s interacting with the dye.”

Improved resolution. The addition of adaptive optics, to correct for wave­front aberrations, improved the resolu­tion achievable with ICG and scanning laser ophthalmoscopy to approximately micrometers, sufficient to visualize and even quantify cells in the outer retinal layers, Dr. Tam said.

Moreover, by subtracting the light emitted by the dye in RPE cells, the re­searchers were able to detect the weaker fluorescent signal emitted by the tiny vessels of the choriocapillaris, he said. “This ability to see the choriocapillaris is not currently possible with conven­tional ICG,” he said.

Additional findings. The researchers also reported the following:

• After 23 healthy subjects received an intravenous injection of ICG, the dye was rapidly taken up by the RPE cells, peaking within several seconds. Further exploration of this dynamic process could lead to insights about drug pharmacokinetics in the retina, the researchers wrote.
• A second, smaller peak occurred a mean of 18.31 seconds (SD ± 2.98 seconds) after the first peak, reflecting the dye’s recirculation from the sys­temic circulation. Additional injections did not affect the subjects’ individual recirculation times. The researchers suggested that this might eventually allow retinal recirculation times to be used as an individualized biomarker for monitoring systemic vascular perfusion.
• RPE cell spacing and the flow voids in the choriocapillaris averaged 3.1 and 3.7 times larger, respectively, than they were in the tightly packed cone photo­receptor layer. Variations in these ratios over time might eventually enable individualized tracking, at a cellular level, of retinal disease progression, the researchers wrote.
• In a single patient with retinitis pig­mentosa, AO-ICG showed intact RPE and choriocapillaris layers underneath areas where photoreceptors had been lost. The borders between areas with healthy and absent photoreceptors were abrupt, rather than gradual. “This data provides a powerful tool for revealing the cellular status of disease in the living human eye,” they wrote.

Other applications. Improved im­aging is seen as a major need for the ad­vancement of regenerative therapies for eye disease, according to the NEI, which currently funds five imaging projects through its Audacious Goals Initiative. Adaptive optics is being used to im­prove other types of advanced retinal imaging, including multiply-scattered light imaging, and angiography using optical coherence tomography. Insights from these and other techniques, such as conventional angiography, will be complementary to AO-ICG, Dr. Tam said.

“We see our combined approach as allowing us to start to validate some of the intriguing findings that we see with conventional ICG. We want to go back to existing data and then collect data in new ways and interpret it all in ways that we didn’t think about previously,” he said.

The ultimate goal would be to apply the cellular-level discoveries from AO-ICG to already familiar imaging modalities, Dr. Tam said.

“Generalizing our results to standard ICG is something we’re very interested in. I think that as we start to compare our AO technique with standard ICG, we can take from the concepts that we’ve learned with AO-ICG and start to apply that toward clinical practice.”

—Linda Roach

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1 Jung H et al. Commun Biol. Published online Nov. 14, 2018.

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Relevant financial disclosures—Dr. Tam: None.

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