Currently, there is no high resolution, non-contact technology available for obtaining live images of oxygen saturation (SO2) or melanin in the retina and uveal tract — a potentially valuable tool for early diagnosis of retinal vein occlusions, diabetic retinopathy, and glaucoma. But initial development of photoacoustic remote sensing (PARS) microscopy in 2017 is now leading to new applications of this technology for ophthalmologic use.

PARS uses multicolored lasers to rapidly and accurately image tissue. When PARS is combined with swept-source optical coherence tomography (SS-OCT), the 2 subsystems form a multi-modal system — set up somewhat like an optical microscope — performing noncontact, in vivo structural, and functional imaging. For the first time, an experimental model has been tested with animal eyes, according to a study published in Scientific Reports

Cutaneous microcirculation of mouse ears is similar to human skin and was tested first. The SS-OCT visualized avascular structures in nude mouse ears. Cross-sectional B-scans show epidermis and dermis, auricular cartilage, and adipose tissue; but vasculature is not well-defined. That is where PARS comes in — the lateral resolution of PARS is approximately 2 times higher than the SS-OCT, so smaller vessels are visible. In maximum amplitude projection (MAP) images, microscopic epidermal ridges can be seen.


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In albino mouse eyes, SS-OCT cross-sectional B-scans show anterior segment structures, and when the instrument setting is adjusted for an improved picture of lower segment areas; the iris, crystalline lens, and chamber angle are sharply focused. PARS effectively illustrated the iris’s vasculature, and vessels pictured can also be overlayed onto the en face OCT image. Another capability of PARS is detection of vessels at multiple depth levels, throughout the beam’s depth-of-focus. 

PARS microscopy is effective through layers of vitreous and lens, so it is able to evaluate the posterior chamber, according to researchers. Scans show choroidal layers, and melanin content in the retinal pigment epithelium (RPE). Using 2 excitation wavelengths of 532 nm and 558 nm, PARS detected oxyhemoglobin HbO2 and deoxyhemoglobin Hb by their color absorption spectra. From this, relative HbO2 and Hb concentration can be gauged, and the approximate SO2 estimated.

The system can operate in 2 modes; as 2 standalone subsystems, or as a multimodal unit that produces simultaneous images. In the simultaneous mode, SS-OCT gathers data while the PARS excitation beam is off. PARS resumes at the backward sweep of the swept-source laser. Volumetric and en face visualizations are constructed from 3D data.

This is early research, and yet to come are tests with rats and rabbits, evaluation of larger eyeballs with customized lenses, and work toward quantifying oxygen metabolic rate. PARS-SS-OCT offers a potential “noninvasive, simultaneous, and accurate measurement of functional details in the ophthalmic tissue and can assist ophthalmologists with the diagnostics and treatment of major eye diseases.”

Disclosure: One study author declared affiliations with the biotech, pharmaceutical, and/or device companies. The research was supported by illumiSonics Inc. Please see the original reference for a full list of authors’ disclosures.  

Reference

Hosseinaee Z,  Abbasi N, Pellegrino N, et.al. Functional and structural ophthalmic imaging using noncontact multimodal photoacoustic remote sensing microscopy and optical coherence tomography. Sci Rep. Published online June 1, 2021. doi:10.1038/s41598-021-90776-5