Abstract
.Clinical imaging techniques for the anterior segment of the eye provide excellent anatomical information, but molecular imaging techniques are lacking. Molecular photoacoustic imaging is one option to address this need, but implementation requires use of contrast agents to distinguish molecular targets from background photoacoustic signals. Contrast agents are typically selected based on a priori knowledge of photoacoustic properties of tissues. However, photoacoustic properties of anterior ocular tissues have not been studied yet. Herein, anterior segment anatomy and corresponding photoacoustic signals were analyzed in brown and blue porcine eyes ex vivo. Measured photoacoustic spectra were compared to known optical absorption spectra of endogenous chromophores. In general, experimentally measured photoacoustic spectra matched expectations based on absorption spectra of endogenous chromophores reported in the literature, and similar photoacoustic spectra were observed in blue and brown porcine eyes. However, unique light–tissue interactions at the iris modified photoacoustic signals from melanin. Finally, we demonstrated how the measured PA spectra established herein can be used for one application of molecular PA imaging, detecting photoacoustically labeled stem cells in the anterior segment for glaucoma treatment.
Highlights
The most-investigated approaches for noninvasive assessment of internal ocular structures have been ultrasound (US) and optical imaging techniques.[1,2,3,4,5,6,7,8,9] Clinical use of ultrasound biomicroscopy and optical coherence tomography (OCT) are well established in ophthalmology.[1,2,10] OCT provides excellent anatomical information by measuring changes in backscattered light.[6]
Our work focuses on extending molecular PA imaging to the anterior segment of the eye, i.e., tissues anterior to the vitreous humor
Measured PA spectra were compiled based on distinct PA spectra observed in the anterior and posterior iris, trabecular meshwork (TM), and sclera
Summary
The most-investigated approaches for noninvasive assessment of internal ocular structures have been ultrasound (US) and optical imaging techniques.[1,2,3,4,5,6,7,8,9] Clinical use of ultrasound biomicroscopy and optical coherence tomography (OCT) are well established in ophthalmology.[1,2,10] OCT provides excellent anatomical information by measuring changes in backscattered light.[6] Advances in OCT allow real-time imaging, widefield imaging, higher resolution, and assessment of other tissue optical properties, such as birefringence.[10,11,12,13,14] Overall OCT research has drastically improved the quality of anatomical imaging in ophthalmology. High background signals from surrounding tissue still make it difficult to distinguish molecular targets. Researchers have invented other clever solutions, including magnetomotive OCT and US, spectroscopic-OCT, pump-probe OCT, and photothermal OCT.[17,18,19]
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