Hu Z, Wang X, Liu Q, Paulus YM (2015) Photoacoustic Imaging in Ophthalmology. Int J Ophthalmol Eye Res 03(8), 126-132. 126 http://scidoc.org/IJOES.php International Journal of Ophthalmology & Eye Science (IJOES) ISSN 2332-290X Photoacoustic Imaging in Ophthalmology Review Article Hu Z 1,2 , Wang X 3 , Liu Q 2 , Paulus YM 1,3* 1 Department of Ophthalmology and Visual Sciences, University of Michigan, Ann Arbor, MI, USA. 2 Department of Ophthalmology, The First Affliated Hospital of Nanjing Medical University, Nanjing, China. 3 Department of Bioengineering, University of Michigan, Ann Arbor, MI, USA. *Corresponding Author: Yannis M. Paulus MD, Department of Ophthalmology and Visual Sciences, University of Mich- igan, 1000 Wall Street, Ann Arbor, MI 48105, USA. Tel: +1-734-763-8122 Fax: +1-810-694-5295 E-mail: ypaulus@med.umich.edu Received: July 27, 2015 Accepted: September 04, 2015 Published: September 09, 2015 Citation: Hu Z, Wang X, Liu Q, Paulus YM (2015) Photoacoustic Im- aging in Ophthalmology. Int J Ophthalmol Eye Res 03(8), 126-132. doi: http://dx.doi.org/10.19070/2332-290X-1500027 Copyright: Paulus YM © 2015. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution and reproduction in any medium, provided the original author and source are credited. Introduction Retinal imaging technology is rapidly advancing in parallel with new pharmacotherapies to treat neovascularization along with stem cell and gene therapies. Well-established imaging instru- ments include color fundus photography, optical coherence to- mography (OCT) [1], fuorescein and indocyanine green angi- ography (FA and ICGA) [2, 3], fundus autofuorescence [4], and B scan ultrasonography. Spectral-domain OCT (SD-OCT) and swept-source OCT can achieve cross-sectional images to deter- mine the layer of the retina involved with high axial-resolution. However, OCT uses interferometry and the contrast is based on the photons backscattered from the biological tissue. Thus, OCT mainly provide the information about scattering properties of retina. In clinic, ophthalmic OCT often creates an anatomic image but does not give physiologic or functional information, such as hemoglobin oxygen saturation (sO 2 ) in fundus vessels and melanin concentration in retinal pigment epithelium (RPE) and choroid. Newly developed OCT angiography allows for visualiza- tion of the vasculature but without allowing for determination of vessel leakage. Though microvascular circulation is visualized by angiography, the limitations of FA and ICGA are the need of an intravenous injection of a contrast agent. The extrinsic contrast agent makes angiography an invasive imaging method and it is not considered for patients with allergic concerns. In addition, there is usually a limited function period for the contrast agent after injection. Photoacoustic (PA) imaging, as a hybrid biomedical imaging method, exploits both optical and acoustical properties and pro- vides functional and structural information. With technical ad- vances in laser sources, computer signals processing, and ultra- sonic detectors, PA imaging has been drawing increasing attention in biological research and clinical practice in the last decade. Since Wang et al [5] frst mapped the vascular structure and functional cerebral hemodynamic changes in the rat brain with PA imaging, remarkable achievements have been made in the felds of cardiol- ogy [6-8], musculoskeletal [9, 10], sentinel lymph nodes [11, 12] and oncology [13, 14], particularly in breast cancer [15, 16]. The eye is considered very suitable for the application of PA imaging due to its accessibility, optical properties, and endogenous light absorbing molecules. As photoacoustic signals are generated by the absorption of pulsed laser illumination, images of the physi- ologically specifc optical absorption contrast in eye can thus be provided, which is not available in all existing ophthalmic imaging modalities. Additional advantages over other imaging modalities are that PA imaging can provide images with relatively high spatial resolution non-invasively. Photoacoustic microscopy (PAM), one of the major branches of PA imaging, has found its application in in vivo imaging of retinal and choroid vessels [17, 18], and RPE cells [19, 20], as well as the metabolism of the retina [21, 22]. Abstract Photoacoustic (PA) imaging is a rapidly developing new biomedical imaging modality. Unlike existing ophthalmic imaging technology such as fundus photography, optical coherence tomography, fuorescein and indocyanine green angiography, and ultrasound, photoacoustic imaging combines the advantages of optical excitation and acoustic detection. The light- absorbing hemoglobin and melanin can be detected noninvasively and sensitively by photoacoustic imaging. This paper reviews recent developments of photoacoustic imaging in ophthalmological research, from basic principles to applications. Emphasis is given to the photoacoustic ophthalmoscopy and its multimodal imaging capability. Keywords: Photoacoustic Imaging; Photoacoustic Ophthalmoscopy (PAOM); Ultrasound; Multimodal Imaging; Func- tional Imaging.