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R. H. Silverman, F. Kong, Y.-C. Chen, H. O. Lloyd, D. J. Coleman; Photoacoustic Imaging of Ocular Tissues. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4277.
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Ultrasound (US) and optical coherence tomography (OCT) are imaging modalities that detect discontinuities in acoustic impedance and refractive index, respectively. Because these both relate to density inhomogeneity, US and OCT images, despite differences in resolution, share a qualitatively similar appearance. Photoacoustic imaging (PAI) is a technique in which a short optical pulse is directed at a tissue and a broadband US signal is generated as a consequence of thermoelastic expansion. PAI images depict optical absorption, which is independent of the characteristics imaged by US and OCT.
We used a frequency-doubled passively Q-switched Cr,Nd:YAG microchip laser emitting at 532-nm or 1064-nm as the light source. 5-ns pulses emitted at a repetition rate of 500-Hz were coupled to a photonic crystal fiber with a core diameter of 25-um and focused through a micro-lens to the focal plane of the US transducer. The laser pulse energy was 1-uJ. We fabricated two probes: the first had optical and 35-MHz acoustic beams intersecting at a common focus at a 30-degree angle; the second used a custom 20-MHz ring transducer with coaxial optical and acoustic beams. Laser spot sizes of 15- and 30-microns were achieved at 532- and 1064-nm, respectively. We acquired US and PAI data at 532- and 1064-nm wavelengths on whole and sectioned ex vivo pig eyes.
Because of the high-scattering of the sclera, PAI imaging required introduction of the laser beam through the cornea and pupil. PAI images were generally sensitive to pigmented structures such as the choroid, iris and ciliary body, although both the surface of the crystalline lens and zonular fibres were visualized. Penetration was higher at 1064-nm than at 532-nm, but sensitivity was lower.
PAI detects tissue properties that are independent of the those visualized by US or OCT. While optical focusing in not a requirement for PAI, it is advantageous for imaging of thin ocular tissue layers. Because the mean free path for photons is on the order of 1-mm in biological tissue, focusing allows laser spot size to be smaller than that of the ultrasound the beam over this depth. In the present study of ex vivo ocular tissues, melanin was the primary optical absorber, but in vivo the microcirculation will be seen since hemoglobin absorbs in both the visible and near infrared. PAI data acquired at multiple optical wavelengths will allow detection of melanin, oxy- and deoxy-hemoglobin and possibly other pigments present in the eye with co-registration against conventional US images.
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