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Ronald H. Silverman, Raksha Urs, Harriet O. Lloyd, Jeffrey A. Ketterling, Fanting Kong, Y. C. Chen; Photoacoustic Detection of Acoustic Radiation Force-Induced Displacements in Ocular Tissues. Invest. Ophthalmol. Vis. Sci. 2011;52(14):2875. doi: https://doi.org/.
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Tissue elasticity can be assessed by measurement of stress-strain relationships occurring under mechanical stretching or compression. Such methods, however, are impractical in vivo. Absorption of ultrasound can induce tissue compression in vivo, with displacements determined from phase-resolved pulse-echo ultrasound data. This method, however, is non-specific in regards to the tissue structures undergoing displacement. In this report we describe a new technique utilizing photoacoustics to track displacements produced by acoustic radiation force in the iris and retina.
The probe consisted of a 20 MHz ultrasound transducer with a central aperture through which an optical fiber introduced 532 nm laser pulses. Pulses were 1 µJ in intensity, 5 nsec in duration and spaced at 2 msec intervals. A lens brought the laser spot (10 µm diameter) to a common focus with the ultrasound beam. We excited the transducer with 20-MHz interleaved monocycles (for pulse/echo) and tonebursts (to generate force) at a 90% duty cycle. Total duration of the push sequence was 12 msec. We examined the digitized photoacoustic and pulse/echo data to determine the magnitude and time-course of displacements in iris and retina in a fresh ex vivo rabbit eye.
At 100 and 150 mV excitations radiation force was 62 Wcm-2 and 97 Wcm-2, respectively. At 100 and 150 mV respectively, we observed iris displacements averaging 26.5 and 76.7 microns photoacoustically versus 24.9 and 71.3 microns by pulse echo. For the retina, the displacements at 100 and 150 mV were 14.6 and 25.3 microns photoacoustically versus 12.4 and 25.8 by pulse/echo. Pulse/echo displacements were more difficult to discern due to their broadness and anatomic non-specificity.
For the ex vivo case examined here, the photoacoustic signal was produced by absorption of 532 nm light by melanin. The photoacoustic signal is advantageous for measurement of displacement because of its highly broadband character, which offers the potential for detection of smaller displacements than would be possible by pulse/echo, and because the photoacoustic signal is only generated by specific molecules. In the case of the retina, this would be primarily the pigment epithelium. This technique offers an avenue towards investigation of the elastic properties of the retina in future in vivo studies.
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