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Kazuhiro Kurokawa, Shuichi Makita, Young-Joo Hong, Yoshiaki Yasuno; Micro-displacement Measurement for Laser-irradiated Tissue Using OCT. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1623.
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To visualize the thermal change of laser-irradiated tissue, we developed a method to measure micro-displacements during laser irradiation using optical coherence tomography (OCT). Our proposed method enables two-dimensional and spatially-resolved displacement measurement.
Five retinal regions of 2 porcine eyes, which are enucleated within 12 hours after death, are irradiated by a 532-nm coagulation laser. Laser power is 0.2 W, and its exposure time is 0.02 sec. The coagulation laser and OCT illumination beam are coaxially aligned, and a sequence of B-scans is taken at the same region on the sample using a spectral-domain OCT (SD-OCT). The SD-OCT uses 1-μm SLD light source with a center wavelength of 1.02 μm, a spectral bandwidth of 100 nm, and A-scan rate is 91,911 A-lines/s. The image size of a B-scan is 1.5 mm (1024 pix) × 1.8 mm (512 pix) in lateral and axial directions. The displacement maps between B-scans measured before and during laser-irradiation are calculated, where the B-scan before the laser-irradiation is used as a reference. For each image during the irradiation, two-dimensional displacements from the reference B-scan are obtained by using local image-correlation-coefficients and Doppler phase-shifts. Specifically, the magnitude of displacement is given by the correlation coefficients and the axial displacement is obtained from Doppler-phase shifts. In this analysis, the variation of the optical resolution, which is a critical factor for the correlation coefficient analysis, was characterized from the target images and used for the analysis.
The spatially-resolved micro-displacement was observed in 3 of 5 retinal regions. The other 2 regions had too large displacement to be correctly measured. The irradiated tissue radially and axially displaced and expanded from the irradiation spots. A representative case is shown in Figure. Green color represents radial displacement, and red color represents axial displacement. The largest radial displacements were observed at laser irradiated region of RPE. On the other hands, the largest axial displacements were observed at that of inner retina. This indicates the tissue-dependent expansion and deformation.
We developed a method to visualize spatially-resolved and two-dimensional micro-displacements of the laser irradiated tissue. This enables further understanding of thermal changes during laser treatment.
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