Measurement of the Retina and Choroid Diffusion Coefficients During TTT

Abstract

Purpose:: Diffusing-wave-spectroscopy (DWS) is a technique able to monitor microscopic movements in turbid media. In this study, we report the first observation of a DWS signal recorded in-vivo on the ocular fundus of a rabbit eye during a laser thermal treatment.

Methods:: A modified slit-lamp with a 30/70 beam splitter and a custom made fibers adapters including the collimation lenses, equipped with a standard therapy laser diode, was used. The motion of the scattered sites in the fundus was varied using transpupillary thermotherapy (TTT) for 60 seconds with a power settings of 200 mW. The scattered light was then guided by the single mode fiber to a single photon counting module (SPCM-AQR-14-FC, Perkin Elmer). The electrical signal from the SPCM was processed by a digital correlator (FLEX99S160B, Correlator.com) to acquire the normalized intensity autocorrelation function. Thus the APD photodiode was substituted by a HeNe laser in order to define the collection area. The distance between illumination and collection spots was estimated to be about 2 mm. Immediately before TTT, the APD photodiode was reconnected to the receiving fiber and three autocorrelation functions were acquired each 5 seconds. Next, we started the laser thermal treatment. During the 60-second NIR laser exposure, an autocorrelation functions was acquired each 5 seconds for a total of 12 readings. After the TTT three autocorrelation functions were acquired each 5 seconds.

Results:: Autocorrelation functions exhibit a similar trend and a common plateau in the delay time range (200ms, 1000ms). The mean-squared displacements of the scattering sites in the short and long delay range were calculated for all the acquired autocorrelation functions. We fit a linear function to in the long delay range and a quadratic function in the short delay range, then we evaluated the diffusion coefficient of the scatterers in the retina and choroid layer. The values estimated at the different stages of the experiment show as in the retina the temperature induced a motion of the scatterers that increases monotonically during the treatment. The choroidal diffusion coefficient induced by the temperature elevation shows a very well defined sharp step-down.

Conclusions:: A correlation was found between DWS signal and motion of the scatterers on the ocular fundus, therefore this technique may be used to evaluate useful parameters during laser treatments.

Keywords: age-related macular degeneration • choroid • choroid: neovascularization