In-vivo Depth-resolved Estimation of Tissue Properties: OCT-derived Attenuation Coefficient Images

Abstract

 

Purpose

 

An important optical tissue property is its attenuation coefficient, which is correlated with various diseases including glaucoma. Here, we present a novel method to determine attenuation coefficients for each pixel in an optical coherence tomography (OCT) volume. We also evaluated the attenuation coefficients for the retinal nerve fiber layer (RNFL) in normal and glaucomatous eyes.

 

Methods

 

A forward scattering model of light propagation in a turbid medium and the detection of the scattered light was developed. It assumes scattering of a constant part of the attenuated light towards the OCT detector and attenuation of all light within the imaging depth range. Inverting the model enables iterative depth-resolved estimation of attenuation coefficients from the observed OCT signal. Volumetric OCT scans from 10 normal and 8 (moderate) glaucomatous eyes were obtained by a Spectralis OCT system (Heidelberg Engineering). Attenuation coefficients were calculated for every pixel in the volume. Average RNFL attenuation coefficients resulting from this new method were compared between normal and glaucomatous groups. It was also compared with a previous method that used the retinal pigment epithelium (RPE) as a reference layer (IOVS 2012;53:6102-8).

 

Results

 

Figure 1 shows an example OCT B-scan and its corresponding attenuation coefficient image, expressed in a physical unit of measurement. Shadowing artifacts (blue arrows) due to blood vessels were largely removed. RNFL attenuation coefficients were highly statistically significantly different (P<0.0001) between normal and glaucomatous eyes (Fig. 2, top). They were highly consistent with the RPE-reference method (Fig. 2, bottom), with correlation coefficients of 0.99 (all eyes), 0.99 (normal eyes) and 0.94 (glaucomatous eyes).

 

Conclusions

 

The attenuation coefficient was estimated for every pixel in an OCT volume. As a tissue property, the resulting attenuation coefficients do not exhibit artifacts that are present in conventional OCT data. RNFL attenuation coefficients are significantly different for normal and glaucomatous eyes. Attenuation coefficient images provide new ways of processing and analyzing OCT data.

 

 

Fig.1 Conventional OCT images (top) are converted to attenuation coefficients (bottom).

 

 

Fig.2 (top) Average RNFL attenuation coefficient for healthy and glaucomatous eyes. (bottom) Comparison of reference free method to RPE-reference method.

 

Keywords: 552 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • 688 retina • 551 imaging/image analysis: non-clinical