In Vivo Measurement of the Attenuation Coefficient of the Sclera and Ciliary Muscle from Transscleral Optical Coherence Tomography images

Abstract

Purpose : Images of accommodation-induced changes in the ciliary muscle acquired using Optical Coherence Tomography (OCT) can provide insight into the mechanism of accommodation. However, the inner boundary of the ciliary body near the apex is often difficult to visualize due to signal loss. The purpose of this study was to quantify the attenuation coefficients of the sclera and ciliary muscle in vivo to better understand the mechanism of signal loss in OCT images of the ciliary muscle.

Methods : Following an IRB-approved protocol, the ciliary muscle was imaged transsclerally in the unaccommodated state in the left eye of eighteen healthy subjects (39.2 ± 10.5 years old) using a Spectral-Domain OCT system (1320nm, 28,000 A-lines/s, 7.5 µm axial resolution, and 2.5 mm axial range, Thorlabs Telesto, Newton, NJ) coupled with an accommodation module (Ruggeri et al, Biomed Opt Exp, 2016: 1351-1364). For each subject, we acquired one image consisting of 897 A-lines. The A-line corresponding to the inner apex was selected in the ciliary muscle image from each subject. The boundaries of the sclera and ciliary muscle in the selected A-line were determined from visual inspection of the OCT images (Figure, left). The attenuation coefficients of the sclera and ciliary muscle were calculated from the slopes of linear fits of the data points of the corresponding regions of the axial reflectivity profile (Figure, right). Calculation of the attenuation coefficient was repeated five times on the same image and the average value was calculated for each subject.

Results : The average attenuation coefficient across subjects was 3.08 ± 2.03 mm^-1 for the sclera and 3.30 ± 0.53 mm^-1 for the ciliary muscle

Conclusions : The study demonstrates the feasibility of measuring the attenuation coefficient of the sclera and ciliary muscle in vivo from OCT images. The results suggest that there are significant inter-individual variations in the scleral attenuation coefficient that may be due to differences in scleral collagen ultrastructure and vasculature.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.

 

Figure. Left: OCT image of the ciliary muscle and sclera. Blue line corresponds to the A-line passing through the inner apex. Right: Linear fits of the axial intensity profile in the sclera and ciliary muscle. The attenuation coefficients for this subject were determined to be 4.75 mm-1 and 3.52 mm-1 for the sclera and ciliary muscle, respectively.