Anterior chamber width measurement by high-speed optical coherence tomography

Abstract

Objective: To measure anterior chamber (AC) width and other dimensions relevant to the sizing of phakic intraocular lenses (IOLs) with a high-speed optical coherence tomography (OCT) system.

Design: Cross-sectional observational study.

Participants: Both eyes of 20 normal volunteers.

Methods: A novel high-speed (4000 axial scans/second) OCT prototype was developed for anterior segment scanning. The system uses long wavelength (1310 nm) for deeper angle penetration, rectangular scanning for undistorted imaging, and short image acquisition time (0.125 seconds) to reduce motion error. Three horizontal cross-sectional OCT images (15.5 mm wide and 6 mm deep) of the anterior segment were obtained from each eye with real-time image display to guide centration on the corneal apex. Image processing software was developed to correct for image warping resulting from index transitions. Anterior chamber dimensions were measured using computer calipers by 3 expert raters (ophthalmologists). Analysis of variance was used to determine interrater, interimage, right versus left eye, and intersubject standard deviation (SD) of OCT measurements.

Main outcome measures: Anterior chamber width (recess to recess), AC depth, and crystalline lens vault as measured by OCT; external white-to-white (WTW) corneal diameter (CD) as measured by Holladay-Godwin gauge.

Results: The mean AC width was 12.53+/-0.47 mm (intereye SD), and the mean corneal diameter was 11.78+/-0.57 mm. Optical coherence tomography measurement of AC width has good repeatability from image to image (SD, 0.134 mm), but there was significant difference between raters (SD, 0.215 mm). Estimation of AC width from WTW CD by linear regression was relatively inaccurate (residual SD, 0.41 mm). The mean AC depth was 2.99+/-0.323 mm (intereye SD), with repeatability of less than 0.001 mm (interimage SD), and the mean crystalline lens vault was 0.39+/-0.27 mm with 0.023 mm repeatability.

Conclusions: Reproducible OCT AC biometry was demonstrated using a high-speed OCT prototype. Further improvement in reproducibility may be achieved by automating the measurements with a computer. Direct OCT AC width measurement may improve sizing of angle-supported AC IOLs over conventional estimation by WTW CD. The measurement of AC depth and lens vault also may be useful for other types of phakic AC IOLs.

Figure 1

One of the authors (MRC) demonstrating the use of the corneal and anterior segment optical coherence tomography (OCT) prototype. The OCT hardware is contained within the black box on top of the slit lamp. The OCT image is displayed on the monitor on the right, and the charge-coupled device video is on the monitor on the left.

Figure 2

Horizontal optical coherence tomography section of the anterior segment after computational correction for index transitions. The anterior chamber (AC) width is measured angle recesses. Anterior chamber depth is the anteroposterior (AP) distance from the corneal endothelium to the lens apex. Lens vault (not drawn) is the AP distance between the lens apex and the recess-to-recess line.

Figure 3

Optical coherence tomography image of the iridocorneal angle demonstrating the scleral spur and angle recess.

Figure 4

Plot of the optical coherence tomography (OCT) anterior chamber width versus white-to-white corneal diameter by eye. Each point is the average of measurements obtained from each eye. The linear regression line and equation are shown.