A newly developed peripheral anterior chamber depth analysis system: principle, accuracy, and reproducibility

Abstract

Aim: To develop a new, non-contact system for measuring anterior chamber depth (ACD) quantitatively, and to investigate its accuracy as well as interobserver and intraobserver reproducibility.

Methods: The system scanned the ACD from the optical axis to the limbus in approximately 0.5 second and took 21 consecutive slit lamp images at 0.4 mm intervals. A computer installed program automatically evaluated the ACD, central corneal thickness (CT), and corneal radius of curvature (CRC) instantly. A dummy eye was used for investigating measurement accuracy. The effects of CT and CRC on the measurement results were examined using a computer simulation model to minimise measurement errors. Three examiners measured the ACD in 10 normal eyes, and interobserver and intraobserver reproducibility was analysed.

Results: The ACD values measured by this system were very similar to theoretical values. Increase of CRC and decrease in CT decreased ACD and vice versa. Data calibration using evaluated CT and CRC successfully reduced measurement errors. Intraobserver and interobserver variations were small. Their coefficient variation values were 7.4% (SD 2.3%) and 6.7% (0.7%), and these values tended to increase along the distance from the optical axis.

Conclusion: The current system can measure ACD with high accuracy as well as high intraobserver and interobserver reproducibility. It has potential use in measuring ACD quantitatively and screening subjects with narrow angle.

Figure 1

Configuration of the SPAC system and the photograph of the SPAC equipment. The system is composed of a slit lamp microscope, a CCD camera, a computer, and a printer. The patient is instructed to gaze into a fixation lamp. The light from the slit lamp is illuminated towards the subject’s eye at an angle of 60 degrees from the temporal side, and moved along a track in the temporal direction. Twenty one consecutive slit lamp images are captured with the CCD camera, the image data are instantaneously analysed by the computer, and the results are printed out by the printer. CCD  =  charged coupled device, PC  =  personal computer. The photograph of the SPAC equipment is shown at the bottom right.

Figure 2

Determination of margins of cornea and iris. Representative SPAC images of a female with a narrow angle. (A) Twenty one consecutive images (right panel), eye monitoring window (lower left panel), and values (upper left panel) are displayed. (B) Sample image for determining PACD and corneal thickness. A slit lamp image (first row) and a histogram of the number of white pixels calculated from the slit lamp image (second row). Two major peaks indicate cornea (left) and iris (right). Determined margins of the cornea and the iris (third row). Margins are superimposed on the slit lamp image (fourth row). (C) An installed program creates a plot of PACD profile. (D) Drawing of the anterior ocular segment. Bold lines indicate boundaries of corneal epithelium, corneal endothelium, and iris surface.

Figure 3

Determination of ACD. A photographed image (A) is converted to a histogram (B) and an installed program automatically determined margins of corneal epithelium, corneal endothelium, and iris surface (C). A superimposed image shows that the determined margins agree well with the margins of the cornea and the iris shown on the original images (D).

Figure 4

Estimation of corneal radius of curvature. Slit lamp ray a: image taken while focusing on the corneal peak on the optical axis; slit lamp ray b: image taken while moving at a distance D towards the posterior along the optical axis; θ: an assumptive angle between the optical axis and the illuminating slit lamp light ray without refraction by the cornea; h: scanning distance between image a and image b; R: corneal radius of curvature.

Figure 5

Structure of dummy eye and ACD measurement: Lx is SPAC measured ACD at a position L.

Figure 6

Interobserver coefficient variance in ACD measurement.