Optimal number of angle images for calculating anterior angle volume and iris volume measurements

Abstract

Purpose: We determined the optimal number of angle images required to obtain reliable measurements of trabecular-iris circumferential volume (TICV) and iris volume (IV) using swept-source Fourier domain anterior segment optical coherence tomography (SSFD-ASOCT) scans in narrow angle eyes.

Methods: Scleral spur landmarks (SSL) were manually identified on ASOCT angle images from 128 meridians from each of 24 eyes with chronic primary angle closure (PAC) spectrum of disease. The anterior and posterior corneal curves, and the anterior and posterior iris surfaces were identified automatically by the anterior chamber analysis and interpretation (ACAI) software, then manually examined and edited by the reader if required. Trabecular-iris circumferential volume at 750 μm from SSL (TICV750) and IV were subsequently calculated using varying numbers of angle images. Threshold error was determined to be less than the lower 95% confidence limit of mean absolute percent error (MAPE) of the change in TICV or IV resulting from laser peripheral iridotomy, which would be 17% for TICV and 5% for IV, based on previous studies. The optimal number of angle images was the smallest number of images where MAPE was less than this threshold for TICV and IV.

Results: A total of 32 equally-spaced angle images (16 meridians) was required to estimate TICV750 and 16 angle images (8 meridians) to estimate IV. Both were within 4.6% and 1.6% of MAPE, respectively.

Conclusions: It is possible to determine TICV and IV parameters reliably in narrow angles without evaluating all 128 meridians obtained with SSFD-ASOCT.

Figure 1

Trabecular-iris circumferential volume. A 3D ASOCT angle image exhibiting TISA750 (light green space) and TICV750 (darker green spaces), along with the SSL (red circle), iris (yellow), and cornea (violet line). Adapted from Rigi M, Blieden LS, Nguyen D, et al. Trabecular-iris circumference volume in open angle eyes using swept-source Fourier domain anterior segment optical coherence tomography. J Ophthalmol. 2014;2014:590978.

Figure 2

Iris volume. (A) The 2D and (B) 3D ASOCT angle images exhibiting borders for IV calculation. Iris cross-sectional area is the area bordered by anterior iris surface, anterior surface of the demarcation line created by the reflection of the posterior layer of pigment epithelium, and the line drawn from SSL and parallel to visual axis. R_ICSA is represented by the green line. (A). Iris volume is the integrated iris cross-sectional area measured around the angle (B). The SSL appears above the iris plane in the 3D view of this open-angle eye.