Abstract
Purpose: :
To determine the accuracy and repeatability of optical thickness measures of an ultra-high resolution optical coherence tomographer (UHR-OCT) using an appropriate refractive index phantom as a gold standard.
Methods: :
The custom, high-speed UHR-OCT operating in the 1µm spectral range was used to obtain two-dimensional images of a series of phantom lenses. The UHR-OCT system provides resolution of 3.2µm (axial) and 10µm (lateral) in corneal tissue and image acquisition rate of 47,000 A-scan/s. The corneal phantoms were a series of 22 custom-made rigid contact lenses (Plano power; BCOR 8.60mm) with refractive index 1.376 (±0.0005 @589m; Optical Polymer Research, Inc., Gainesville, FL, USA). The central thickness of the lenses ranged between 104-764µm, measured using a precision mechanical gauge (Vigor GA-715; Japan). For each rigid lens, 10 2-D images (1000 A-scans x 512 pixels) were obtained. An automatic segmentation algorithm was used to determine the optical thickness of the glass phantoms from the UHR-OCT tomograms. All values of optical thickness were obtained as an average of 200 A-scans in the central section of the 2-D image. Accuracy was assessed using Deming regression, correlation coefficients and Difference vs Means plots. Method comparison and repeatability was assessed using intraclass correlation coefficients (ICC).
Results: :
The optical and physical thickness measurements were almost perfectly correlated (Bootstrapped ρ=0.998). Accuracy The mean difference between the physical and optical thickness was -0.467±3.50µm. The UHR-OCT optical thickness tended towards very slightly smaller values for physical thickness measures less than 400µm and very slightly larger values for thickness' >400µm. Deming regression optical-physical thickness slope was 0.99±0.001. Repeatability ICCs were close to 1.0 for agreement between multiple measures of the same phantom. Method comparison exhibited a mean difference between the highest and lowest repeated measure of -0.23±0.09µm.
Conclusions: :
Our protype UHR-OCT is an accurate and repeatable device under optimal circumstances. On the basis of these estimates, it is anticipated that human corneal morphometry will be accurate.
Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • imaging/image analysis: clinical • cornea: clinical science