Comparison of Eye Length Measurement using Partial Coherence Interferometry and Clinical Ultrasound

Abstract

Abstract: : Purpose: Partial Coherence Interferometry (PCI) is an optical, non-contact, method of measuring ocular axial length using interference patterns from partially coherent light reflected from various layers of the fundus. The PCI device measures optical path lengths that need to be converted to geometric eye length by calibrating with an estimate of the refractive index of the eye. The purpose of this study was to generate an estimate, for our instrument, of the refractive index of the ocular media that allows easy comparison with geometric path lengths recorded by standard clinical ophthalmic ultrasound. Methods: Thirty-nine adult subjects, with best corrected visual acuity of 20/40 or better participated. Subjects fixated on an alignment beam in the PCI with their right eye. Four sets of sixteen measurements were taken with a 0.8 second measurement scan (E.L. Francis, ARVO Abstract #358, 2002). Ophthalmic ultrasound measurements using the AB5500 A/B Scan (Sonomed, Lake Success, NY) device followed PCI measurement. After topical anesthesia, subjects fixated straight ahead and four ultrasound probe measurements were taken using standard clinical technique. Results: The mean refractive error was –2.8D (range –8.75D to +0.5D, SE 0.408). Mean optical axial length (front of the cornea to RPE/Bruch’s interface) obtained using PCI was 33.146 mm (SE 0.230) and mean geometric axial length was 24.045 mm (SE 0.175) using ophthalmic ultrasound. Because clinical ultrasound provides a measure from corneal probe to ILM, we added 0.200 mm to the ultrasound geometric length to yield a converted geometric length. The ratio of PCI optical length to converted geometric length was 1.367; slope of the regression line between the two was similar. Conclusions: Eye length measurements obtained from our instrument are comparable to geometric eye length measured by gold standard clinical ultrasound when the refractive index used to convert optical length to axial length is 1.367.

Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, S • refraction • myopia