Abstract
Purpose :
Measuring and tracking eye length during regular eye exams is highly desirable, especially due to the increase in myopic eyes around the world. Here we report on using the coherence revival (CR) property of swept-source external cavity tunable laser (SS-ECTL) and extended depth imaging to measure axial eye length.
Methods :
A prototype PLEX® Elite 9000 (ZEISS, Dublin, CA) is used to image a human subject with 6 mm axial depth and 16 mm lateral field-of-view. The system uses a dual mode (100/200 kHz) SS-ECTL (Axsun Technologies, Billerica, MA). Due to the CR property of the source, we obtain ghost images of any object at a distance which is an integer multiple of the cavity round-trip length (Lc). Fig. 1 (a) shows the conceptual idea of CR based eye length measurement where the CR image of cornea will show up at different locations based upon relationship Le = (Lc-(corneal offset + retinal offset))/n where n is the refractive index of eye and Le is the geometrical eye length. We aligned the subject to ensure that the CR of corneal reflex is within the imaging window of the system. For comparison, axial length was also measured using a commercial system IOL Master 700 (Zeiss, Jena, Germany). To increase the range of eye lengths that can be imaged for axial eye length measurement, we modified the data acquisition system to capture a 32 mm (including mirror image) axial depth image. We used a custom built human retina mimicking test eye to combine extended depth measurement capability with the CR based axial length measurement.
Results :
Fig. 1(b) shows 6 mm axial depth OCT scan of a human subject showing corneal reflex in a retinal scan. Knowing the corneal offset, retinal offset, and cavity length, the eye length is calculated to be 24.35 mm compared to 23.94 measured on the IOL Master 700. The small offset could be due to an approximation used in our method regarding refractive index. Fig. 2 shows a 32 mm axial depth image of a test eye where the axial length is measured to be 26.1 mm compared to 25.8 mm from data sheet.
Conclusions :
We have shown the feasibility of measuring axial eye length using CR feature of SS-ECTL. We have expanded the idea with extended depth imaging to include all possible eye lengths. The proposed method will enable axial eye length measurement during a normal OCT scan.
This abstract was presented at the 2019 ARVO Imaging in the Eye Conference, held in Vancouver, Canada, April 26-27, 2019.