OCT imaging of the choroid (HIRESOMI)

Researcher: Livui Neagu
Supervisors: A. Podoleanu & G. Dobre (University of Kent); A.B. Lobo Ribeiro & Jose Salcedo (Multiwave Photonics).
Project Funding: HIRESOMI

Project Outline

In applying OCT to diagnose conditions of the retina it is essential to image and distinguish details of the choroid. However, this essential structure of the retina is below the retinal pigment epithelium, which has a high reflectivity allowing only a small amount of light to pass through. In order to penetrate better into the choroid, longer wavelengths should be used, as longer wavelengths lead to less scattering and better penetration. However, for wavelengths longer than 950 nm in the eye, attenuation due to water absorption builds up. There is, however, a valley in the absorption after the peak at 970 nm, at around 1040 nm where the attenuation is less.

In this project, we developed a new OCT system using two acousto optic frequency shifters and a broadband source centred on 1050 nm wavelength, which provides a highly efficient power configuration. This consists of an optical circulator and at least two couplers in a symmetric arrangement for the object and reference arm. The OCT microscope system was driven by a broadband light source from Multiwave Photonics. The OCT system was based on a Mach-Zennder interferometer configuration. We acquired optical coherence tomography en-face images as well as cross-section images using an InGaAs balance detector.

Schematic diagram of the en-face OCT system: LS- Laser source (Multiwave Photonics); 80/20 – coupler, 50/50- couplers; PC- polarisation controller; I- isolator; AMF- acousto-optic frequency shifter; TS- 3D translation stages, MO- X10 microscope objectives; FG- frame grabber.