Most traditional ophthalmic OCT imaging systems suffer from the large aberrations within the eye and do not allow sufficient lateral resolution to provide cellular level images of the human retina in vivo. Adaptive optics can compensate for image aberrations, while optimizing the beam spot size at specific depths.
Adaptive optics (AO) cameras equipped with time-domain and Fourier domain optical coherence tomography (FD-OCT) have achieved unprecedented 3D imaging resolution in the human retina in-vivo. Spectrometer based FD-OCT has been gaining more attention due to its substantially higher acquisition speed without loss in sensitivity. However, new OCT system architectures, in particular swept source OCT (SS-OCT), may offer additional benefits that are attractive for retinal imaging. The long coherence length (>5mm) of the swept source supports a large depth measurement range and the balanced detection scheme allows high collection efficiency of sample signals. Recently a group of researchers at the University of Indiana, School of Optometry, constructed an AO SS-OCT system to compare with their existing AO FD-OCT system, in an attempt to achieve enhanced imaging resolution and evaluate imaging efficacy in human retinal in vivo.
A schematic of the AO SS-OCT camera is shown in Figure 1 below. The AO sub-system consisted of a Shack-Hartmann wavefront sensor and an AOptix mirror that dynamically corrects the ocular aberrations over a 6.6 mm human pupil at up to 25 Hz, while the large stroke of the AOptix mirror provides rapid focusing in the retina. The SS-OCT subsystem consisted of a Thorlabs swept source laser, a Michelson interferometer with balanced detection, and custom data acquisition and processing software. The laser has a center wavelength of 850 nm with a 35 nm bandwidth (at 3dB), and delivered 300 µW into the eye with 20,000 A-scans/s. Axial resolution in air and sensitivity of the AO swept source OCT were 9.4 µm and 96 dB, respectively.
Figure 1: Schematic of Adaptive Optic Swept Source OCT System
Thorlabs’ OCT system software provided 32 Hz real-time image display and data file management on the hard drive. 3D volume and B-scan images up to 1° in diameter were acquired at retinal eccentricities of 2° and 7° on two subjects with the focus ranging from the nerve fiber layer to the photoreceptors. Axial resolution and sensitivity measurements were performed on a model eye.
Measurement results are shown in Figure 2. Individual cone photoreceptors, nerve bundles and small blood vessels were visible at both 2° and 7°. These results show that the combination of AO and swept source OCT provides high-resolution images of the retina similar to AO FD-OCT. The SS-OCT system also offers advantages including low sensitive decay over a 3 mm imaging depth, turn key operation of the swept source, and user-friendly software.
Figure 2 shows the en-face images extracted from the OCT volume data recorded with the AO SS-OCT system. The images are acquired at 2° and 7° eccentricity, thus providing the definition needed to resolve individual cone photoreceptors. The images are displayed using a linear intensity scale.
Image Courtesy: Prof. Donald T. Miller, School of Optometry, Indiana University, Bloomington, Indiana.
1. B. Cense, et al., “Retinal Imaging at 850nm with Swept Source Optical Coherence Tomography and Adaptive Optics,” ARVO 2007.
2. Y. Zhang, et al., Optics Express. 14, 4380 (2006)