Optical Coherence Tomography: Newer Techniques, Newer Machines

Introduction Since 1991, when optical coherence tomography (OCT) was first introduced as a tool for investigation in ophthalmology, OCT has become the mainstay in retinal imaging and, in some situations, has supplanted fundus fluoresce in angiography which is regarded, even today, as the gold standard for investigations in choroidal and retinal disorders. By definition, OCT represents a non-invasive method for cross-sectional imaging of the internal retinal structures through the detection of optical reflections and echo time delays of light, using low coherence interferometry to subsequently produce in vivo two-dimensional images of internal tissue microstructure. Initially, the time domain (TD) technology was used (Stratus OCT, Carl Zeiss Meditec), which employed a mobile reference arm mirror that sequentially measured light echoes from time delays with an acquisition speed of 400 A scans/ second and axial resolution of 810 μm. TD OCT was limited by longer acquisition times (due to a mobile reference mirror), limited image sampling (which potentially overlooked smaller lesions), motion artifacts and lack of patient cooperation. Fourier domain or spectral domain (FD/SD) OCT succeeded the TD OCT. With a central wavelength of 800–850 nm, a stationary reference arm, a high-speed spectrometer, and a charge-coupled device (CCD) line scan camera these OCTs had an increased acquisition speed of 25,000–52,000 A scans/second, with axial resolution of 37 μm, significantly improved signal-to-noise ratio, reduction in motion artifacts, increased area of retinal coverage and better visualization of individual retinal layers. They were also able to create three-dimensional (3D) topographic maps with precise registration. Imaging resolution and penetration was further enhanced by averaging and enhanced depth imaging (EDI). However, axial resolution and acquisition speed in SD OCT were limited by the limitations of CCD line scan cameras and SD OCT was still subject to motion artifacts, segmentation artifacts and interinstrument comparability. Limited resolution due to infrared radiation absorption by ocular structures, limited axial and lateral resolution due to image scattering from ocular structures and restricted numerical aperture of the optical system respectively, in both TD and SD OCT, were the major disadvantages that prompted researchers to look for newer technologies. SD-OCT scanning laser ophthalmoscopy systems Currently, the machines that are using the confocal scanning laser ophthalmoscopy (cSLO) system are the Optical Coherence Tomography RS-3000 Advance from NIDEK, Cirrus HD-OCT 5000 with FastTrac, OCT/SLO Combination Imaging System from Optos, OPKO Spectral OCT/ SLO Combo Imaging System and the Spectralis OCT. We will be discussing the Spectralis OCT in details along with the added technologies for better imaging.