![]() However, OCT has also been used for many other biomedical applications, including cancer detection– but only by way of providing spatial/structural information. This plot is known as an A-scan.OCT has been particularly successful in ophthalmology due to the optical transparency of the eye in the near-infrared region of the spectrum, thus allowing visualization of retinal layers for diagnosing diseases and assisting in surgeries (example image below). n, between the scatterers and the reference mirror (labeled depth on the figure).This process enables use of a fast Fourier transform (FFT which requires linear sampling along the x-axis) to reveal the optical path length difference, ΔOPL = (z_s – z_r) ![]() The interferometric signal (i.e., interferogram) is recorded as a function of wavelength and resampled to a linear wavenumber vector, k = 2π / λ. In this particular example light is scattered off of three different locations in the sample (m = 1, 2, and 3) and is then recombined with the reference field at the beamsplitter (BS) and detected using a spectrometer. The figure above illustrates a typical FD-OCT system using a Michelson interferometer geometry, where a light source with a wide bandwidth, S(λ), is split into a sample field, E_S, and reference field, E_R then E_S is incident on a sample with refractive index (RI) n, and E_R is reflected off a reference mirror. Schematic of OCT system and signal acquisition Then, multiple A-scans are acquired to generate an OCT image (this process is again more closely related to that of ultrasound). ![]() Thus, a Fourier transform of the interferometric signal yields an A-scan, typically with an axial resolution of a few micrometers and imaging depths of 1-2 mm. As the name suggests, FD-OCT samples the frequency space of samples, and is hence somewhat analogous to the signal acquired by magnetic resonance imaging (MRI) where the bandwidth of the source dictates the spatial resolution and the sampling frequency dictates the imaging field of view. in 1991, OCT has gone through a number of configurations, of which Frequency domain (FD-) OCT has become the most widely implemented due to the improved acquisition speed and sensitivity. (c) 2004 Society of Photo-Optical Instrumentation Engineers.Since its introduction by Huang et al. ![]() Knowledge of the retardation and the slow axis distribution of the cornea might improve nerve fiber polarimetry for glaucoma diagnostics and could be useful for diagnosing different types of pathologies of the cornea. The retardation increases in a radial direction and with depth the slow axis varies in the transversal direction. We present maps of retardation and the distribution of slow axis orientation of the human cornea in longitudinal cross-sections and en face images obtained at the back surface of the cornea. While the retardation information is encoded in the amplitude ratio of the two interferometric signals, the axis orientation is encoded entirely in their phase difference. Using an algorithm based on a Hilbert transform, it is possible to calculate the retardation and the slow axis orientation of the sample with only a single A-scan per transversal measurement location. We used a two-channel PS-OCT system employing a phase-sensitive recording of the interferometric signals in two orthogonal polarization channels. PS-OCT was used to measure and image retardation and birefringent axis orientation of in vitro human cornea. Polarization-sensitive optical coherence tomography (PS-OCT) is a functional extension of OCT that can image birefringent properties of a biological sample. Optical coherence tomography (OCT) is an emerging technology for high-resolution, noncontact imaging of transparent and scattering media.
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