Erich Götzinger

Speckle reduction in optical coherence tomography by frequency compounding

We are investigating the possibility of a frequency compounding method for speckle reduction in optical coherence tomography. The method is based on incoherent summation of the magnitudes of two independent interferometric signals, which were recorded at two different center wavelengths simultaneously. We derive the corresponding statistics and compare the theoretical results with measurements obtained in a uniformly scattering sample. Finally we demonstrate our method by comparing images of human skin recorded in vivo with and without frequency compounding. The compounding method results in an increased contrast and improved image quality without loss of resolution.

High speed spectral domain polarization sensitive optical coherence tomography of the human retina

We developed a high-speed polarization sensitive optical coherence tomography (PS-OCT) system for retinal imaging based on spectral domain OCT. The system uses two spectrometers, one for each polarization channel, that operate in parallel at 20000 A-lines/s each. It provides reflectivity, retardation, and cumulative optic axis orientation simultaneously. We present our instrument and discuss the requirements for the alignment of the two spectrometers specific for our setup. We show 2D spectral domain PS-OCT images and - to the best of our knowledge - the first 3D spectral domain PS-OCT data sets in form of fly-through movies and volume rendered data sets recorded in human retina in vivo.

Retinal pigment epithelium segmentation by polarization sensitive optical coherence tomography

We present a new method for identifying and segmenting the retinal pigment epithelium (RPE) in polarization sensitive optical coherence tomography (PS-OCT) images of the human retina. Contrary to previous, intensity based segmentation algorithms, our method uses an intrinsic tissue property of the RPE: its depolarizing, or polarization scrambling effect on backscattered light. Two different segmentation algorithms are presented and discussed: a simpler algorithm based on retardation data, and a more sophisticated algorithm based on local variations of the polarization state calculated from averaged Stokes vector elements. By using a state of the art spectral domain PS-OCT instrument, we demonstrate the method in healthy and diseased eyes.

Imaging of polarization properties of human retina in vivo with phase resolved transversal PS-OCT

Recently, we developed a phase resolved polarization sensitive OCT system based on transversal scanning. This system was now improved and adapted for retinal imaging in vivo. We accelerated the image acquisition speed by a factor of 10 and adapted the system for light sources emitting at 820nm. The improved instrument records 1000 transversal lines per second. Two different scanning modes enable either the acquisition of high resolution B-scan images containing 1600x500 pixels in 500ms or the recording of 3D data sets by C-scan mode imaging. This allows acquiring a 3D-data set containing 1000x100x100 pixels in 10 seconds. We present polarization sensitive B-scan images and to the best of our knowledge, the first 3D-data sets of retardation and fast axis orientation of fovea and optic nerve head region in vivo. The polarizing and birefringence properties of different retinal layers: retinal pigment epithelium, Henles fiber layer, and retinal nerve fiber layer are studied.

Measurement and imaging of birefringent properties of the human cornea with phase-resolved, polarization-sensitive optical coherence tomography.

Optical coherence tomography (OCT) is an emerging technology for high-resolution, noncontact imaging of transparent and scattering media. Polarization-sensitive optical coherence tomography (PS-OCT) is a functional extension of OCT that can image birefringent properties of a biological sample. PS-OCT was used to measure and image retardation and birefringent axis orientation of in vitro human cornea. We used a two-channel PS-OCT system employing a phase-sensitive recording of the interferometric signals in two orthogonal polarization channels. 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. 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. 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. The retardation increases in a radial direction and with depth; the slow axis varies in the transversal direction. 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.

Full range complex spectral domain optical coherence tomography without additional phase shifters.

We demonstrate a new full range complex spectral domain optical coherence tomography (FRC SD-OCT) method. Other than FRC SD-OCT systems reported in literature, which employed devices such as electro-/acousto optic modulators or piezo-driven mirrors providing the phase modulations necessary for retrieval of the complex-valued signal, the system presented works without any additional phase shifting device. The required phase shift is introduced by the galvanometer scanner used for transversally scanning the sample beam. By means of a slight displacement of the probe beam with respect to the scanning mirrors pivot axis, the sample arm length and thus the phase is continuously modulated as the beam is scanned in lateral direction. From such modulated spectral data, the complex-valued data yielding a twofold increase of accessible depth range can be calculated using an algorithm based on the Hilbert transform. To demonstrate the performance of our method quantitative measurements of the suppression of mirror images as a function of induced phase shift were performed. In order to validate the FRC SD-OCT technique for high-speed imaging of biological tissue, we present full-range images of the human anterior chamber in vivo.

High speed full range complex spectral domain optical coherence tomography

We present a high speed full range spectral domain optical coherence tomography system. By inserting a phase modulator into the reference arm and recording of every other spectrum with a 90 degrees phase shift (introduced by the phase modulator) we are able to distinguish between negative and positive optical path differences with respect to the reference mirror. A modified two-frame algorithm eliminates the problem of suppressing symmetric structure terms in the final image. To demonstrate the performance of our method we present images of the anterior chamber of the human eye in vivo recorded with an A-scan rate of 10000 depth profiles per second.

Measurement and imaging of water concentration in human cornea with differential absorption optical coherence tomography

We present measurements of water absorption in human cornea in vitro with a differential absorption optical coherence tomography (DAOCT) technique. This technique uses two OCT images recorded simultaneously with two different light sources, one centered within (1488nm) and one centered outside (1312nm) of a water absorption band. We investigated the cornea under different conditions: First, a series of OCT images was recorded at different hydration states of the cornea, starting from a normally hydrated cornea to an almost completely dehydrated cornea. To investigate the influence of scattering on our measurements, the dehydrated cornea was re-hydrated with Deuterium oxide, which shows similar optical properties like water, but negligible absorption in the used wavelength region, and a similar series of OCT images was recorded. For a quantitative analysis, we averaged the OCT signals over adjacent A-Scans and performed a linear regression analysis of the logarithmic OCT signals versus imaging depth in the cornea for each wavelength. The difference of the slopes corresponds to the difference in the absorption coefficient, if the difference in the scattering coefficient is negligible. With the known difference in the absorption cross section it is possible to calculate the mean water concentration of the cornea.

Visualization of microvasculature by dual-beam phase-resolved Doppler optical coherence tomography

We present a dual-beam Doppler optical coherence tomography system for visualizing the microvasculature within the retina. The sample arm beams from two identical spectral domain optical coherence tomography (SD-OCT) systems are combined such that there is a small horizontal offset between them at the retina. Thereby we record two tomograms which are slightly separated in time. Phase-resolved Doppler analysis is performed between these two data sets. This system allows blood capillary imaging with high flow sensitivity and variable velocity range. To demonstrate the performance of our system we present images of the microvascular network around the fovea and around the optic nerve head of the human eye.

Bidirectional Doppler Fourier-domain optical coherence tomography for measurement of absolute flow velocities in human retinal vessels

We describe a bidirectional color Doppler Fourier-domain optical coherence tomography system capable of measuring absolute velocities of moving scatterers by illuminating the sample with two linearly and orthogonally polarized beams, incident at a known angle on the sample. The velocity calculation is independent of the exact orientation of the velocity vector in the detection plane. First measurements were performed on a rotating disk driven at well-defined velocities and tilted by various small angles. Our results indicate a high correlation between preset and calculated velocities (correlation coefficient 0.999) and the independency of these velocities from the tilting angle of the disk. We demonstrate that bidirectional color Doppler optical coherence tomography allows for the measurement of absolute blood flow values in vivo in human retinal vessels.