Low Optical Coherence Tomography Research Articles

Discrepancy in Loss of Macular Perfusion Density and Ganglion Cell Layer Thickness in Early Glaucoma

To identify characteristics of patients with early open-angle glaucoma exhibiting greater macular perfusion density (PD) loss compared with macular ganglion cell layer (GCL) thickness loss. Cross-sectional study. Optical coherence tomography (OCT) imaging of the optic nerve head and macula was conducted in patients and healthy control subjects. Minimum rim width, retinal nerve fiber layer and GCL thickness, and PD from OCT angiography were derived. Only high-quality images were included. For direct comparison, raw PD and GCL thickness values in patients were converted to relative age-corrected loss values based on data from controls. Demographic and ocular variables related to greater PD loss compared with GCL thickness loss were identified with multivariate logistic regression. Data from 89 patients (median mean deviation with the 24-2 and 10-2 tests, Humphrey Field Analyzer:-1.96 dB and-1.49 dB, respectively) and 54 controls were analyzed. Sixty-three (71%) patients had relatively more GCL thickness loss, whereas 26 (29%) had relatively more PD loss. More PD loss was associated with lower OCT and OCT-angiography signal strength (odds ratio [95% confidence interval], 0.64 [0.40, 0.96] and 0.60 [0.38, 0.86], per dB, respectively), thicker retinal nerve fiber layer thickness (1.08 [1.01, 1.16] per μm), and female sex (6.57 [1.25, 48.79]). Less than one-third of patients with early glaucoma had more loss of perfusion compared with conventional structural loss in the macula. Even within a range of high-quality images, lower signal strength may be at least partially responsible for apparent perfusion loss.

An FPGA based all-in-one function generator, lock-in amplifier and auto-relockable PID system

We report an all-in-one waveform generator, lock-in amplifier and Proportional-Integral-Differential (PID) controller, embedded in a single Field Programmable Gate Array (FPGA). The PID controller is advanced with a novel automatic relocking mechanism, which is capable of self-finding and relocking at the desired setpoint upon unlocking of the PID due to disturbances. The instrument is designed in such a way that these devices can either be used individually or simultaneously. Digital implementation via software processing of the associated modules is used and the firmware is embedded in an FPGA IC, which makes it compact and reconfigurable. The instrument consists of a hardware for establishing external linkage and a Python based computer-controlled interface to control it from a remote PC as well as for acquiring and plotting relevant data. A steep roll-off of the filters (6 dB/octave to 24 dB/octave), low noise density (30 nV/√Hz @ 100 kHz) in the lock-in amplifier and a PID bandwidth of 100 kHz makes it useful for wide range of applications. Versatility of the instrument is demonstrated in three different experiments, (i) Auger electron spectroscopy, (ii) low coherence optical tomography and (iii) laser frequency stabilization followed by self identification of the setpoint upon unlocking and automatic relocking to that.

Cross-Sectional Imaging of Materials Structure Using PS-OCT

Optical low-coherence tomography is a measurement technique for non-contact and non-destructive investigation of materials inner structure. Nowadays, this method is highly applied in medical treatment especially in dermatology and ophthalmology. During our research we have developed an optical low-coherence tomography system with polarization state analysis for structure examination of a broad range of technical materials. In this paper we present our recent measurements including polarization sensitive analysis. Those measurements were taken for semitransparent and highly scattering materials like polymer layers and polymer optical retarders, anticorrosion protective coatings or abrasive paper. We proved that with the aid of developed polarization sensitive optical coherence tomography system it is possible to investigate birefringence principles of materials as well as to improve visualization contrast of cross-sectional images.

Estimate of the minimal coherence length of probe optical radiation in interferometry

The minimal coherence length of probe optical radiation sufficient for formation of a homogeneous interference structure is estimated. The estimate is based on the analysis of the interference structure in the intensity distribution of the field scattered by rough surfaces and point objects and also formed in interferometers. Analysis was performed for the field intensity detected for the time T > 10τc (under the condition that the coherence time of the probe radiation is τc > 3/ω0, where ω0 is the central frequency of the emission spectrum). It is shown that the minimal coherence length Lc of the probe radiation, at which the homogeneous stratified interference structure of the scattered field can be still formed, is 8λ (λ is the central wavelength). The possibility of using this result for determining the maximal information content of the method of low-coherence optical tomography is analysed.

Effect of photons of different scattering orders on the formation of a signal in optical low-coherence tomography of highly scattering media

The influence of photons of different scattering orders on the formation of a detected signal in optical low-coherence tomography (OCT) is considered. The scattering orders are estimated by analysing the spatial distribution of the probability density for the effective optical paths of detected photons calculated by the Monte Carlo method. The influence of photons with different scattering orders on the formation of a signal is estimated quantitatively depending on the optical properties of the medium under study. The results of numerical simulations are interpreted within the framework of possible applications of OCT for non-invasive diagnostics of the human skin and other highly scattering random media. It is shown by the example of calculation of OCT signals from model biological tissues that the OCT method gives reliable information on their internal structure from optical depths up to 0.3 mm.

ANALYSIS OF 3D TRANSFER FUNCTIONS OF OPTICAL COHERENCE TOMOGRAPHY

With the consideratioin of both defocused and abaxial cases,the analytical expressions for the 3D transfer function of optical low coherence tomography are derived in this paper.The theoretical stud-ies are concerned with the resolution-related factors,which furnish evidences for the designation and optimization of the systems.According to our study,the axis resolution would increase linearly with the spectral bandwidth,while the signal level would decrease linearly.In the case of defocused scan-ning,although longer focus length and narrower beam aperture could result in an absolute decrement of the resolution,they give an even better defocused imaging property in the whole scanning region.The detected waveform and the actual image of the optical coherence tomography are also given.

Imaging of laser thermokeratoplasty lesions by optical low coherence tomography and polarization microscopy after Sirius Red staining

Information on the extent and degree of the thermal effect produced is of great importance for control of the laser dosage in laser thermokeratoplasty (LTK) and for postoperative follow-up. We investigated on acute LTK effects which information images obtained by optical low coherence tomography (OCT) offer compared to those obtained by polarization microscopy. Porcine eyes were irradiated through a 400 microns quartz fiber using light from a laser diode emitting up to 300 mW at a wavelength of 1.86 microns. Thermal lesions of varying strength were scanned using an experimental OCT device with about 25 microns lateral and 20 microns axial resolution. Histologic evaluation of the scanned areas was done by polarization microscopy after Sirlus-Red staining, and similar lesions were also analyzed by TEM. Both methods differentiated three damage zones a transition zone, a zone of moderate coagulation, and a central zone of strong coagulation. In the transition zone, increased birefringence was seen in polarization microscopy, which correlated with increased light scattering seen in the OCT images. In the moderately coagulated zone, a decrease in birefringence was associated with an even stronger increase of the OCT signal. In the central zone, a loss of the fibrillar tissue structure was observed, which led to a complete loss of birefringence and a strong reduction of the OCT signal. Although OCT does not provide the detailed information on thermal changes of tissue seen by the histologic method, it offers information on the extent and degree of tissue changes without preparation artifacts and provides a non-invasive method of immediate and follow-up control of LTK lesions. A quantitative analysis of changes in corneal thickness and curvature is much simpler than by a slit lamp. Time-resolved measurements of corneal light scattering may be used for on-line control of the laser-light dosage during LTK.

Optical low-coherence reflectometry to enhance monte Carlo modeling of skin

Optical low-coherence reflectometry and confocal microscopy images were taken of the rat dorsal skin flap window model. Blood vessel depths and diameters measured with the two techniques, and preparation thickness determined from reflectometry images, are in reasonable agreement with measurements from histologic sections. Blood vessels appear as areas of low signal when constant-depth reflectometry images are taken at a depth near the center of a vessel, whereas they appear bright when taken close to the blood–dermis boundary. Doppler shift plus increased light absorption in blood, and the blood–dermis index of refraction mismatch, are discussed as possible causes of the dark- and bright-appearing vessels, respectively. One reflectometry image was used to generate an input grid for a novel Monte Carlo analysis program that is capable of determining the light distribution and heat generation @J/m3# within complex blood vessel geometries. The feasibility of imaging skin blood vessel accurately with optical low-coherence tomography, and using the acquired knowledge of blood vessels structure to create more realistic Monte Carlo analyses is demonstrated by the results of the study.

Low-coherence optical tomography in turbid tissue: theoretical analysis

On the basis of white-light interferometry and statistical optics, a theoretical model for low-coherence optical tomography is presented that establishes the relation of interference modulation with path-length-resolved reflectance and that can provide analytical expressions and numerical solutions by means of a Fourier transform. The Monte Carlo technique is used to simulate the path-length-resolved reflectance from different multilayer tissue phantoms. Theoretical analyses and preliminary experimental results suggest that, unlike time-resolved spectroscopy, low-coherence optical tomography detects the local relative variations of path-length-resolved reflectance from the turbid tissues.