Optical coherence angiography (OCA) enables visualisation of three-dimensional micro-vasculature from optical coherence tomography data volumes. Typically, various statistical methods are used to discriminate static tissue from blood flow within vessels. In this paper, we introduce a new method that relies upon the beating heart frequency to isolate blood vessels from the surrounding tissue. Vascular blood flow is assumed to be more strongly modulated by the heart-beat compared to surrounding tissue and therefore short-time Fourier transform of sequential measurements can discriminate the two. Furthermore, it is demonstrated that adjacent B-Scans within an OCT data volume can provide the required sampling frequency. As such, the technique can be considered to be a spatially mapped variation of photoplethysmography (PPG), whereby each image voxel operates as a PPG detector. This principle is demonstrated using both a model system and in vivo for monitoring the vascular changes effected by traumatic brain injury in mice. In vivo measurements were acquired at an A-Scan rate of 10kHz to form a 500x500x512 (lateral x lateral x axial) pixel volume, enabling sequential sampling of the mouse heart rate in an expected range of 300-600 bpm. One of the advantages of this new OCA processing method is that it can be used in conjunction with existing algorithms as an additional filter for signal to noise enhancement.
Traumatic brain injury (TBI) results in direct vascular disruption, triggering edema, and reduction in cerebral blood flow. Therefore, understanding the pathophysiology of brain microcirculation following TBI is important for the development of effective therapies. Optical coherence angiography (OCA) is a promising tool for evaluating TBI in rodent models. We develop an approach to OCA that uses the heart-rate frequency to discriminate between static tissue and vasculature. This method operates on intensity data and is therefore not phase sensitive. Furthermore, it does not require spatial overlap of voxels and thus can be applied to pre-existing datasets for which oversampling may not have been explicitly considered. Heart-rate sensitive OCA was developed for dynamic assessment of mouse microvasculature post-TBI. Results show changes occurring at 5-min intervals within the first 50 min of injury.
Dental enamel mineral loss is multifactorial and is consequently explored using a variety of in vitro models. Important factors include the presence of acidic pH and its specific ionic composition, which can both influence lesion characteristics. Optical coherence tomography (OCT) has been demonstrated as a promising tool for studying dental enamel demineralization. However, OCT-based characterization and comparison of demineralization model dynamics are challenging without a consistent experimental environment. Therefore, an automated four-dimensional OCT system was integrated with a multispecimen flow cell to measure and compare the optical properties of subsurface enamel demineralization in different models. This configuration was entirely automated, thus mitigating any need to disturb the specimens and ensuring spatial registration of OCT image volumes at multiple time points. Twelve bovine enamel disks were divided equally among three model groups. The model demineralization solutions were citric acid (pH 3.8), acetic acid (pH 4.0), and acetic acid with added calcium and phosphate (pH 4.4). Bovine specimens were exposed to the solution continuously for 48 h. Three-dimensional OCT data were obtained automatically from each specimen at a minimum of 1-h intervals from the same location within each specimen. Lesion dynamics were measured in terms of the depth below the surface to which the lesion extended and the attenuation coefficient. The net loss of surface enamel was also measured for comparison. Similarities between the dynamics of each model were observed, although there were also distinct characteristic differences. Notably, the attenuation coefficients showed a systematic offset and temporal shift with respect to the different models. Furthermore, the lesion depth curves displayed a discontinuous increase several hours after the initial acid challenge. This work demonstrated the capability of OCT to distinguish between different enamel demineralization models by making dynamic quantitative measurements of lesion properties. This has important implications for future applications in clinical dentistry.
This study aimed to determine the feasibility of using optical coherence elastography to measure internal displacements during the curing phase of a light-activated, resin-based composite material. Displacement vectors were spatially mapped over time within a commercial dental composite. Measurements revealed that the orientation of cure-induced displacement vectors varied spatially in a complex manner; however, each vector showed a systematic evolution with time. Precision of individual displacements was estimated to be ∼1 to 2 μm, enabling submicrometer time-varying displacements to be detected.
Point spread function (PSF) phantoms based on unstructured distributions of sub-resolution particles in a transparent matrix have proven effective for evaluating resolution and its spatial variation in optical coherence tomography (OCT) systems. Measurements based on PSF phantoms have the potential to become a standard test method for consistent, objective and quantitative inter-comparison of OCT system performance. Towards this end, we have evaluated three PSF phantoms and investigated their ability to compare the performance of four OCT systems. The phantoms are based on 260-nm-diameter gold nanoshells, submicron-diameter iron oxide particles and 1.5-micron-diameter silica particles. The OCT systems included spectral-domain and swept source systems in free-beam geometries as well as a time-domain system in both free-beam and fiberoptic probe geometries. Results indicated that iron oxide particles and gold nanoshells were most effective for measuring spatial variations in the magnitude and shape of PSFs across the image volume. The intensity of individual particles was also used to evaluate spatial variations in signal intensity uniformity. Significant system-to-system differences in resolution and signal intensity and their spatial variation were readily quantified. The phantoms proved useful for identification and characterization of irregularities such as astigmatism. Particle concentrations of 5000 per cubic millimeter or greater provided accurate determination of performance metrics. Our multi-system inter-comparison provides evidence of the effectiveness of PSF-phantom-based test methods for comparison of OCT system resolution and signal uniformity.
The recent expansion of clinical applications for optical coherence tomography (OCT) is driving the development of
approaches for consistent image acquisition. There is a simultaneous need for time-stable, easy-to-use imaging targets
for calibration and standardization of OCT devices. We present calibration targets consisting of three-dimensional
structures etched into nanoparticle-embedded resin. Spherical iron oxide nanoparticles with a predominant particle
diameter of 400 nm were homogeneously dispersed in a two part polyurethane resin and allowed to harden overnight.
These samples were then etched using a precision micromachining femtosecond laser with a center wavelength of 1026
nm, 100kHz repetition rate and 450 fs pulse duration. A series of lines in depth were etched, varying the percentage of
inscription energy and speed of the translation stage moving the target with respect to the laser. Samples were imaged
with a dual wavelength spectral-domain OCT system (λ=800nm, ▵λ≈180nm, and λ=1325nm, ▵λ≈100nm) and point-spread function of nanoparticles within the target was measured.
In Fourier domain optical coherence tomography (FD-OCT), a large amount of interference data needs to be resampled from the wavelength domain to the wavenumber domain prior to Fourier transformation. We present an approach to optimize this data processing, using a graphics processing unit (GPU) and parallel processing algorithms. We demonstrate an increased processing and rendering rate over that previously reported by using GPU paged memory to render data in the GPU rather than copying back to the CPU. This avoids unnecessary and slow data transfer, enabling a processing and display rate of well over 524,000 A-scan/s for a single frame. To the best of our knowledge this is the fastest processing demonstrated to date and the first time that FD-OCT processing and rendering has been demonstrated entirely on a GPU.
We present a new method for quantitative visualization of premalignant oral epithelium called scattering attenuation microscopy (SAM). Using low-coherence interferometry, SAM projects measurements of epithelial optical attenuation onto an image of the tissue surface as a color map. The measured attenuation is dominated by optical scattering that provides a metric of the severity of oral epithelial dysplasia (OED). Scattering is sensitive to the changes in size and distribution of nuclear material that are characteristic of OED, a condition recognized by the occurrence of basal-cell-like features throughout the epithelial depth. SAM measures the axial intensity change of light backscattered from epithelial tissue. Scattering measurements are obtained from sequential axial scans of a 3-D tissue volume and displayed as a 2-D SAM image. A novel segmentation method is used to confine scattering measurement to epithelial tissue. This is applied to oral biopsy samples obtained from 19 patients. Our results show that imaging of tissue scattering can be used to discriminate between different dysplastic severities and furthermore presents a powerful tool for identifying the most representative tissue site for biopsy.
In this study, we have chosen to implement a Monte Carlo simulation of an OCT system in order to investigate elements of the underlying physics of OCT images. Of particular interest is the signal decay primarily attributable to optical scattering, refractive index variations, including index matching, and how these compare with the influence of layer anisotropy.
In this work we present contrast measurement phantoms for optical coherence tomography. In this initial study, contrast
has been evaluated from OCT images of a USAF-1951 bar target, buried beneath scattering layers of different density.
Preliminary results indicate that scattering does not degrade imaging contrast significantly, however further work is
required to verify these findings. This work has important implications from a quality control perspective as well for
OCT inter-comparisons at, for example, different wavebands.
In this paper a novel method for determining refractive indices of a multi-layered samples using low coherence
interferometry (LCI), developed at the National Physical Laboratory, UK, is introduced. Conventional Optical
Coherence Tomography (OCT) utilises a lateral scanning optical probe beam to construct a depth resolved image of the
sample under investigation. All interfaces are detected in optical path length, resulting in an image depending on the
refractive index of all prior layers. This inherent ambiguity in optical and geometric path length reduces OCT images to
purely qualitative ones. We have demonstrated that by optically probing the sample at multiple angles we can determine
bulk refractive index of layers throughout plane parallel samples. This method improves upon current approaches of
extracting refractive index parameters from multi-layered samples as no prior geometrical information is required of the
sample and the phase index for each layer is obtained as opposed to the group index. Consequently the refractive index
result for each layer is independent of the refractive index of surrounding layers. This technique also improves on
conventional measurements, as it is less susceptible to error due to surface defects. This technique is easily implemented,
and can easily be modified to obtain in situ measurements. Investigating a silica test piece and comparing the refractive
index obtained by that of standard critical angle refractometry has validated the robustness of the technique.
Two significant figures of merit for optical coherence tomography (OCT) systems are the axial and transverse resolutions.
Transverse resolution has been defined using the Rayleigh Criterion or from Gaussian beam optics. The axial resolution
is generally defined in terms of the coherence length of a Gaussian shaped source. Whilst these definitions provide a
useful mathematical reference they are somewhat abstracted from the three dimensional resolution that is encountered
under practical imaging conditions. Therefore, we have developed a three-dimensional resolution target and measurement
methodology that can be used to calibrate the three-dimensional resolution of OCT systems.
Dental amalgam is being increasingly replaced by Light-activated resin-based dental composites. However, these materials
are limited by inefficient setting reactions as a function of depth, constraining the maximum extent of cure and reducing
biocompatibility. In this paper we demonstrate a novel metrological tool for dynamic monitoring of refractive index
and thickness change through curing resins using spectral-domain optical coherence tomography. We present real-time
measurements from pre- to post-cure of a series of un-filled bisphenol-A diglycidyl ether dimethacrylate (bisGMA) and
triethylene glycol dimethacrylate (TEGDMA) resins with different inhibitor concentrations. Our results demonstrate that
refractive index measurements are sensitive to the extent of cure of such resins and that the inhibitor concentration strongly
affects the cure dynamics and final extent of cure.
The flow of culture medium through a mechanically stimulated
cell-seeded tissue scaffold is a factor influencing not only
the transport of essential nutrients and waste product removal but also impacting on the degradation kinetics of the scaffold.
Being able to map spatial and temporal changes in fluid flow behaviour is key to the development of improved bioreactors
and tissue scaffold designs, especially for the new generation of multiple tissue reactors. In this paper we demonstrate
the excellent metrological benefits of fast Doppler optical coherence tomography for time-lapse characterisation of tissue
scaffolds placed in a dynamic flow environment.
A novel dual angle optical coherence tomography (OCT) method is developed that has been termed stereoscopic OCT,
highlighting the similarities between this technique and stereoscopic ranging. OCT images are obtained at two angles of
incidence with respect to the surface of a layered phantom. From these measurements, optical path lengths are determined
for each layer that are used to calculate the refractive index and physical thickness of each layer directly from Snell's law.
This method may prove to be useful for characterising the bulk optical properties of biological material in vivo, that are
presently not well known or understood.
Light-activated resin-based dental composites are increasingly replacing dental amalgam. However, these materials are limited by inefficient setting reactions as a function of depth that constrain the maximum extent of cure. Insufficient curing can contribute to an overall reduction in biocompatibility of the material. We demonstrate dynamic refractive index measurements of a commercial dental composite throughout cure using spectral domain low coherence interferometry. Our results show a linear relationship between the change in refractive index and polymerization-induced reduction in physical thickness during light-activated curing. This relationship between the optical and physical density demonstrates the potential of this technique as a unique noninvasive tool for measurement of the conversion degree of curing dental composite materials.
We demonstrate a high-speed tuneable, continuous wave laser source at 1550nm for Fourier domain OCT imaging. The light source was based on a pigtailed semiconductor optical amplifier and a diffraction grating and polygon mirror for fast frequency tuning. This source provides frequency scan rate of up to 20kHz over a wavelength range of 80nm (60nm FWHM) at the central wavelength of 1550nm, offering an axial resolution of ~17 microns.
In this article we present a novel approach to quantitative imaging by Fourier domain optical coherence tomography. Using an eigenanalysis technique, a matrix model of the sample under test is fitted to real spectral data to extract layer dependent refractive index and thickness values. We demonstrate this method experimentally for a simple test artefact, made from silica test slides and highly scattering dental composite.
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