92 research outputs found
Reciprocity relation for the vector radiative transport equation and its application to diffuse optical tomography with polarized light
We derive a reciprocity relation for vector radiative transport equation
(vRTE) that describes propagation of polarized light in multiple-scattering
media. We then show how this result, together with translational invariance of
a plane-parallel sample, can be used to compute efficiently the sensitivity
kernel of diffuse optical tomography (DOT) by Monte Carlo simulations.
Numerical examples of polarization-selective sensitivity kernels thus computed
are given.Comment: 5 pages, 3 figure
Propagation of coherent polarized light in turbid highly scattering medium
Within the framework of further development of unified Monte Carlo code for the needs of biomedical optics and biophotonics, we present an approach for modeling of coherent polarized light propagation in highly scattering turbid media, such as biological tissues. The temporal coherence of light, linear and circular polarization, interference, and the helicity flip of circularly polarized light due to reflection at the medium boundary and/ or backscattering events are taken into account. To achieve higher accuracy in the results and to speed up the modeling, the implementation of the code utilizes parallel computing on NVIDIA graphics processing units using Compute Unified Device Architecture. The results of the simulation of coherent linearly and circularly polarized light are presented in comparison with the results of known theoretical studies and the results of alternative modelings
Efficient inversion strategies for estimating optical properties with Monte Carlo radiative transport models
Significance: Indirect imaging problems in biomedical optics generally require repeated evaluation of forward models of radiative transport, for which Monte Carlo is accurate yet computationally costly. We develop an approach to reduce this bottleneck, which has significant implications for quantitative tomographic imaging in a variety of medical and industrial applications.Aim: Our aim is to enable computationally efficient image reconstruction in (hybrid) diffuse optical modalities using stochastic forward models.Approach: Using Monte Carlo, we compute a fully stochastic gradient of an objective function for a given imaging problem. Leveraging techniques from the machine learning community, we then adaptively control the accuracy of this gradient throughout the iterative inversion scheme to substantially reduce computational resources at each step.Results: For example problems of quantitative photoacoustic tomography and ultrasound-modulated optical tomography, we demonstrate that solutions are attainable using a total computational expense that is comparable to (or less than) that which is required for a single high-accuracy forward run of the same Monte Carlo model.Conclusions: This approach demonstrates significant computational savings when approaching the full nonlinear inverse problem of optical property estimation using stochastic methods
On-Chip Cavity Optomechanical Coupling
On-chip cavity optomechanics, in which strong co-localization of light and
mechanical motion is engineered, relies on efficient coupling of light both
into and out of the on-chip optical resonator. Here we detail our particular
style of tapered and dimpled optical fibers, pioneered by the Painter group at
Caltech, which are a versatile and reliable solution to efficient on-chip
coupling. First, a brief overview of tapered, single mode fibers is presented,
in which the single mode cutoff diameter is highlighted. The apparatus used to
create a dimpled tapered fiber is then described, followed by a comprehensive
account of the procedure by which a dimpled tapered fiber is produced and
mounted in our system. The custom-built optical access vacuum chambers in which
our on-chip optomechanical measurements are performed are then discussed.
Finally, the process by which our optomechanical devices are fabricated and the
method by which we explore their optical and mechanical properties is
explained. It is our expectation that this manuscript will enable the novice to
develop advanced optomechanical experiments.Comment: 31 pages, 9 figure
On the inverse problem in optical coherence tomography
We examine the inverse problem of retrieving sample refractive index information in the context of optical coherence tomography. Using two separate approaches, we discuss the limitations of the inverse problem which lead to it being ill-posed, primarily as a consequence of the limited viewing angles available in the reflection geometry. This is first considered from the theoretical point of view of diffraction tomography under a weak scattering approximation. We then investigate the full non-linear inverse problem using a variational approach. This presents another illustration of the non-uniqueness of the solution, and shows that even the non-linear (strongly scattering) scenario suffers a similar fate as the linear problem, with the observable spatial Fourier components of the sample occupying a limited support. Through examples we demonstrate how the solutions to the inverse problem compare when using the variational and diffraction-tomography approaches
Diffusing-wave polarimetry for tissue diagnostics
We exploit the directional awareness of circularly and/or elliptically polarized light propagating within media which exhibit high numbers of scattering events. By tracking the Stokes vector of the detected light on the Poincaŕe sphere, we demonstrate its applicability for characterization of anisotropy of scattering. A phenomenological model is shown to have an excellent agreement with the experimental data and with the results obtained by the polarization tracking Monte Carlo model, developed in house. By analogy to diffusing-wave spectroscopy we call this approach diffusing-wave polarimetry, and illustrate its utility in probing cancerous and non-cancerous tissue samplesin vitro for diagnostic purposes
Fabrication of High-Aspect Ratio Nanogratings for Phase-based X-ray Imaging
Diffractive optical elements such as periodic gratings are fundamental
devices in X-ray imaging - a technique that medical, material science and
security scans rely upon. Fabrication of such structures with high aspect
ratios at the nanoscale creates opportunities to further advance such
applications, especially in terms of relaxing X-ray source coherence
requirements. This is because typical grating-based X-ray phase imaging
techniques (e.g., Talbot self-imaging) require a coherence length of at least
one grating period and ideally longer. In this paper, the fabrication
challenges in achieving high aspect-ratio nanogratings filled with gold are
addressed by a combination of laser interference and nanoimprint lithography,
physical vapor deposition, metal assisted chemical etching (MACE), and
electroplating. This relatively simple and cost-efficient approach is unlocked
by an innovative post-MACE drying step with hexamethyldisilazane, which
effectively minimizes the stiction of the nanostructures. The theoretical
limits of the approach are discussed and, experimentally, X-ray nanogratings
with aspect ratios >40 demonstrated. Finally, their excellent diffractive
abilities are shown when exposed to a hard (12.2 keV) monochromatic x-ray beam
at a synchrotron facility, and thus potential applicability in phase-based
X-ray imaging.Comment: 27 pages, 5 figures in main text, plus supporting informatio
Young Edinburgh Action presents..... YEA, WE CAN!
The creation of this briefing paper was a collaborative project between a group of young people from YEA and an MSc Childhood Studies student from the University of Edinburgh as part of a placement based dissertation. Keeping in the spirit of YEA, to create this briefing paper we used this opportunity to create a questionnaire to find out about how young people who participate in YEA projects feel about their participation. The questionnaires were sent out to all former and active members of YEA and we used the responses to inform this briefing paper
Blood sampling: two sides to the story
This study aimed to investigate why there is variability in taking blood. A multi method Pilot study was completed in four National Health Service Scotland hospitals. Human Factors/Ergonomics principles were applied to analyse data from 50 observations, 15 interviews and 12-months of incident data from all Scottish hospitals. The Functional Resonance Analysis Method (FRAM) was used to understand why variability may influence blood sampling functions. The analysis of the 61 pre blood transfusion sampling incidents highlighted limitations in the data collected to understand factors influencing performance. FRAM highlighted how variability in the sequence of blood sampling functions and the number of practitioners involved in a single blood sampling activity was influenced by the working environment, equipment, clinical context, work demands and staff resources. This pilot study proposes a realistic view of why blood sampling activities vary and proposes the need to consider the system’s resilience in future safety management strategies
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