Optical transfer diagnosis differentiating benign and malignant pigmented lesions in a simulated primary care practice.

BACKGROUND: The detection of melanoma poses a substantial challenge, particularly for primary care providers (PCPs) who may have limited training in discriminating between suspicious and benign melanocytic lesions. The noninvasive optical transfer diagnosis (OTD) method was designed to be used by PCPs in their decision-making process. OBJECTIVES: To assess the potential of the OTD method by developing, training and validating an OTD indication algorithm for automated discrimination between benign melanocytic lesions and malignant lesions, based on a set of 712 lesions. METHODS: The authors performed in vivoOTD capture and subsequent analysis of 712 pigmented lesions. Of the lesions, 415 were clinically and dermoscopically benign and 297 were dermoscopically suspicious or equivocal. After image capture, all suspicious or equivocal lesions were biopsied and examined histopathologically. RESULTS: Of the 297 suspicious or equivocal lesions, histopathological findings revealed 80 to be malignant (64 melanomas, 13 basal cell carcinomas and 3 squamous cell carcinomas). OTD misdiagnosed one of the 80 malignant lesions as benign (sensitivity, 99%). OTD specificity was 93% for the dermoscopically benign lesions, 73% for all lesions included in the study and 36% for the clinically suspicious but histopathologically benign lesions. CONCLUSIONS: High sensitivity and specificity, as provided by OTD in this preliminary study, would help PCPs reduce the number of referrals for dermatology consultation, excision or biopsy. Further studies are planned for screening patients in a primary care setting, with comparisons of OTD results with biopsy or dermoscopy results.

Long-term comparisons of UV index values derived from a NILU-UV instrument, NWS, and OMI in the New York area.

A comparison is presented of UV index (UVI) values obtained under different cloud conditions from a Norwegian Institute for Air Research UV (NILU-UV) instrument, the ozone monitoring instrument (OMI) onboard the Aura satellite, and the National Weather Service (NWS) model for the time period of 2010-2014. The NILU-UV irradiance meter is a ground-based, multi-channel, moderate bandwidth filter instrument. UVI values derived from measurements by a NILU-UV instrument deployed in the New York area (40.74°N, -74.03°E) to monitor the erythemal UV radiation from 2010 to present is compared to UVI values derived from OMI measurements and predicted by the NWS model. OMI overestimated the UVI values by 13.06% for all cloud conditions compared with the UVI values derived from measurements by the NILU-UV instrument. The heavier the cloud cover, the higher the overestimation. The mean relative difference between the UVI derived from the NWS model and from NILU-UV measurements was 11.54%. The UVI prediction by NWS was also overestimated under cloudy conditions. Under overcast conditions the NWS predictions of UVI values differ significantly from those derived from NILU-UV measurements, yielding a correlation of only 0.8025 and a mean relative difference of 28.25%.

Comparisons of three NILU-UV instruments deployed at the same site in the New York area.

The Norwegian Institute for Air Research ultraviolet (NILU-UV) irradiance meter is a ground-based, multichannel, moderate bandwidth filter instrument that measures irradiances at ultraviolet (UV) and visible wavelengths with five channels in the UV (302, 312, 320, 340, and 380 nm) and one channel in the visible (400-700 nm) part of the spectrum. Minute-by-minute irradiances recorded in these channels are used to infer the total ozone column (TOC) amount, and a radiation modification factor (RMF) designed to have a value close to 100 under cloud-free conditions. The performance of three NILU-UV instruments deployed side-by-side in the New York area (40.74°N, -74.03°E) is assessed, and derived TOC values are compared with those derived from the ozone monitoring instrument (OMI) deployed on NASA's AURA satellite. Based on about three years of data, it was found that the three instruments yielded similar TOC values that were in close agreement with those derived from the OMI. The relative difference in TOC values derived from the three NILU-UV instruments was generally less than 2.5%. Cloud cover affects the accuracy of the inferred TOC, but reliable values can be obtained in the presence of clouds, although the accuracy deteriorates under heavy overcast conditions with RMF values smaller than 65 (low cloud transmittance).

Comparison of discrete ordinate and Monte Carlo simulations of polarized radiative transfer in two coupled slabs with different refractive indices.

A comparison is presented of two different methods for polarized radiative transfer in coupled media consisting of two adjacent slabs with different refractive indices, each slab being a stratified medium with no change in optical properties except in the direction of stratification. One of the methods is based on solving the integro-differential radiative transfer equation for the two coupled slabs using the discrete ordinate approximation. The other method is based on probabilistic and statistical concepts and simulates the propagation of polarized light using the Monte Carlo approach. The emphasis is on non-Rayleigh scattering for particles in the Mie regime. Comparisons with benchmark results available for a slab with constant refractive index show that both methods reproduce these benchmark results when the refractive index is set to be the same in the two slabs. Computed results for test cases with coupling (different refractive indices in the two slabs) show that the two methods produce essentially identical results for identical input in terms of absorption and scattering coefficients and scattering phase matrices.

Modeling optical properties of human skin using Mie theory for particles with different size distributions and refractive indices.

We used size distributions of volume equivalent spherical particles with complex refractive index to model the inherent optical properties (IOPs) in four different layers of human skin at ten different wavelengths in the visible and near-infrared spectral bands. For each layer, we first computed the size-averaged absorption coefficient, scattering coefficient, and asymmetry factor for the collection of particles in a host medium using Mie theory and compared these IOPs in each layer with those obtained from a bio-optical model (BOM). This procedure was repeated, using an optimization scheme, until satisfactory agreement was obtained between the IOPs obtained from the particle size distribution and those given by the BOM. The size distribution as well as the complex refractive index of the particles, obtained from this modeling exercise, can be used to compute the phase matrix, which is an essential input to model polarized light transport in human skin tissue.

Impact of particulate oceanic composition on the radiance and polarization of underwater and backscattered light.

We use a Monte Carlo model to investigate how the particulate oceanic composition affects the radiance, the linear polarization, and the circular polarization of underwater and backscattered light. The Mueller matrices used in our simulations were computed using the T-matrix method. They are significantly different for organic and inorganic particles. Our Monte Carlo simulations show that these differences have a significant impact on the underwater and backscattered light, and that it may be possible to determine the ratio between the amounts of organic and inorganic particles from measurements of the full Stokes vector.

Retrieval of the physiological state of human skin from UV-Vis reflectance spectra - a feasibility study.

We test the feasibility of using an accurate radiative transfer model for the coupled air-tissue system in conjunction with a classic inversion scheme based on Bayesian optimal estimation theory for retrieval of parameters describing the physiological state of human skin. To that end, we analyse ultraviolet and visible reflectance spectra from human skin measured before, immediately after, and on each day for two weeks after photodynamic treatment with the hexyl ester of ALA and exposure to red light (632 nm). For the first time, we show that it is possible to perform a simultaneous retrieval of the melanosome concentration in both the basal and the upper layers of the epidermis.

Optical detection and monitoring of pigmented skin lesions.

A method is presented for discriminating between malignant and benign pigmented skin lesions based on multispectral and multi-angle images. It is discussed how to retrieve maps of physiology properties and morphometric parameters from recorded images using a bio-optical model, radiative transfer calculations, and nonlinear inversion, and how to employ automated zooming to extract lesion and surrounding masks. Training and validation of a classification scheme for separation between benign and malignant tissue yielded sensitivity/specificity ranging from 97%/97% for application to a small dataset comprised of lesions not used for training and validation to 99%/93% for application to a larger dataset.

Quantitative results in ultrasonic tomography of large objects using line sources and curved detector arrays.

A tomographic reconstruction technique valid for line sources, curved detector arrays, and large object is presented. For acquisitions involving a curved detector array, inverse diffraction is first used to propagate the field back to a straight line and then the standard filtered backpropagation (FBP) algorithm is employed to reconstruct the image. Using inverse diffraction the measured field can be accurately propagated all the way back to the reconstruction area. Thus an essential improvement is obtained compared to using the approximate backpropagation of Rytov data contained in the FBP algorithm, which becomes inaccurate when the distance from the measurement surface to the reconstruction area is large. This technique is applied to measured data and it is shown that it gives reconstructions of high quality, both with respect to geometry and velocity. It is also shown that, when the illuminating wave is cylindrical rather than plane, segmentation of the image can be used in combination with inverse diffraction and FBP reconstruction to obtain high-quality images of large objects.

Diffraction by circular apertures. 1: Method of linear phase and amplitude approximation.

Two methods are presented for efficient computation of the wave field that results when a spherical wave is diffracted by circular apertures. One method is based on the Kirchhoff diffraction integral, the other on the boundary-diffraction-wave (BDW) integral. In each method the integration domain is divided into sub-domains, and the amplitude and phase within each subdomain are linearized to make an analytical integration possible. Explicit and simple formulas are derived that specify the number of subdomains needed to obtain a desired accuracy for a given geometry and wavelength. Also we determine the number of subdomains needed in the BDW integral to obtain a sufficient accuracy in the vicinity of the shadow boundary. The speed of computation of each method is compared with that using direct numerical integration. As an illustration, the BDW method is used to compute the image field of a holographic lens.

Scattering by simple and nonsimple shapes by the combined method of ray tracing and diffraction: application to circular cylinders.

The combined method of ray tracing and diffraction (CMRD) is an efficient and accurate technique for computing the scattered field in focal regions of optical systems. Here we extend the CMRD concept so it can be used to compute fields scattered by objects of simple as well as nonsimple shapes. To that end we replace the scattering object by an equivalent, planar phase object; use ray tracing to determine its location, aperture area, amplitude distribution, and phase distribution; and use standard Kirchhoff diffraction theory to compute the field scattered by the equivalent phase object. To illustrate the practical use of the CMRD we apply it to a two-dimensional problem in which a plane or cylindrical wave is normally incident upon a circular cylinder. For this application we determine the range of validity of the CMRD by comparing its results for the scattered field with those obtained by use of an exact eigenfunction expansion.

Validity of diffraction tomography based on the first born and the first rytov approximations.

Using computer simulations we examine the ranges of validity of the first Born and first Rytov approximations employed in diffraction tomography. To that end we apply the filtered backpropagation(FBP) algorithm in conjunction with the first Born approximation and the hybrid FBP algorithm in conjunction with the first Rytov approximation. We find that the range of validity of the first Born approximation is approximately 3 times smaller than that of the first Rytov approximation and that the range of validity of each approximation can be expressed in terms of the product of the refractive-index difference between the object and the background and the size of the object. Also, we establish precise criteria for the validity of diffraction tomography within each of these two approximations. For the first Rytov approximation the validity of the hybrid FBP algorithm is found to be limited by phase-unwrapping problems.

Scalar and electromagnetic diffraction point-spread functions.

We describe an accurate technique for computing the diffraction point-spread function for optical systems. The approach is based on the combined method of ray tracing and diffraction, which implies that the computation is accomplished in a two-step procedure. First, ray tracing is employed to compute the wave-front error in a reference plane on the image side of the system and to determine the shape of the vignetted pupil. Next the Rayleigh-Sommerfeld diffraction theory, combined with the Kirchhoff approximation and the Stamnes-Spjelkavik-Pedersen method for numerical integration, is applied to compute the field in the region of the image. The method does not rely on small-angle approximations and works well for a pupil of general shape. Both scalar and electromagnetic computations are discussed and numerical results are presented.

Transmission of a Gaussian beam into a biaxial crystal.

We derive integral representations that are suitable for studying the transmission of an electromagnetic Gaussian beam through a plane interface that lies between an isotropic medium and a biaxially anisotropic crystal for the case in which the interface normal is along one of the principal axes of the crystal. To that end, we use recently developed exact solutions for the transmitted fields to derive explicit expressions for the corresponding dyadic Green's functions as well as integral representations that are suitable for asymptotic analysis and efficient numerical evaluation.

Transmission of a two-dimensional Gaussian beam into a uniaxial crystal.

We study the transmission of a two-dimensional (2-D) TM Gaussian beam through a plane interface between an isotropic medium (e.g., air) and a uniaxially anisotropic crystal. The optic axis of the crystal is taken to be in the plane of incidence but is arbitrarily oriented relative to the interface normal. We show that, in the paraxial approximation, a nontruncated transmitted 2-D TM Gaussian beam inside a uniaxial crystal can be expressed in a form similar to that of a scalar Gaussian beam that propagates in a homogeneous medium. We also show that the transmitted beam corresponding to an incident 2-D TM Gaussian beam with its main propagation direction along the interface normal is tilted inside the crystal by the same angle as is the transmitted axial ray that corresponds to a normally incident ray.

Diffracted-ray approach to the fast evaluation of diffractive lenses.

We employ a combination of asymptotic methods to speed up the computation of fields in the focal region of a diffractive lens (DL). The DL is treated locally as a linear grating with a slowly varying period and groove orientation. We employ rigorous electromagnetic diffraction theory locally to obtain the field just behind the DL. A simple diffracted-ray formula is derived for the field in the focal region of the DL at observation points that are not in the immediate vicinity of the optical axis. A careful study of the range of validity of this formula is made. For observation points that are not in the immediate vicinity of the optical axis the new algorithm is 3 x 10(5) times faster than the application of numerical integration to the double integrals involved and approximately 1000-1200 times faster than a recently published algorithm based on using asymptotic theory to replace the double integral with a single integral.

Comparison of exact and asymptotic results for the focusing of electromagnetic waves through a plane interface.

Recently, exact Kirchhoff solutions and the corresponding asymptotic solutions for the focusing of electromagnetic waves through a plane interface between two different dielectrics were reported. But the computation of exact results takes a long time because it requires the quadruple integration of a rapidly oscillating integrand. By using asymptotic techniques to perform two of the integrations, one can reduce the computing time dramatically. Therefore it is important to establish the accuracy and the range of validity of the asymptotic technique. To that end, we compare the exact and the asymptotic results for high-aperture, near-field focusing systems with a total distance from the aperture to the focal point of a few wavelengths and with a distance from the aperture to the interface as small as a fraction of a wavelength. The systems examined have f-numbers in the range from 0.6 to 0.9 and Fresnel numbers in the range from 0.4 to 3.5. Our results show that the accuracy of the asymptotic method increases with the aperture-interface distance when the aperture-focus distance is kept fixed and that it increases with the aperture-focus distance when the aperture-interface distance is kept fixed. To an accuracy of 7.8%, the asymptotic techniques are valid for aperture-interface distances as small as 0.5lambda as long as the total distance from the aperture to the focal point exceeds 8lambda. It is also shown that an accuracy of better than 1% can be obtained for the same aperture-interface distance of 0.5lambda and for interface-observation-point distances as small as 0.1lambda as long as the total distance from the aperture to the focal point exceeds 12lambda. By use of the asymptotic technique the computing time is reduced by a factor of 10(3).

Reflection and refraction of an arbitrary electromagnetic wave at a plane interface separating an isotropic and a biaxial medium.

Exact solutions are obtained for the reflected and transmitted fields resulting when an arbitrary electromagnetic field is incident on a plane interface separating an isotropic medium and a biaxially anisotropic medium in which one of the principal axes is along the interface normal. From our exact solutions for the reflected fields resulting when a plane TE or TM wave is incident on the plane interface, it can be inferred that the reflected field contains both a TE and a TM component. This gives a change in polarization that can be utilized to determine the properties of the biaxial medium. The time-harmonic solution for the reflected field is in the form of two quadruple integrals, one of which is a superposition of plane waves polarized perpendicular to the plane of incidence and the other a superposition of plane waves polarized parallel to the plane of incidence. The time-harmonic solution for the transmitted field is also in the form of two quadruple integrals. Each of these is a superposition of extraordinary plane waves with displacement vectors that are perpendicular to the direction of phase propagation.

Comparison of the filtered backpropagation and the filtered backprojection algorithms for quantitative tomography.

We compare the filtered backpropagation algorithm with the filtered backprojection algorithm for reconstructing the complex refractive-index distribution of semitransparent, cylindrical objects. Before reconstruction, the recorded scattered light is propagated back to the reconstruction area by inverse diffraction. Our comparison is based on computer-simulated data, and experimental optical data obtained from fibers with step-index, graded-index, and uniform-index distributions. The results show that both the filtered backpropagation algorithm and the filtered backprojection algorithm can produce accurate reconstructions of the complex refractive-index distribution as long as the weak-scattering approximation is valid. The good agreement between the results obtained from these two reconstruction algorithms indicates that the errors introduced by the assumption of straight-line propagation inside the object are negligible compared with those introduced by the weak-scattering approximation.

Encircled energy for systems with centrally obscured circular pupils.

The encircled energy is computed for large f/No. aberration-free annular aperture systems with linear obscuration ratios between 0.0 and 0.9. The computations are based on an algorithm due to Hopkins. The results are displayed graphically as contour lines showing the fraction of the total energy which falls within small circles centered on the optical axis in selected receiving planes parallel to the geometric focal plane.