Defocused Spatially Offset Raman Spectroscopy in Media of Different Optical Properties for Biomedical Applications Using a Commercial Spatially Offset Raman Spectroscopy Device (original) (raw)
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Spatially Offset Raman Spectroscopy—How Deep?
Analytical Chemistry
Spatially offset Raman spectroscopy (SORS) is a technique for interrogating the subsurface composition of turbid samples noninvasively. This study generically addresses a fundamental question relevant to a wide range of SORS studies, which is how deep SORS probes for any specific spatial offset when analyzing a turbid sample or, in turn, what magnitude of spatial offset one should select to probe a specific depth. This issue is addressed by using Monte Carlo simulations, under the assumption of negligible absorption, which establishes that the key parameter governing the extent of the probed zone for a point-like illumination and point-like collection SORS geometry is the reduced scattering coefficient of the medium. This can either be deduced from literature data or directly estimated from a SORS measurement by evaluating the Raman intensity profile from multiple spatial offsets. Once this is known, the extent of the probed zone can be determined for any specific SORS spatial offset using the Monte Carlo simulation results presented here. The proposed method was tested using experimental data on stratified samples by analyzing the signal detected from a thin layer that was moved through a stack of layers using both non-absorbing and absorbing samples. The proposed simple methodology provides important additional information on SORS measurements with direct relevance to a wide range of SORS applications including biomedical, pharmaceutical, security, forensics, and cultural heritage.
Turbidity-Corrected Raman Spectroscopy for Blood Analyte Detection
Analytical Chemistry, 2009
A major challenge in quantitative biological Raman spectroscopy, particularly as applied to transcutaneous Raman spectroscopy measurements, is overcoming the deleterious effects of scattering and absorption (turbidity). The Raman spectral information is distorted by multiple scattering and absorption events in the surrounding medium, thereby diminishing the prediction capability of the calibration model. To account for these distortions, we present a novel analytical method, that we call turbidity-corrected Raman spectroscopy (TCRS), which is based on the photon migration approach and employs alternate acquisition of diffuse reflectance and Raman spectra. We demonstrate that, upon application of TCRS, the widely varying Raman spectra observed from a set of tissue phantoms having the same concentration of Raman scatterers but different turbidities has a tendency to collapse onto a single spectral profile. Furthermore, in a prospective study that employs physical tissue models with varying turbidities and randomized concentrations of Raman scatterers and interfering agents, a 20% reduction in prediction error is obtained by applying the turbidity correction procedure to the observed Raman spectra.
Analytical Chemistry
We propose a new method for estimating the reduced scattering coefficient, μ s ′, of turbid homogeneous samples using Spatially Offset Raman Spectroscopy (SORS). The concept is based around the variation of Raman signal with SORS spatial offset that is strongly μ s ′-dependent, as such, permitting the determination of μ s ′. The evaluation is carried out under the assumptions that absorption is negligible at the laser and Raman wavelengths and μ s ′ is approximately the same for those two wavelengths. These conditions are often satisfied for samples analyzed in the NIR region of the spectrum where SORS is traditionally deployed. Through a calibration procedure on a PTFE model sample, it was possible to estimate the μ s ′ coefficient of different turbid samples with an error (RMSEP) below 18%. The knowledge of μ s ′ in the NIR range is highly valuable for facilitating accurate numerical simulations to optimize illumination and collection geometries in SORS, to derive in-depth information about the properties of SORS measurements or in other photon applications, dependent on photon propagation in turbid media with general impact across fields such as biomedical, pharmaceutical, security, forensic, and cultural sciences.
Applied Spectroscopy, 2017
This study demonstrates experimentally a method to enable prediction of depth of a chemical species buried in a turbid medium by using transmission Raman spectroscopy alone. The method allows the prediction of the depth of a single, chemically distinct layer within a turbid matrix by performing two measurements, with and without a beam enhancing element, or “photon diode.” The samples employed consisted of two different polymers, of total thickness 3.6 mm, whose optical properties are loosely relevant to pharmaceutical applications. A polymer layer of low-density polyethylene (LDPE) was placed at different positions within multiple layers of the polytetrafluoroethylene (PTFE) matrix and Raman spectra were recorded in each case. Both univariate and multivariate analyses were utilized to determine whether the depth of the LDPE layer could be predicted using the obtained data. The best-achieved RMSE of prediction was 4.2% of the total sample size (i.e., +/− 0.15 mm) with the multivaria...
Journal of biophotonics, 2016
The broad range of applications of spatially-offset Raman spectroscopy (SORS) were found to involve samples having only marginal differences in Raman cross-sections between the surface and subsurface targets. We report the results of a feasibility study to evaluate the potential of the approach to identify the presence of a very low Raman-active turbid sample placed inside a highly Raman-active diffusely scattering matrix. Paraffin sandwiched tissue blocks prepared by embedding slices of chicken muscle tissue into solid paraffin blocks were employed as representative samples for the study. It was found that in contrast to the several millimetres of probing depth reported in the earlier applications, the Raman signatures of tissue were best recovered when it was located beneath the surface of the paraffin block at a depth of around a millimetre, beyond which the quality of recovery was increasingly poorer. However, the probing depth could be further increased by increasing the thickn...
Quantitative Raman spectroscopy in turbid media
Journal of Biomedical Optics, 2010
Intrinsic Raman spectra of biological tissue are distorted by the influences of tissue absorption and scattering, which significantly challenge signal quantification. A combined Raman and spatially resolved reflectance setup is introduced to measure the absorption coefficient a and the reduced scattering coefficient s Ј of the tissue, together with the Raman signals. The influence of a and s Ј on the resonance Raman signal of -carotene is measured at 1524 cm −1 by tissue phantom measurements and Monte Carlo simulations for a = 0.01 to 10 mm −1 and s Ј=0.1 to 10 mm −1. Both methods show that the Raman signal drops roughly proportional to 1/ a for a Ͼ 0.2 mm −1 in the measurement geometry and that the influence of s Ј is weaker, but not negligible. Possible correction functions dependent on the elastic diffuse reflectance are investigated to correct the Raman signal for the influence of a and s Ј, provided that a and s Ј are measured as well. A correction function based on the Monte Carlo simulation of Raman signals is suggested as an alternative. Both approaches strongly reduce the turbidity-induced variation of the Raman signals and allow absolute Raman scattering coefficients to be determined.
Raman Spectroscopy and Related Techniques in Biomedicine
Sensors, 2010
In this review we describe label-free optical spectroscopy techniques which are able to non-invasively measure the (bio)chemistry in biological systems. Raman spectroscopy uses visible or near-infrared light to measure a spectrum of vibrational bonds in seconds. Coherent anti-Stokes Raman (CARS) microscopy and stimulated Raman loss (SRL) microscopy are orders of magnitude more efficient than Raman spectroscopy, and are able to acquire high quality chemically-specific images in seconds. We discuss the benefits and limitations of all techniques, with particular emphasis on applications in biomedicine-both in vivo (using fiber endoscopes) and in vitro (in optical microscopes).
In-vivo Raman spectroscopy: from basics to applications
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For more than two decades, Raman spectroscopy has found widespread use in biological and medical applications. The instrumentation and the statistical evaluation procedures have matured, enabling the lengthy transition from ex-vivo demonstration to in-vivo examinations. This transition goes hand-in-hand with many technological developments and tightly bound requirements for a successful implementation in a clinical environment, which are often difficult to assess for novice scientists in the field. This review outlines the required instrumentation and instrumentation parameters, designs, and developments of fiber optic probes for the in-vivo applications in a clinical setting. It aims at providing an overview of contemporary technology and clinical trials and attempts to identify future developments necessary to bring the emerging technology to the clinical end users. A comprehensive overview of in-vivo applications of fiber optic Raman probes to characterize different tissue and disease types is also given.