Quantification of the reduced scattering coefficient and phase-function-dependent parameter γ of turbid media using multidiameter single fiber reflectance spectroscopy: experimental validation

Diffuse photon remission associated with the center-illuminated-area-detection geometry: Part I, an approach to the steady-state model

Diffuse photon remission associated with the center-illuminated-area-detection (CIAD) geometry has been useful for non-contact sensing and may inform single-fiber reflectance (SfR). This series of work advances model approaches that help enrich the understanding and applicability of the photon remission by CIAD. The general approach is to derive the diffuse photon remission by the area integration of the radially resolved diffuse reflectance while limiting the analysis to a medium exhibiting only the Heyney-Greenstein (HG) scattering phase function. Part I assesses the steady-state photon remission in CIAD over a reduced scattering scaled diameter of μ s ' d _{a r e a} ∈[0.5×10^-3,10³] that covers the range extensively modeled for SfR. The corresponding radially resolved diffuse reflectance is obtained by concatenating an empirical expression for the semi-ballistic region near the point-of-illumination and a formula utilizing a master-slave dual-source scheme over the semi-diffusive to a diffusive regime while being constrained by an extrapolated zero-boundary condition. The terminal algebraic photon remission is examined against Monte Carlo simulations for an absorption coefficient over [0.001,1]m m ^-1, a reduced scattering coefficient over [0.01,1000]m m ^-1, a HG scattering anisotropy factor within [0.5,0.95], and a diameter of the area of collection ranging [50,1000]µm. The algebraic model is also applied to phantom data acquired over a ∼2c m non-contact CIAD configuration and with a 200 µm SfR probe. The model approach will be extended in a subsequent work towards the time-of-flight characteristics of CIAD.

Characterizing factors influencing calibration and optical property determination in quantitative reflectance spectroscopy to improve standardization

SIGNIFICANCE

The combination of reflectance and fluorescence spectroscopy allows the determination of tissue optical properties and the calculation of the intrinsic fluorescence in vivo. These parameters can discriminate between tissues and may allow the discrimination of malignant from benign tissue. While this approach has significant clinical potential, the lack of standardization and quality assessment prevents the upscaling of research.

AIM

Investigate which factors influence device calibration and tissue optical property determination. Improve system quality assessment and allow upscaling of the clinical research using multidiameter single fiber reflectance/singe fiber fluorescence spectroscopy.

APPROACH

Two studies, one phantom based on uniform calibrations and skin measurements and a clinical study including clinical calibrations. The first validates the effect of factors under identical conditions and the effect of calibration quality on the optical property determination of skin. The second shows the effect of different system configurations and the performance of the system and probe over an extended period.

RESULTS

Phantom calibrations showed stability over a period of 20 weeks except for a 16-week-old intralipid phantom which showed a significant difference (at least p  =  0.0032) for all five probes evaluated. For clinical calibrations, only the fiber tree had a significant influence (probe 4: p  <  0.000001 and probe 5: p  =  0.00038) on the calibration quality. Interestingly, no degradation of probe performance was detected over a period of 21 months despite the exposure to stress during clinical measurements. Calibration quality affected μs' and the power law scattering exponent, but the degree of the influence was different per fiber.

CONCLUSIONS

Intralipid phantom quality and fiber tree performance are the main factors influencing the calibration quality. Probe and user performance did not show any effect, which makes the upscaling of research to multicenter trials easier. A high-quality assessment procedure should be implemented to track changes during clinical trials.

In-Vivo Optical Monitoring of the Efficacy of Epidermal Growth Factor Receptor Targeted Photodynamic Therapy: The Effect of Fluence Rate

Targeted photodynamic therapy (PDT) has the potential to improve the therapeutic effect of PDT due to significantly better tumor responses and less normal tissue damage. Here we investigated if the efficacy of epidermal growth factor receptor (EGFR) targeted PDT using cetuximab-IRDye700DX is fluence rate dependent. Cell survival after treatment with different fluence rates was investigated in three cell lines. Singlet oxygen formation was investigated using the singlet oxygen quencher sodium azide and singlet oxygen sensor green (SOSG). The long-term response (to 90 days) of solid OSC-19-luc2-cGFP tumors in mice was determined after illumination with 20, 50, or 150 mW·cm-2. Reflectance and fluorescence spectroscopy were used to monitor therapy. Singlet oxygen was formed during illumination as shown by the increase in SOSG fluorescence and the decreased response in the presence of sodium azide. Significantly more cell death and more cures were observed after reducing the fluence rate from 150 mW·cm-2 to 20 mW·cm-2 both in-vitro and in-vivo. Photobleaching of IRDye700DX increased with lower fluence rates and correlated with efficacy. The response in EGFR targeted PDT is strongly dependent on fluence rate used. The effectiveness of targeted PDT is, like PDT, dependent on the generation of singlet oxygen and thus the availability of intracellular oxygen.

Multidiameter single-fiber reflectance spectroscopy of heavily pigmented skin: modeling the inhomogeneous distribution of melanin

When analyzing multidiameter single-fiber reflectance (MDSFR) spectra, the inhomogeneous distribution of melanin pigments in skin tissue is usually not accounted for. Especially in heavily pigmented skins, this can result in bad fits and biased estimation of tissue optical properties. A model is introduced to account for the inhomogeneous distribution of melanin pigments in skin tissue. In vivo visible MDSFR measurements were performed on heavily pigmented skin of type IV to VI. Skin tissue optical properties and related physiological properties were extracted from the measured spectra using the introduced model. The absorption of melanin pigments described by the introduced model demonstrates a good correlation with the co-localized measurement of the well-known melanin index.

Optical pre-screening for laryngeal cancer using reflectance spectroscopy of the buccal mucosa

A new approach in early cancer detection focuses on detecting field cancerization (FC) instead of the tumor itself. The aim of the current study is to investigate whether reflectance spectroscopy can detect FC in the buccal mucosa of patients with laryngeal cancer. The optical properties of the buccal mucosa of patients were measured with multidiameter single-fiber reflectance spectroscopy. The blood oxygen saturation and blood volume fraction were significantly lower in the buccal mucosa of laryngeal cancer patients than in non-oncologic controls. The data of these two parameters were combined to form a single 'biomarker α', which optimally discriminates these two groups. Alpha was lower in the laryngeal cancer group (0.28) than the control group (0.30, p = 0.007). Alpha could identify oncologic patients with a sensitivity of 78% and a specificity of 74%. These results might be the first step toward optical pre-screening for laryngeal cancer.

Review of structured light in diffuse optical imaging

Diffuse optical imaging probes deep living tissue enabling structural, functional, metabolic, and molecular imaging. Recently, due to the availability of spatial light modulators, wide-field quantitative diffuse optical techniques have been implemented, which benefit greatly from structured light methodologies. Such implementations facilitate the quantification and characterization of depth-resolved optical and physiological properties of thick and deep tissue at fast acquisition speeds. We summarize the current state of work and applications in the three main techniques leveraging structured light: spatial frequency-domain imaging, optical tomography, and single-pixel imaging. The theory, measurement, and analysis of spatial frequency-domain imaging are described. Then, advanced theories, processing, and imaging systems are summarized. Preclinical and clinical applications on physiological measurements for guidance and diagnosis are summarized. General theory and method development of tomographic approaches as well as applications including fluorescence molecular tomography are introduced. Lastly, recent developments of single-pixel imaging methodologies and applications are reviewed.

Optical detection of field cancerization in the buccal mucosa of patients with esophageal cancer

Introduction

Esophageal cancer is an increasingly common type of neoplasm with a very poor prognosis. This prognosis could improve with more early tumor detection. We have previously shown that we can use an optical spectroscopy to detect field cancerization in the buccal mucosa of patients with laryngeal cancer. The aim of this prospective study was to investigate whether we could detect field cancerization of buccal mucosa of patients with esophageal squamous cell carcinoma (ESCC) and esophageal adenocarcinoma (EAC).

Methods

Optical measurements were performed in vivo using a novel optical technique: multidiameter single-fiber reflectance (MDSFR) spectroscopy. MDSFR spectra were acquired by a handheld probe incorporating three fiber diameters. Multiple absorption and scattering parameters that are related to the physiological and ultrastructural properties of the buccal mucosa were derived from these spectra. A linear discriminant analysis of the parameters was performed to create a combined biomarker σ to discriminate oncologic from non-oncologic patients.

Results

Twelve ESCC, 12 EAC, and 24 control patients were included in the study. The median value of our biomarker σ was significantly higher in patients with ESCC (2.07 [1.93–2.10]) than control patients (1.86 [1.73–1.95], p = 0.022). After cross-validation σ was able to identify ESCC patients with a sensitivity of 66.7% and a specificity of 70.8%. There were no significant differences between the EAC group and the control group.

Conclusion

Field cancerization in the buccal mucosa can be detected using optical spectroscopy in ESCC patients. This may be the first step towards non-invasive ESCC cancer screening.

Single fiber reflectance spectroscopy calibration

To accurately determine sample optical properties using single fiber reflectance spectroscopy (SFR), an absolute calibration of the reflectance is required. We investigated two SFR calibration methods, using a calibrated mirror and using the Fresnel reflection at the fiber tip as a reference. We compared these to commonly used calibration methods, using either Intralipid-20% in combination with Monte Carlo simulations or Spectralon as a reference. The Fresnel reflection method demonstrated the best reproducibility and yielded the most reliable result. We therefore recommend the Fresnel reflection method for the measured absolute reflectance calibration of SFR.

Modeling subdiffusive light scattering by incorporating the tissue phase function and detector numerical aperture

To detect small-scale changes in tissue with optical techniques, small sampling volumes and, therefore, short source–detector separations are required. In this case, reflectance measurements are not adequately described by the diffusion approximation. Previous studies related subdiffusive reflectance to ? or ? , which parameterize the phase function. Recently, it was demonstrated that ? predicts subdiffusive reflectance better than ? , and that ? becomes less predictive for lower numerical apertures (NAs). We derive and evaluate the parameter R p NA , which incorporates the NA of the detector and the integral of the phase function over the NA in the backward and forward directions. Monte Carlo simulations are performed for overlapping source/detector geometries for a range of phase functions, reduced scattering coefficients, NAs, and source/detector diameters. R p NA improves prediction of the measured reflectance compared to ? and ? . It is, therefore, expected that R p NA will improve derivation of optical properties from subdiffusive measurements.

Toward optical guidance during endoscopic ultrasound-guided fine needle aspirations of pancreatic masses using single fiber reflectance spectroscopy: a feasibility study

Endoscopic ultrasound-guided fine needle aspirations (EUS-FNA) of pancreatic masses suffer from sample errors and low-negative predictive values. Fiber-optic spectroscopy in the visible to near-infrared wavelength spectrum can noninvasively extract physiological parameters from tissue and has the potential to guide the sampling process and reduce sample errors. We assessed the feasibility of single fiber (SF) reflectance spectroscopy measurements during EUS-FNA of pancreatic masses and its ability to distinguish benign from malignant pancreatic tissue. A single optical fiber was placed inside a 19-gauge biopsy needle during EUS-FNA and at least three reflectance measurements were taken prior to FNA. Spectroscopy measurements did not cause any related adverse events and prolonged procedure time with ? 5 ?? min . An accurate correlation between spectroscopy measurements and cytology could be made in nine patients (three benign and six malignant). The oxygen saturation and bilirubin concentration were significantly higher in benign tissue compared with malignant tissue (55% versus 21%, p = 0.038 ; 166 ?? ? mol / L versus 17 ?? ? mol / L , p = 0.039 , respectively). To conclude, incorporation of SF spectroscopy during EUS-FNA was feasible, safe, and relatively quick to perform. The optical properties of benign and malignant pancreatic tissue are different, implying that SF spectroscopy can potentially guide the FNA sampling.

Estimation of optical properties by spatially resolved reflectance spectroscopy in the subdiffusive regime

We propose and objectively evaluate an inverse Monte Carlo model for estimation of absorption and reduced scattering coefficients and similarity parameter ? from spatially resolved reflectance (SRR) profiles in the subdiffusive regime. The similarity parameter ? carries additional information on the phase function that governs the angular properties of scattering in turbid media. The SRR profiles at five source-detector separations were acquired with an optical fiber probe. The inverse Monte Carlo model was based on a cost function that enabled robust estimation of optical properties from a few SRR measurements without a priori knowledge about spectral dependencies of the optical properties. Validation of the inverse Monte Carlo model was performed on synthetic datasets and measured SRR profiles of turbid phantoms comprising molecular dye and polystyrene microspheres. We observed that the additional similarity parameter ? substantially reduced the reflectance variability arising from the phase function properties and significantly improved the accuracy of the inverse Monte Carlo model. However, the observed improvement of the extended inverse Monte Carlo model was limited to reduced scattering coefficients exceeding ?15??cm?1, where the relative root-mean-square errors of the estimated optical properties were well within 10%.

Quantifying phase function influence in subdiffusively backscattered light

Light backscattering at short source-detector separations is considerably influenced by the scattering phase function of a turbid medium. We seek to more precisely relate a medium's subdiffusive backscattering to the angular scattering characteristics of its microstructure. First, we demonstrate the inability of the scattering asymmetry g1 = < cos θ > to predict phase function influence on backscattering and reveal ambiguities related to the established phase function parameter γ. Through the use of high-order similarity relations, we introduce a new parameter that more accurately relates a scattering phase function to its subdiffusive backscattering intensity. Using extensive analytical forward calculations based on solutions to the radiative transfer equation in the spatial domain and spatial frequency domain, we demonstrate the superiority of our empirically derived quantifier σ over the established parameter γ.

In vivo assessment of diet-induced rat hepatic steatosis development by percutaneous single-fiber spectroscopy detects scattering spectral changes due to fatty infiltration

This study explores percutaneous single-fiber spectroscopy (SfS) of rat livers undergoing fatty infiltration. Eight test rats were fed a methionine-choline-deficient (MCD) diet, and four control rats were fed a normal diet. Two test rats and one control rat were euthanized on days 12, 28, 49, and 77 following initiation of the diet, after percutaneous SfS of the liver under transabdominal ultrasound guidance. Histology of each set of the two euthanized test rats showed mild and mild hepatic lipid accumulations on day 12, moderate and severe on day 28, severe and mild on day 49, and moderate and mild on day 77. Livers with moderate or higher lipid accumulation generally presented higher spectral reflectance intensity when compared to lean livers. Livers of the eight test rats on day 12, two of which had mild lipid accumulation, revealed an average scattering power of 0.37±0.14 in comparison to 0.07±0.14 for the four control rats (p<0.01 ). When livers of the test rats with various levels of fatty infiltration were combined, the average scattering power was 0.36±0.15 0.36±0.15 in comparison to 0.14±0.24 of the control rats (0.05<p<0.1). Increasing lipid accumulation in concentration and size seemed to cause an increase of the scattering power prior to increasing total spectral reflectance.

Detecting structural information of scatterers using spatial frequency domain imaging

We demonstrate optical phantom experiments on the phase function parameter γ using spatial frequency domain imaging. The incorporation of two different types of scattering particles allows for control of the optical phantoms’ microscopic scattering properties. By laterally structuring areas with either TiO2 or Al2O3 scattering particles, we were able to obtain almost pure subdiffusive scattering contrast in a single optical phantom. Optical parameter mapping was then achieved using an analytical radiative transfer model revealing the microscopic structural contrast on a macroscopic field of view. As part of our study, we explain several correction and referencing techniques for high spatial frequency analysis and experimentally study the sampling depth of the subdiffusive parameter γ.

Sources of variability in the quantification of tissue optical properties by multidiameter single-fiber reflectance and fluorescence spectroscopy

Recently, a multidiameter single-fiber reflectance and fluorescence spectroscopy device has been developed that enabled us to extract the autofluorescence of tissue that is corrected for the optical properties. Such a system has been incorporated in the population-based Rotterdam Study to investigate the autofluorescence of the skin. Since the device will be used by different operators over many years, it is essential that the results are comparable between users. It is, however, unclear how different methods of handling the probe might influence the outcome. Variability of blood oxygen saturation, blood volume fraction and vessel diameter, average gamma, reduced scattering coefficient at 800 nm, and integrated intrinsic fluorescence measured in three volunteers were assessed within and between eight untrained users. A variability of less than one standard deviation from the group mean was defined as an acceptable limit. Three mature volunteers were also included to assess the intrauser variability of mature skin. The variation in the measured parameters suggests that variation is dominated by tissue heterogeneity. Most users measured within one standard deviation of the group mean. Notably, corrected intrinsic fluorescence showed low intra- and interuser variability. These results strongly suggest that variability is mostly caused by tissue heterogeneity and is not user induced.

Correction for tissue optical properties enables quantitative skin fluorescence measurements using multi-diameter single fiber reflectance spectroscopy

BACKGROUND AND OBJECTIVE

Fluorescence measurements in the skin are very much affected by absorption and scattering but existing methods to correct for this are not applicable to superficial skin measurements.

STUDY DESIGN/MATERIALS AND METHODS

The first use of multiple-diameter single fiber reflectance (MDSFR) and single fiber fluorescence (SFF) spectroscopy in human skin was investigated. MDSFR spectroscopy allows a quantification of the full optical properties in superficial skin (μa, μs' and γ), which can next be used to retrieve the corrected - intrinsic - fluorescence of a fluorophore Qμa,x(f). Our goal was to investigate the importance of such correction for individual patients. We studied this in 22 patients undergoing photodynamic therapy (PDT) for actinic keratosis.

RESULTS

The magnitude of correction of fluorescence was around 4 (for both autofluorescence and protoporphyrin IX). Moreover, it was variable between patients, but also within patients over the course of fractionated aminolevulinic acid PDT (range 2.7-7.5). Patients also varied in the amount of protoporphyrin IX synthesis, photobleaching percentages and resynthesis (>100× difference between the lowest and highest PpIX synthesis). The autofluorescence was lower in actinic keratosis than contralateral normal skin (0.0032 versus 0.0052; P<0.0005).

CONCLUSIONS

Our results clearly demonstrate the importance of correcting the measured fluorescence for optical properties, because these vary considerably between individual patients and also during PDT. Protoporphyrin IX synthesis and photobleaching kinetics allow monitoring clinical PDT which facilitates individual-based PDT dosing and improvement of clinical treatment protocols. Furthermore, the skin autofluorescence can be relevant for diagnostic use in the skin, but it may also be interesting because of its association with several internal diseases.

Sub-diffusive scattering parameter maps recovered using wide-field high-frequency structured light imaging

This study investigates the hypothesis that structured light reflectance imaging with high spatial frequency patterns [Formula: see text] can be used to quantitatively map the anisotropic scattering phase function distribution [Formula: see text] in turbid media. Monte Carlo simulations were used in part to establish a semi-empirical model of demodulated reflectance ([Formula: see text]) in terms of dimensionless scattering [Formula: see text] and [Formula: see text], a metric of the first two moments of the [Formula: see text] distribution. Experiments completed in tissue-simulating phantoms showed that simultaneous analysis of [Formula: see text] spectra sampled at multiple [Formula: see text] in the frequency range [0.05-0.5] [Formula: see text] allowed accurate estimation of both [Formula: see text] in the relevant tissue range [0.4-1.8] [Formula: see text], and [Formula: see text] in the range [1.4-1.75]. Pilot measurements of a healthy volunteer exhibited [Formula: see text]-based contrast between scar tissue and surrounding normal skin, which was not as apparent in wide field diffuse imaging. These results represent the first wide-field maps to quantify sub-diffuse scattering parameters, which are sensitive to sub-microscopic tissue structures and composition, and therefore, offer potential for fast diagnostic imaging of ultrastructure on a size scale that is relevant to surgical applications.

Shifting focus in optical image-guided cancer therapy

Cancer patients could benefit from a surgical procedure that helps the surgeon to determine adequate tumor resection margins. Systemic injection of tumor-specific fluorescence agents with subsequent intraoperative optical imaging can guide the surgeon in this process. However, tumor heterogeneity hampers tumor-specific targeting. In addition, determination of adequate resection margins can be very challenging due to invasive tumor strands that are difficult to resolve and because of the confounding effect of variations in tissue optical properties in the surgical margin. We provide an overview of the "classic approach" of imaging tumor-specific targets or tumor-associated pathophysiological processes, and explain the limitations of these targeting strategies. It is proposed that problems of tumor heterogeneity can theoretically be circumvented by shifting focus of tumor targeting towards the follicle-stimulating hormone receptor (FSHR). Furthermore, we discuss why objective determination of resection margins is required to improve resection of the invasive strands, a goal that may be achieved by targeting the FSHR. When invasive strands would nevertheless extend beyond such a standardized resection margin, we suggest that adjuvant photodynamic therapy would be a very suitable therapeutic regimen. Finally, we describe how point optical spectroscopy can be used to scrutinize suspect tissue that is difficult to differentiate from normal tissue by measuring the local tissue optical properties to recover a local intrinsic fluorescence measurement.

Extraction of intrinsic fluorescence from single fiber fluorescence measurements on a turbid medium: experimental validation

The detailed mechanisms associated with the influence of scattering and absorption properties on the fluorescence intensity sampled by a single optical fiber have recently been elucidated based on Monte Carlo simulated data. Here we develop an experimental single fiber fluorescence (SFF) spectroscopy setup and validate the Monte Carlo data and semi-empirical model equation that describes the SFF signal as a function of scattering. We present a calibration procedure that corrects the SFF signal for all system-related, wavelength dependent transmission efficiencies to yield an absolute value of intrinsic fluorescence. The validity of the Monte Carlo data and semi-empirical model is demonstrated using a set of fluorescent phantoms with varying concentrations of Intralipid to vary the scattering properties, yielding a wide range of reduced scattering coefficients (μ's = 0-7 mm (-1)). We also introduce a small modification to the model to account for the case of μ's = 0 mm (-1) and show its relation to the experimental, simulated and theoretically calculated value of SFF intensity in the absence of scattering. Finally, we show that our method is also accurate in the presence of absorbers by performing measurements on phantoms containing red blood cells and correcting for their absorption properties.

In vivo quantification of the scattering properties of tissue using multi-diameter single fiber reflectance spectroscopy

Multi diameter single fiber reflectance (MDSFR) spectroscopy is a non-invasive optical technique based on using multiple fibers of different diameters to determine both the reduced scattering coefficient (μs') and a parameter γ that is related to the angular distribution of scattering, where γ = (1-g2)/(1-g1) and g1 and g2 the first and second moment of the phase function, respectively. Here we present the first in vivo MDSFR measurements of μs'(λ) and γ(λ) and their wavelength dependence. MDSFR is performed on nineteen mice in four tissue types including skin, liver, normal tongue and in an orthotopic oral squamous cell carcinoma. The wavelength-dependent slope of μs'(λ) (scattering power) is significantly higher for tongue and skin than for oral cancer and liver. The reduced scattering coefficient at 800 nm of oral cancer is significantly higher than of normal tongue and liver. Gamma generally increases with increasing wavelength; for tumor it increases monotonically with wavelength, while for skin, liver and tongue γ(λ) reaches a plateau or even decreases for longer wavelengths. The mean γ(λ) in the wavelength range 400-850 nm is highest for liver (1.87 ± 0.07) and lowest for skin (1.37 ± 0.14). Gamma of tumor and normal tongue falls in between these values where tumor exhibits a higher average γ(λ) (1.72 ± 0.09) than normal tongue (1.58 ± 0.07). This study shows the potential of using light scattering spectroscopy to optically characterize tissue in vivo.

Use of a coherent fiber bundle for multi-diameter single fiber reflectance spectroscopy

Multi-diameter single fiber reflectance (MDSFR) spectroscopy enables quantitative measurement of tissue optical properties, including the reduced scattering coefficient and the phase function parameter γ. However, the accuracy and speed of the procedure are currently limited by the need for co-localized measurements using multiple fiber optic probes with different fiber diameters. This study demonstrates the use of a coherent fiber bundle acting as a single fiber with a variable diameter for the purposes of MDSFR spectroscopy. Using Intralipid optical phantoms with reduced scattering coefficients between 0.24 and 3 mm(-1), we find that the spectral reflectance and effective path lengths measured by the fiber bundle (NA = 0.40) are equivalent to those measured by single solid-core fibers (NA = 0.22) for fiber diameters between 0.4 and 1.0 mm (r ≥ 0.997). This one-to-one correlation may hold for a 0.2 mm fiber diameter as well (r = 0.816); however, the experimental system used in this study suffers from a low signal-to-noise for small dimensionless reduced scattering coefficients due to spurious back reflections within the experimental system. Based on these results, the coherent fiber bundle is suitable for use as a variable-diameter fiber in clinical MDSFR quantification of tissue optical properties.