The purpose of this study is to investigate the effects of external factors – including environmental conditions (e.g., ambient temperature, ambient relative humidity), and deployment parameters (e.g., working distance, viewing angle, setting temperature of external temperature reference source (ETRS)) – on the use of infrared thermographs (IRTs) for elevated body temperature (EBT) detection. These effects were theoretically evaluated through computer simulation and experimentally evaluated through bench tests. Overall, the insights into the influence of these factors provided by this study may improve IRT performance and thus enhance the potential of IRTs as countermeasures for high-quality point-of-care EBT screening.
Infrared thermographs (IRTs) have been used for fever screening during infectious disease epidemics. However, their performance is inconsistent in literature, due to wide quality/implementation variations. We overview standards and FDA guidance for IRT performance evaluation, implementation, and regulation policies. Additionally, we present results from a large-scale clinical study of fever-screening IRTs and discuss impact of consensus guidelines and facial measurement location on performance. We found that: high-quality IRTs implemented according to international standards can help to accurately measure temperature; current standards can be improved to further enhance IRT performance. Overall, fever screening is only one element in infectious disease detection.
Significance: Infrared thermographs (IRTs) have been used for fever screening during infectious disease epidemics, including severe acute respiratory syndrome, Ebola virus disease, and coronavirus disease 2019 (COVID-19). Although IRTs have significant potential for human body temperature measurement, the literature indicates inconsistent diagnostic performance, possibly due to wide variations in implemented methodology. A standardized method for IRT fever screening was recently published, but there is a lack of clinical data demonstrating its impact on IRT performance.
Aim: Perform a clinical study to assess the diagnostic effectiveness of standardized IRT-based fever screening and evaluate the effect of facial measurement location.
Approach: We performed a clinical study of 596 subjects. Temperatures from 17 facial locations were extracted from thermal images and compared with oral thermometry. Statistical analyses included calculation of receiver operating characteristic (ROC) curves and area under the curve (AUC) values for detection of febrile subjects.
Results: Pearson correlation coefficients for IRT-based and reference (oral) temperatures were found to vary strongly with measurement location. Approaches based on maximum temperatures in either inner canthi or full-face regions indicated stronger discrimination ability than maximum forehead temperature (AUC values of 0.95 to 0.97 versus 0.86 to 0.87, respectively) and other specific facial locations. These values are markedly better than the vast majority of results found in prior human studies of IRT-based fever screening.
Conclusion: Our findings provide clinical confirmation of the utility of consensus approaches for fever screening, including the use of inner canthi temperatures, while also indicating that full-face maximum temperatures may provide an effective alternate approach.
KEYWORDS: Infrared radiation, Black bodies, Body temperature, Minimum resolvable temperature difference, Data processing, Image acquisition, Standards development
Infrared thermographs (IRTs) have been implemented for mass fever screening in public areas such as airports during outbreaks of infectious disease pandemics such as Ebola virus disease, yet the approach has not been entirely successful. There has been increasing evidence in the literature that IRTs can provide greater accuracy in estimating core body temperature, if qualified systems are used and appropriate procedures are consistently applied. In this study, we addressed the issue of system qualification by implementing and evaluating a battery of test methods for objective and quantitative performance assessment of two commercial IRTs based on a recent international standard (IEC 80601-2-59). We evaluated stability and drift, image uniformity, minimum resolvable temperature difference, and measurement accuracy of the IRTs and illustrated how experimental and data processing procedures affect results. For instance, we demonstrated that offset temperature compensation, achieved using an external blackbody, is essential to meet the standard’s recommendations for temperature drift and stability. Additionally, we identified methods that can be implemented to optimize IRT evaluation. As an example, we identified a less burdensome approach to characterize image uniformity with a single image acquisition of a uniform blackbody. Overall, the insights into thermograph standardization and acquisition methods provided by this study may improve the utility of this technology and aid in comparing IRT performance, thus improving the potential for high quality disease pandemic countermeasures.
Infrared thermography (IRT) – a non-contact, non-invasive technique – has been used for mass screenings to identify febrile individuals at transportation nodes (e.g., airports) during infectious disease pandemics such as SARS (Severe Acute Respiratory Syndrome), H1N1 virus, and Ebola outbreaks. Despite the potential of IRTs, the field lacks a well-established consensus methodology to ensure temperature measurement accuracy and reliability. This study aims to investigate the use of IRTs in a controlled setting to determine the effectiveness of IRT and the most reliable facial region for estimation of core temperature. We conducted a large clinical study, acquiring facial thermographs of 1,109 febrile and non-febrile subjects using Screening Thermographs (STs). Regression analyses between the reference oral temperature and different areas of the face, specifically the forehead and canthi, were carried out. The coefficients of determination of each regression were compared to determine how well facial and core body temperatures were correlated. Receiver operating characteristic (ROC) curves were constructed to compare the effectiveness of using different facial areas to identify febrile patients. Results show that the maximum temperature of the overall face has the best linear trend, followed by the maximum temperature at the inner canthus region. Both of these values show better correlations than forehead temperatures, which are commonly used as a target by non-contact infrared thermometers. For any chosen facial area, the maximum temperature collected always showed a stronger correlation than a specific point in that area. Results indicate that IRT performance is substantially approved when applying optimal measurement methodology.
Mobile phone cameras employ sensors with near-infrared (NIR) sensitivity, yet this capability has not been exploited for biomedical purposes. Removing the IR-blocking filter from a phone-based camera opens the door to a wide range of techniques and applications for inexpensive, point-of-care biophotonic imaging and sensing. This study provides proof of principle for one of these modalities – phone-based NIR fluorescence imaging. An imaging system was assembled using a 780 nm light source along with excitation and emission filters with 800 nm and 825 nm cut-off wavelengths, respectively. Indocyanine green (ICG) was used as an NIR fluorescence contrast agent in an ex vivo rodent model, a resolution test target and a 3D-printed, tissue-simulating vascular phantom. Raw and processed images for red, green and blue pixel channels were analyzed for quantitative evaluation of fundamental performance characteristics including spectral sensitivity, detection linearity and spatial resolution. Mobile phone results were compared with a scientific CCD. The spatial resolution of CCD system was consistently superior to the phone, and green phone camera pixels showed better resolution than blue or green channels. The CCD exhibited similar sensitivity as processed red and blue pixels channels, yet a greater degree of detection linearity. Raw phone pixel data showed lower sensitivity but greater linearity than processed data. Overall, both qualitative and quantitative results provided strong evidence of the potential of phone-based NIR imaging, which may lead to a wide range of applications from cancer detection to glucose sensing.
Near-infrared fluorescence (NIRF) imaging has gained much attention as a clinical method for enhancing visualization of cancers, perfusion and biological structures in surgical applications where a fluorescent dye is monitored by an imaging system. In order to address the emerging need for standardization of this innovative technology, it is necessary to develop and validate test methods suitable for objective, quantitative assessment of device performance. Towards this goal, we develop target-based test methods and investigate best practices for key NIRF imaging system performance characteristics including spatial resolution, depth of field and sensitivity. Characterization of fluorescence properties was performed by generating excitation-emission matrix properties of indocyanine green and quantum dots in biological solutions and matrix materials. A turbid, fluorophore-doped target was used, along with a resolution target for assessing image sharpness. Multi-well plates filled with either liquid or solid targets were generated to explore best practices for evaluating detection sensitivity. Overall, our results demonstrate the utility of objective, quantitative, target-based testing approaches as well as the need to consider a wide range of factors in establishing standardized approaches for NIRF imaging system performance.
Thermal modalities represent the only currently viable mass fever screening approach for outbreaks of infectious disease pandemics such as Ebola and SARS. Non-contact infrared thermometers (NCITs) and infrared thermographs (IRTs) have been previously used for mass fever screening in transportation hubs such as airports to reduce the spread of disease. While NCITs remain a more popular choice for fever screening in the field and at fixed locations, there has been increasing evidence in the literature that IRTs can provide greater accuracy in estimating core body temperature if appropriate measurement practices are applied – including the use of technically suitable thermographs. Therefore, the purpose of this study was to develop a battery of evaluation test methods for standardized, objective and quantitative assessment of thermograph performance characteristics critical to assessing suitability for clinical use. These factors include stability, drift, uniformity, minimum resolvable temperature difference, and accuracy. Two commercial IRT models were characterized. An external temperature reference source with high temperature accuracy was utilized as part of the screening thermograph. Results showed that both IRTs are relatively accurate and stable (<1% error of reading with stability of ±0.05°C). Overall, results of this study may facilitate development of standardized consensus test methods to enable consistent and accurate use of IRTs for fever screening.
Fever screening based on infrared thermographs (IRTs) is a viable mass screening approach during infectious disease pandemics, such as Ebola and Severe Acute Respiratory Syndrome (SARS), for temperature monitoring in public places like hospitals and airports. IRTs have been found to be powerful, quick and non-invasive methods for detecting elevated temperatures. Moreover, regions medially adjacent to the inner canthi (called the canthi regions in this paper) are preferred sites for fever screening. Accurate localization of the canthi regions can be achieved through multi-modality registration of infrared (IR) and white-light images. Here we propose a registration method through a coarse-fine registration strategy using different registration models based on landmarks and edge detection on eye contours. We have evaluated the registration accuracy to be within ± 2.7 mm, which enables accurate localization of the canthi regions.
With improved diagnostic capabilities and complex optical designs, endoscopic technologies are advancing. As one of the several important optical performance characteristics, geometric distortion can negatively affect size estimation and feature identification related diagnosis. Therefore, a quantitative and simple distortion evaluation method is imperative for both the endoscopic industry and the medical device regulatory agent. However, no such method is available yet. While the image correction techniques are rather mature, they heavily depend on computational power to process multidimensional image data based on complex mathematical model, i.e., difficult to understand. Some commonly used distortion evaluation methods, such as the picture height distortion (DPH) or radial distortion (DRAD), are either too simple to accurately describe the distortion or subject to the error of deriving a reference image. We developed the basic local magnification (ML) method to evaluate endoscope distortion. Based on the method, we also developed ways to calculate DPH and DRAD. The method overcomes the aforementioned limitations, has clear physical meaning in the whole field of view, and can facilitate lesion size estimation during diagnosis. Most importantly, the method can facilitate endoscopic technology to market and potentially be adopted in an international endoscope standard.
Photoacoustic Tomography (PAT) employs high-power near-infrared (near-IR) laser pulses to generate structural and functional information on tissue chromophores up to several centimeters below the surface. Such insights may facilitate detection of breast cancer – the most common cancer in women. PAT mammography has been the subject of extensive research, including techniques based on exogenous agents for PAT contrast enhancement and molecular specificity. However, photothermal safety risks of PAT due to strong chromophores such as epidermal melanin and plasmonic nanoparticles have not been rigorously studied. We have used computational and experimental approaches to elucidate highly dynamic optical-thermal processes during PAT. A Monte Carlo model was used to simulate light propagation at 800 and 1064 nm in a multi-layer breast tissue geometry with different epidermal pigmentation levels and a tumorsimulating inclusion incorporating nanoparticles. Energy deposition results were then used in a bioheat transfer model to simulate temperature transients. Experimental measurements involved multi-layer hydrogel phantoms with inclusions incorporating gold nanoparticles. Phantom optical properties were measured using the inverse adding-doubling technique. Thermal imaging was performed as phantoms were irradiated with 5 ns near-IR pulses. Scenarios using 10 Hz laser irradiation of breast tissue containing various nanoparticle concentrations were implemented experimentally and computationally. Laser exposure levels were based on ANSI/IEC limits. Surface temperature measurements were compared to corresponding simulation data. In general, the effect of highly pigmented skin on temperature rise was significant, whereas unexpectedly small levels of temperature rise during nanoparticle irradiation were attributed to rapid photodegradation. Results provide key initial insights into light-tissue interactions impacting the safety and effectiveness of PAT.
Increasing numbers of devices are emerging which involve biophotonic imaging on a mobile platform. Therefore, effective test methods are needed to ensure that these devices provide a high level of image quality. We have developed novel phantoms for performance assessment of near infrared fluorescence (NIRF) imaging devices. Resin molding and 3D printing techniques were applied for phantom fabrication. Comparisons between two imaging approaches – a CCD-based scientific camera and an NIR-enabled mobile phone – were made based on evaluation of the contrast transfer function and penetration depth. Optical properties of the phantoms were evaluated, including absorption and scattering spectra and fluorescence excitation-emission matrices. The potential viability of contrastenhanced biological NIRF imaging with a mobile phone is demonstrated, and color-channel-specific variations in image quality are documented. Our results provide evidence of the utility of novel phantom-based test methods for quantifying image quality in emerging NIRF devices.
Endoscopy is a well-established paradigm in medical imaging, and emerging endoscopic technologies such as high resolution, capsule and disposable endoscopes promise significant improvements in effectiveness, as well as patient safety and acceptance of endoscopy. However, the field lacks practical standardized test methods to evaluate key optical performance characteristics (OPCs), in particular the geometric distortion caused by fisheye lens effects in clinical endoscopic systems. As a result, it has been difficult to evaluate an endoscope’s image quality or assess its changes over time. The goal of this work was to identify optimal techniques for objective, quantitative characterization of distortion that are effective and not burdensome. Specifically, distortion measurements from a commercially available distortion evaluation/correction software package were compared with a custom algorithm based on a local magnification (ML) approach. Measurements were performed using a clinical gastroscope to image square grid targets. Recorded images were analyzed with the ML approach and the commercial software where the results were used to obtain corrected images. Corrected images based on the ML approach and the software were compared. The study showed that the ML method could assess distortion patterns more accurately than the commercial software. Overall, the development of standardized test methods for characterizing distortion and other OPCs will facilitate development, clinical translation, manufacturing quality and assurance of performance during clinical use of endoscopic technologies.
Photoacoustic imaging (PAI) has grown rapidly as a biomedical imaging technique in recent years, with key applications in cancer diagnosis and oximetry. In spite of these advances, the literature provides little insight into thermal tissue interactions involved in PAI. To elucidate these basic phenomena, we have developed, validated, and implemented a three-dimensional numerical model of tissue photothermal (PT) response to repetitive laser pulses. The model calculates energy deposition, fluence distributions, transient temperature and damage profiles in breast tissue with blood vessels and generalized perfusion. A parametric evaluation of these outputs vs. vessel diameter and depth, optical beam diameter, wavelength, and irradiance, was performed. For a constant radiant exposure level, increasing beam diameter led to a significant increase in subsurface heat generation rate. Increasing vessel diameter resulted in two competing effects – reduced mean energy deposition in the vessel due to light attenuation and greater thermal superpositioning due to reduced thermal relaxation. Maximum temperatures occurred either at the surface or in subsurface regions of the dermis, depending on vessel geometry and position. Results are discussed in terms of established exposure limits and levels used in prior studies. While additional experimental and numerical study is needed, numerical modeling represents a powerful tool for elucidating the effect of PA imaging devices on biological tissue.
Technological advances in endoscopes, such as capsule, ultrathin and disposable devices, promise significant
improvements in safety, clinical effectiveness and patient acceptance. Unfortunately, the industry lacks test methods for
preclinical evaluation of key optical performance characteristics (OPCs) of endoscopic devices that are quantitative,
objective and well-validated. As a result, it is difficult for researchers and developers to compare image quality and
evaluate equivalence to, or improvement upon, prior technologies. While endoscope OPCs include resolution, field of
view, and depth of field, among others, our focus in this paper is geometric image distortion. We reviewed specific test
methods for distortion and then developed an objective, quantitative test method based on well-defined experimental and
data processing steps to evaluate radial distortion in the full field of view of an endoscopic imaging system. Our
measurements and analyses showed that a second-degree polynomial equation could well describe the radial distortion
curve of a traditional endoscope. The distortion evaluation method was effective for correcting the image and can be
used to explain other widely accepted evaluation methods such as picture height distortion. Development of consensus
standards based on promising test methods for image quality assessment, such as the method studied here, will facilitate
clinical implementation of innovative endoscopic devices.
KEYWORDS: Safety, Thermal modeling, Laser tissue interaction, Tissue optics, Skin, Optical simulations, Photoacoustic spectroscopy, Monte Carlo methods, Data modeling, Breast
Medical diagnostic devices based on photoacoustics represent an emerging area with significant potential for evaluation of brain injury and chemical agent exposure, as well as detection of pandemic diseases and cancer. However, few studies have addressed photothermal safety of these devices which emit high-power laser pulses to generate rapid, selective, yet non-destructive heating of subsurface structures. Towards elucidation of laser-tissue interactions and factors of safety for photothermal injury, we have developed a three-dimensional numerical model including light propagation, heat transfer and thermal damage algorithms. Literature surveys were performed to identify appropriate optical properties and the range of device exposure levels implemented in prior in vivo studies. Initial simulations provided model validation against results from the literature. Simulations were then performed based on breast tissue with discrete blood vessels irradiated by a train of laser pulses (10 Hz) at 800 and 1064 nm. For a constant exposure level, increasing beam diameter from 0.2 to 2.0 cm led to a factor of 2.5 increase in subsurface heat generation rates. Our preliminary modeling results indicate that for a 10 second tissue exposure under standard photoacoustic imaging conditions, irradiance-based safety limits should provide a factor of safety of 6 or greater over exposure levels that induce thermal coagulation. Opticalthermal modeling represents a powerful tool for elucidating photothermal effects relevant to the safety and effectiveness of photoacoustic systems.
Spectral variations in contrast enhancement of mucosal vasculature are a key feature of narrow band imaging (NBI) devices. In prior NBI studies, the enhanced visualization of larger, deeper vessels with green light (e.g., 540 nm) relative to violet light (e.g., 415 nm) has often been attributed to the well-known monotonic decrease in scattering coefficient with wavelength in biological tissues. We have developed and implemented numerical and experimental approaches to elucidate and quantify this and other light-tissue interaction effects relevant to NBI. A Monte Carlo model incorporating vessel-like inclusions with a range of diameters (20 to 400 microns) and depths (20 to 400 microns) was used to predict reflectance and fluence distributions in the tissue and calculate vessel contrast values. These results were compared to experimental measurements based on a liquid phantom with a hemoglobin-filled capillary. By comparing results for cases representing mucosa regions with and without blood, we were able to evaluate the relative significance of absorption and scattering on spectral variations in depth-selectivity. Results indicate that at 415 nm, detection of superficial vasculature with NBI was almost entirely dependent on the absorption coefficient of the blood in the vessel of interest. The enhanced visualization of deep vessels at 540 nm bands relative to 415 nm was due primarily to absorption by the superficial vasculature rather than a decrease in scattering coefficient. While computationally intensive, our numerical modeling approach provides unique insights into the light propagation mechanisms underlying this emerging clinical imaging technology.
Light-tissue interactions that influence vascular contrast enhancement in narrow band imaging (NBI) have not been the subject of extensive theoretical study. In order to elucidate relevant mechanisms in a systematic and quantitative manner we have developed and validated a Monte Carlo model of NBI and used it to study the effect of device and tissue parameters, specifically, imaging wavelength (415 versus 540 nm) and vessel diameter and depth. Simulations provided quantitative predictions of contrast-including up to 125% improvement in small, superficial vessel contrast for 415 over 540 nm. Our findings indicated that absorption rather than scattering-the mechanism often cited in prior studies-was the dominant factor behind spectral variations in vessel depth-selectivity. Narrow-band images of a tissue-simulating phantom showed good agreement in terms of trends and quantitative values. Numerical modeling represents a powerful tool for elucidating the factors that affect the performance of spectral imaging approaches such as NBI.
Narrow-band imaging (NBI) is a spectrally-selective reflectance imaging technique for enhanced visualization of
superficial vasculature. Prior clinical studies have indicated NBI's potential for detection of vasculature abnormalities
associated with gastrointestinal mucosal neoplasia. While the basic mechanisms behind the increased vessel contrast - hemoglobin absorption and tissue scattering - are known, a quantitative understanding of the effect of tissue and device
parameters has not been achieved. In this investigation, we developed and implemented a numerical model of light
propagation that simulates NBI reflectance distributions. This was accomplished by incorporating mucosal tissue layers
and vessel-like structures in a voxel-based Monte Carlo algorithm. Epithelial and mucosal layers as well as blood vessels
were defined using wavelength-specific optical properties. The model was implemented to calculate reflectance
distributions and vessel contrast values as a function of vessel depth (0.05 to 0.50 mm) and diameter (0.01 to 0.10 mm).
These relationships were determined for NBI wavelengths of 410 nm and 540 nm, as well as broadband illumination
common to standard endoscopic imaging. The effects of illumination bandwidth on vessel contrast were also simulated.
Our results provide a quantitative analysis of the effect of absorption and scattering on vessel contrast. Additional
insights and potential approaches for improving NBI system contrast are discussed.
Narrow band imaging (NBI) is a spectrally-selective reflectance imaging technique that is used as an adjunctive
approach to endoscopic detection of mucosal abnormalities such as neoplastic lesions. While numerous clinical studies
in tissue sites such as the esophagus, oral cavity and lung indicate the efficacy of this approach, it is not well
theoretically understood. In this study, we performed Monte Carlo simulations to elucidate the factors that affect NBI
device performance. The model geometry involved a two-layer turbid medium based on mucosal tissue optical
properties and embedded cylindrical, blood-filled vessels at varying diameters and depths. Specifically, we studied the
effect of bandpass filters (415±15 nm, 540±10 nm versus white light), blood vessel diameter (20-400 μm) and depth (30
- 450 μm), wavelength, and bandwidth on vessel contrast. Our results provide a quantitative evaluation of the two
mechanisms that are commonly believed to be the primary components of NBI: (i) the increased contrast provided by
high hemoglobin absorption and (ii) increase in the penetration depth produced by the decrease in scattering with
increasing wavelength. Our MC model can provide novel, quantitative insight into NBI, may lead to improvements in its
performance.
Quantitative data on the fundamental optical properties (OPs) of biological tissue, including absorption and reduced
scattering coefficients are important for elucidating light propagation during optical spectroscopy and facilitating
diagnostic device design and optimization, and may enable rapid detection of early neoplasia. However, systems for in
situ broadband measurement of mucosal tissue OPs in the ultraviolet-visible range have not been realized. In this study,
we evaluated a fiberoptic-based reflectance system, coupled with neural network inverse models (trained with Monte
Carlo simulation data), for measuring OPs in highly attenuating, two-layer turbid media. The experimental system
incorporated a broadband light source, a fiberoptic probe and a CCD camera. The calibration method involved a set of
standard nigrosin-microsphere phantoms as well as a more permanent spectralon phantom for quality assurance testing
and recalibration. The system was experimentally evaluated using two-layer hydrogel phantoms with hemoglobin and
polystyrene microspheres. The effects of tissue top-layer thickness and fitting approaches based on known absorption
and scattering distributions were discussed. With our method, measurements with error less than 28% were obtained in
the wavelength range of 350-630 nm.
We have investigated a potential technique based on spatially resolved reflectance to determine optical properties
(OPs) in two-layer turbid media. Reflectance from two-layer tissue was simulated for a wide range of OP combinations
(μa = 1-22.5, μs' = 5-42.5 cm-1) using a condensed Monte Carlo (MC) model and utilized to train neural network (NN)
inverse models. Experimental data from two-layer tissue phantoms with top layer thicknesses (D) ranging from 0.22 to
0.66 mm were collected at three UV-Vis wavelengths. Estimation accuracy was compared to simulated results with
added noise. The mean error in experimental determination of μa ranged from 1.5 to 5.9 cm-1 and mean error for μs'
ranged from 2.1 to 12.1 cm-1 as a function of D. Although numerous realistic challenges remain, this initial experimental
study of an unconstrained two-layer diffuse reflectance based OP estimation approach provides support that such a
technique has a strong potential to provide accurate in situ measurements in layered tissues.
KEYWORDS: Monte Carlo methods, Optical fibers, Optical properties, Reflectivity, Tissue optics, Neural networks, Tissues, Data modeling, Computer simulations, Photon transport
In order to elucidate light propagation mechanisms involved in optical spectroscopy devices, the optical properties of layered mucosal tissues at ultraviolet and visible wavelengths are needed. Previous approaches to measuring this data have typically been based on spatially-resolved reflectance. However, these approaches have limitations, some of which are not well understood. Therefore, the objectives of this study were (1) to elucidate the relationship between spatially-resolved reflectance distributions and optical properties in two-layer tissue models and (2) introduce and assess an unconstrained approach to optical property measurement. The first part of this study involved calculating reflectance from two-layer tissue for a wide variety of optical property combinations (πa = 1-22.5, πs' = 5-42.5 cm-1) using a Monte Carlo scaling technique. In the second part, a neural network inverse model trained with the aforementioned results was evaluated using simulated reflectance data. This relationship between optical properties and reflectance provides fundamental insights into the strengths, weaknesses and potential limitations of strategies for optical property measurement based on spatially-resolved reflectance. The neural network approach estimated optical property values with a degree of accuracy that depended on the probe geometry (5-, 6-, 10- and 11-fiber probes were simulated). The average error in determination of πa ranged from 15 to 51% and average error for πs' ranged from 8 to 32%. While computationally expensive to develop, neural network models calibrated with simulation data may prove to be a highly effective approach for rapid, unconstrained estimation of the optical properties of two-layer tissues.
Tissue optical properties at ultraviolet A (UVA) and visible (VIS) wavelengths are needed to elucidate light-tissue
interaction effects and optimize design parameters for spectroscopy-based neoplasia detection devices. Toward the goal
of accurate and useful in vivo measurements, we have constructed and evaluated a system for optical property
measurement at UVA-VIS wavelengths. Our approach involves a neural network-based inverse model calibrated with
reflectance datasets simulated using a condensed Monte Carlo approach with absorption coefficients as high as 80 cm-1
and reduced scattering coefficients as high as 70 cm-1. Optical properties can be predicted with the inverse model based
on spatially resolved reflectance measured with a fiberoptic probe. Theoretical evaluation of the inverse model was
performed using simulated reflectance distributions at random optical properties. Experimental evaluation involved the
use of tissue phantoms constructed from bovine hemoglobin and polystyrene microspheres. An average accuracy of
±1.0 cm-1 for absorption coefficients and ±2.7 cm-1 for reduced scattering coefficients was found from realistic phantoms
at five UVA-VIS wavelengths. While accounting for the very high attenuation levels near the 415 nm Soret absorption
band required some modifications, our findings provide evidence that the current approach produces useful data over a
wide range of optical properties, and should be particularly useful for in vivo characterization of highly attenuating
biological tissues.
The lowest detectable fluorescence signal level from biomedical specimens have been determined using a spectrometer, cooled CCD detector, and PIN photodiode with 365 nm UV LED light excitation. The data indicates the PIN photodiodes have adequate sensitivity for detection of tissue fluorescence with a sufficient signal-to-noise ratio. This data is being used to design a "pill-sized" Compact Photonics Explorer (CPE) for in vivo cancer optical diagnostics.
Second harmonic and two photon fluorescence generated by focused 10 fs ultrashort laser pulses were used to image subsurface structures of highly scattering biological tissues. By scanning the focal point of the incident laser beam, 2D sectional images of the tissues subsurface structure properties, such as symmetry and molecular compositions are obtained.
Significant narrowing of both the spectral and temporal profiles of emission radiation from optically pumped dyes was observed in discrete and continuously disordered media such as dilute colloidal dye solutions and densely packed forms of sandy powders and animal tissue treated with rhodamine 640 dye solution. The narrowing of the spectral and temporal response is attributed to laser action arising from feedback of the emission radiation from the surrounding scattering walls into the photoexcited dye regions of the animal and
sandy colloidal disordered media.
An ultrafast optical deflector based on the induced nonlinear refractive index change of an area-modulation laser pulse for low-signal level optical A/D waveform converter has been designed. Using resonance enhanced Kerr nonlinear media, an optical deflector with the deflection time of approximately 1 ns to 100 ns with a 30-ps time resolution may be achieved for low- level input optical signals.
The spectral sub-structures of XPM were measured using a high resolution optical spectral analysis system. The observation is in good agreement with the theoretical prediction. The oscillatory behavior of the XPM spectrum is suggested to be used for a high-bit all-optical XPM A/D converter. With the advent of ultrashort laser sources, it has been established that a light pulse can be used to control the properties of its own or another light pulse by using optical nonlinear processes in a material. This important property can be utilized for the future generation of high-speed optical computations and communications. Over the past several years, the operation of ultrafast optical logic gated using picosecond Sagnac Kerr Interferometer switch and phase conjugation gates have been successfully demonstrated. The third order nonlinear process cross-phase-modulation (XPM), in which the phase of a weak signal pulse is modulated by the index change induced by a strong pump pulse to produce the spectral broadening, provides a novel method to modulated the frequency of optical pulses at THz speed. We propose the ultrafast all optical A/D converter using XPM and report on measurements on the fine spectral structures of the XPM spectra of picosecond laser pulses using high resolution grating spectral analysis system. The observed substructure of the oscillatory XPM spectrum is suggested to be used for increasing the accuracy of all optical XPM A/D converters.
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