Diffuse optical tomographic reconstruction using multifrequency data

We investigated the use of multifrequency diffuse optical tomography (MF-DOT) data for the reconstruction of the optical parameters. The experiments were performed in a 63 mm diameter cylindrical phantom containing a 15 mm diameter cylindrical object. Modulation frequencies ranging from 110 MHz to 280 MHz were used in the phantom experiments changing the contrast in absorption of the object with respect to the phantom while keeping the scattering value the same. The diffusion equation was solved using the finite element method. The sensitivity information from each frequency was combined to form a single Jacobian. The inverse problem was solved iteratively by minimizing the difference between the measurements and forward problem using single and multiple modulation frequency data. A multiparameter Tikhonov scheme was used for regularization. The phantom results show that the peak absorption coefficient in a region of interest was obtained with an error less then 5% using two-frequency reconstruction for absorption contrast values up to 2.2 times higher than background and 10% for the absorption contrast values larger than 2.2. The use of two-frequency data is sufficient to improve the quantitative accuracy compared with the single frequency reconstruction with appropriate selection of these frequencies.

Diffuse optical imaging of the whole head

Near-Infrared Spectroscopy (NIRS) and diffuse optical imaging (DOI) are increasingly used to detect hemodynamic changes in the cerebral cortex induced by brain activity. Until recently, the small number of optodes in NIRS instruments has hampered measurement of optical signals from diverse brain regions. Our new DOI system has 32 detectors and 32 sources; by arranging them in a specific pattern, we can cover most of the adult head. With the increased number of optodes, we can collect optical data from prefrontal, sensorimotor, and visual cortices in both hemispheres simultaneously. We describe the system and report system characterization measurements on phantoms as well as on human subjects at rest and during visual, motor, and cognitive stimulation. Taking advantage of the system's larger number of sources and detectors, we explored the spatiotemporal patterns of physiological signals during rest. These physiological signals, arising from cardiac, respiratory, and blood-pressure modulations, interfere with measurement of the hemodynamic response to brain stimulation. Whole-head optical measurements, in addition to providing maps of multiple brain regions' responses to brain activation, will enable better understandings of the physiological signals, ultimately leading to better signal processing algorithms to distinguish physiological signal clutter from brain activation signals.

Noncontact backscatter-mode near-infrared time-resolved imaging system: Preliminary study for functional brain mapping

To improve the spatial resolution and to obtain the depth information of absorbers buried in highly scattering material, we developed a noncontact backscatter-mode near-infrared time-resolved imaging system (noncontact B-TRIS) that is intended for functional human brain mapping. It consists of mode-locked Ti-sapphire lasers as light sources and a charge-coupled device camera equipped with a time-resolved intensifier as a detector. The system was tested with a white polyacetal phantom as a light-scattering medium and black polyacetal particles as absorbers. Illumination and detection of light through an objective lens system (phi = 150 mm) enabled us to capture images from an area whose diameter is about 70 mm without coming into contact with it. The scattering and absorption coefficients of the white phantom obtained by B-TRIS were similar to those obtained by a conventional time-resolved spectroscopy. Although the imaged diameter of an absorber buried within a phantom was considerably larger than the actual diameter, the center position of the absorber coincided with the actual position with accuracy <2 mm. Furthermore, the depth information can be also detected by the noncontact B-TRIS. These results suggest a potential of noncontact B-TRIS for imaging cognitive human brain function.

Non-invasive measurement of cardiac output: Evaluation of new infrared absorption spectrometer

The mass spectrometer (MS) traditionally has been the instrument of choice for measuring cardiac output (Q (T)) non-invasively using the foreign gas uptake method. However, the size and cost of the MS has hampered widespread adoption of this technique outside of the laboratory. Here, we present results, from six normal human subjects at rest and during exercise, of simultaneous Q (T) measurements by an MS and a new, portable infrared (IR) device developed in our laboratories. These measurements are made using on the open-circuit acetylene uptake method. The IR device measures inspired and end-tidal concentrations of acetylene, sulfur hexafluoride, and carbon dioxide by IR absorption spectroscopy with a 10-90% response time of 43 ms; accurate measurements were made down to sample flow rates of 50 mL min(-1). Excellent correlation [Q (T)(IR)=0.98 Q (T)(MS), R(2)=0.94] was observed between instruments across the range from rest to heavy exercise. These results suggest that the IR device, which is small, light-weight, and rugged may enable the foreign gas uptake method to be used in clinical, field, and point-of-care settings for Q (T) measurement.

Pulse glucometry: A new approach for noninvasive blood glucose measurement using instantaneous differential near-infrared spectrophotometry

We describe a new optical method for noninvasive blood glucose (BGL) measurement. Optical methods are confounded by basal optical properties of tissues, especially water and other biochemical species, and by the very small glucose signal. We address these problems by using fast spectrophotometric analysis in a finger, deriving 100 transmittance spectra per second, to resolve optical spectra (900 to 1700 nm) of blood volume pulsations throughout the cardiac cycle. Difference spectra are calculated from the pulsatile signals, thereby eliminating the effects of bone, other tissues, and nonpulsatile blood. A partial least squares (PLS) model is used with the measured spectral data to predict BGL levels. Using glucose tolerance tests in 27 healthy volunteers, periodic optical measurements were made simultaneously with collection of blood samples for in vitro glucose analysis. Altogether, 603 paired data sets were obtained in all subjects and two-thirds of the data or of the subjects randomly selected were used for the PLS calibration model and the rest for the prediction. Bland-Altman and error-grid analyses of the predicted and measured BGL levels indicated clinically acceptable accuracy. We conclude that the new method, named pulse glucometry, has adequate performance for safe, noninvasive estimation of BGL.

Time-resolved diffusing wave spectroscopy applied to dynamic heterogeneity imaging

We report what is to our knowledge the first observation of a time-resolved diffusing wave spectroscopy (DWS) signal recorded by transillumination through a thick turbid medium: the DWS signal is measured for a fixed photon transit time, which opens the possibility of improving the spatial resolution. This technique could find biomedical applications, especially in mammography.

Modified Versus Producer Milk Calibration: Mid-Infrared Analyzer Performance Validation

Our objective was to determine the validation performance of mid-infrared (MIR) milk analyzers, using the traditional fixed-filter approach, when the instruments were calibrated with producer milk calibration samples vs. modified milk calibration samples. Ten MIR analyzers were calibrated using producer milk calibration sample sets, and 9 MIR milk analyzers were calibrated using modified milk sample sets. Three sets of 12 validation milk samples with all-laboratory mean chemistry reference values were tested during a 3-mo period. Calibration of MIR milk analyzers using modified milk increased the accuracy (i.e., better agreement with chemistry) and improved agreement between laboratories on validation milk samples compared with MIR analyzers calibrated with producer milk samples. Calibration of MIR analyzers using modified milk samples reduced overall mean Euclidian distance for all components for all 3 validation sets by at least 24% compared with MIR analyzers calibrated with producer milk sets. Calibration with modified milk sets reduced the average Euclidian distance from all-laboratory mean reference chemistry on validation samples by 40, 25, 36, and 27%, respectively for fat, anhydrous lactose, true protein, and total solids. Between-laboratory agreement was evaluated using reproducibility standard deviation (s(R)). The number of single Grubbs statistical outliers in the validation data was much higher (53 vs. 7) for the instruments calibrated with producer milk than for instruments calibrated with modified milk sets. The s(R) for instruments calibrated with producer milks (with statistical outliers removed) was similar to data collected in recent proficiency studies, whereas the s(R) for instruments calibrated with modified milks was lower than those calibrated with producer milks by 46, 52, 61, and 55%, respectively for fat, anhydrous lactose, true protein, and total solids.

Calibration of Infrared Milk Analyzers: Modified Milk Versus Producer Milk

Mid-infrared (MIR) milk analyzers are traditionally calibrated using sets of preserved raw individual producer milk samples. The goal of this study was to determine if the use of sets of preserved pasteurized modified milks improved calibration performance of MIR milk analyzers compared with calibration sets of producer milks. The preserved pasteurized modified milk sets exhibited more consistent day-to-day and set-to-set calibration slope and intercept values for all components compared with the preserved raw producer milk calibration sets. Pasteurized modified milk calibration samples achieved smaller confidence interval (CI) around the regression line (i.e., calibration uncertainty). Use of modified milk calibration sets with a larger component range, more even distribution of component concentrations within the ranges, and the lower correlation of fat and protein concentrations than producer milk calibration sets produced a smaller 95% CI for the regression line due to the elimination of moderate and high leverage samples. The CI for the producer calibration sets were about 2 to 12 times greater than the CI for the modified milk calibration sets, depending on the component. Modified milk calibration samples have the potential to produce MIR milk analyzer calibrations that will perform better in validation checks than producer milk-based calibrations by reducing the mean difference and standard deviation of the difference between instrument values and reference chemistry.

Infrared transmission spectrometry for the determination of urea in microliter sample volumes of blood plasma dialysates

Application of mid-infrared spectroscopy for the determination of urea in blood plasma dialysates of microliter sample volumes using a transmission microcell was investigated. Infrared spectra of the dialysates of plasma samples collected from 75 different patients using CMA 60 microdialysis catheters were evaluated with multivariate partial least squares regression. Using the absorbance spectral data from 1520-1420 cm(-1) and 1220-1120 cm(-1), a minimum standard error of prediction (SEP) of 0.88 mg/dL (0.14 mM) was achieved with spectral variable selection. Our findings suggest the feasibility of developing a mid-infrared sensor in combination with micro-fluidics for on-line monitoring of urea in patients undergoing dialysis treatment.

Cerebral oxygenation monitoring: near-infrared spectroscopy

Neurological complications during critical illness remain a frequent cause of morbidity and mortality. To date, monitors of cerebral function including electroencephalography, jugular bulb mixed venous oxygen saturation and transcranial Doppler, either require an invasive procedure and/or are not sensitive enough to effectively identify patients at risk for cerebral hypoxia. Near-infrared spectroscopy is a noninvasive device that uses infrared light, a technique similar to pulse oximetry, to penetrate living tissue and estimate brain tissue oxygenation by measuring the absorption of infrared light by tissue chromophores. The following article reviews the latest technology available to monitor cerebral oxygenation, near-infrared spectroscopy, its advantages and disadvantages, the currently available evidence-based medicine that demonstrates that this technology can identify deficits in cerebral oxygenation, and that monitoring such deficits allows for therapy to reverse cerebral oxygenation issues and thereby prevent long-term neurological sequelae.

Infrared Laser Spectroscopy of Imidazole Complexes in Helium Nanodroplets: Monomer, Dimer, and Binary Water Complexes

Infrared laser spectroscopy has been used to characterize imidazole (IM), imidazole dimer (IMD), and imidazole-water (IMW) binary systems formed in helium nanodroplets. The experimental results are compared with ab initio calculations reported here. Vibrational transition moment angles provide conclusive assignments for the various complexes studied here, including IM, one isomer of IMD, and two isomers of the IMW binary complexes.

Mapping bacterial surface population physiology in real-time: infrared spectroscopy of Proteus mirabilis swarm colonies

We mapped the space-time distribution of stationary and swarmer cells within a growing Proteus mirabilis colony by infrared (IR) microspectroscopy. Colony mapping was performed at different positions between the inoculum and the periphery with a discrete microscope-mounted IR sensor, while continuous monitoring at a fixed location over time used an optical fiber based IR-attenuated total reflection (ATR) sensor, or "optrode." Phenotypes within a single P. mirabilis population relied on identification of functional determinants (producing unique spectral signals) that reflect differences in macromolecular composition associated with cell differentiation. Inner swarm colony domains are spectrally homogeneous, having patterns similar to those produced by the inoculum. Outer domains composed of active swarmer cells exhibit spectra distinguishable at multiple wavelengths dominated by polysaccharides. Our real-time observations agree with and extend earlier reports indicating that motile swarmer cells are restricted to a narrow (approximately 3 mm) annulus at the colony edge. This study thus validates the use of an IR optrode for real-time and noninvasive monitoring of biofilms and other bacterial surface populations.

Optical Properties of Wounds: Diabetic Versus Healthy Tissue

Diffuse photon density wave (DPDW) methodology at Near Infrared frequencies has been used to calculate absorption and scattering from wounds of healthy and diabetic rats. The diffusion equation for semi-infinite media is being used for calculating the absorption and scattering coefficients based on measurements of phase and amplitude with a frequency domain device. Differences observed during the course of healing in the two populations can be correlated to the delayed healing observed in diabetics. These results are encouraging and further work will focus on the implementation of this device to the clinical setting as a monitoring tool in chronic diabetic wounds.

Hollow polycarbonate fiber for Er:YAG laser light delivery

We developed hollow fibers with polycarbonate (PC) capillaries for use as a supporting tube. The PC capillaries were prepared by using a glass-drawing technique. Hollow PC fibers are safer and more flexible than hollow glass fibers because no fragments are released when the fibers are broken in various applications. Inner coating layers of silver and cyclic olefin polymer (COP) enhanced the reflection rate at the Er:YAG laser light wavelength. Using these fibers, we attained low loss for Er:YAG laser light transmission. By adjusting the drawing temperature in the fabrication of the PC capillaries, we created a smooth inner surface and uniform PC capillaries. We also demonstrated low-loss properties for visible pilot beams.

Evaluation of MRI RF probes utilizing infrared sensors

The magnetic resonance imaging (MRI) coil's radio frequency (RF) field distribution has a strong effect on image quality as well as specific absorption rate. In this paper, a method of probing a coil's RF field distribution over any unoccupied region of the coil is presented. This technique is based on the use of infrared sensing. The proposed method was implemented and tested on a high field RF volume coil operating at 340 MHz. Very good agreement was achieved between the infrared measurements and numerical data obtained utilizing an in-house three-dimensional finite-difference time-domain package. The results demonstrate that the proposed technique is practical, robust, and efficient in making accurate measurements of the electric field distributions in loaded and unloaded MRI coils.

Lightweight biometric detection system for human classification using pyroelectric infrared detectors

We use pyroelectric detectors that are differential in nature to detect motion in humans by their heat emissions. Coded Fresnel lens arrays create boundaries that help to localize humans in space as well as to classify the nature of their motion. We design and implement a low-cost biometric tracking system by using off-the-shelf components. We demonstrate two classification methods by using data gathered from sensor clusters of dual-element pyroelectric detectors with coded Fresnel lens arrays. We propose two algorithms for person identification, a more generalized spectral clustering method and a more rigorous example that uses principal component regression to perform a blind classification.

[Progress in tissue optical imaging]

In this review, we introduce the basic principle and technology progress of tissue optical imaging from both diffuse optical imaging and coherence domain imaging, which include the continuous-wave imaging, time-resolved optical tomography, diffuse photon density waves tomography, ultrasound-modulated optical tomography, optical coherence tomography and laser speckle imaging. Applications of optical imaging in brain activity and tissue function are also discussed.

Construction of universal quantitative models for determination of roxithromycin and erythromycin ethylsuccinate in tablets from different manufacturers using near infrared reflectance spectroscopy

Universal quantitative models using NIR reflectance spectroscopy were developed for the analysis of API contents (active pharmaceutical ingredient) in roxithromycin and erythromycin ethylsuccinate tablets from different manufacturers in China. The two quantitative models were built from 78 batches of roxithromycin samples from 18 different manufacturers with the API content range from 19.5% to 73.9%, and 66 batches erythromycin ethylsuccinate tablets from 36 manufacturers with the API content range from 28.1% to 70.9%. Three different spectrometers were used for model construction in order to have robust and universal models. The root mean square errors of cross validation (RMSECV) and the root mean square errors of prediction (RMSEP) of the model for roxithromycin tablets were 1.84% and 1.45%, respectively. The values of RMSECV and RMSEP of the model for erythromycin ethylsuccinate tablets were 2.31% and 2.16%, respectively. Based on the ICH guidelines and characteristics of NIR spectroscopy, the quantitative models were then evaluated in terms of specificity, linearity, accuracy, precision, robustness and model transferability. Our study has shown that it is feasible to build a universal quantitative model for quick analysis of pharmaceutical products from different manufacturers. Therefore, the NIR method could be used as an effective method for quick, non-destructive inspection of medicines in the distribution channels or open market.

Measurement of nitrogen dioxide in cigarette smoke using quantum cascade tunable infrared laser differential absorption spectroscopy (TILDAS)

Although nitrogen dioxide (NO(2)) has been previously reported to be present in cigarette smoke, the concentration estimates were derived from kinetic calculations or from measurements of aged smoke, where NO(2) was formed some time after the puff was taken. The objective of this work was to use tunable infrared laser differential absorption spectroscopy (TILDAS) equipped with a quantum cascade (QC) laser to determine if NO(2) could be detected and quantified in a fresh puff of cigarette smoke. A temporal resolution of approximately 0.16s allowed measurements to be taken directly as the NO(2) was formed during the puff. Sidestream cigarette smoke was sampled to determine if NO(2) could be detected using TILDAS. Experiments were conducted using 2R4F Kentucky Reference cigarettes with and without a Cambridge filter pad. NO(2) was detected only in the lighting puff of whole mainstream smoke (without a Cambridge filter pad), with no NO(2) detected in the subsequent puffs. The measurement precision was approximately 1.0 ppbVHz(-1/2), which allows a detection limit of approximately 0.2 ng in a 35 ml puff volume. More NO(2) was generated in the lighting puff using a match or blue flame lighter (29+/-21 ng) than when using an electric lighter (9+/-3 ng). In the presence of a Cambridge filter pad, NO(2) was observed in the gas phase mainstream smoke for every puff (total of 200+/-30 ng/cigarette) and is most likely due to smoke chemistry taking place on the Cambridge filter pad during the smoke collection process. Nitrogen dioxide was observed continuously in the sidestream smoke starting with the lighting puff.

The removal of Cremophor® EL from paclitaxel for quantitative analysis by HPLC–UV

A novel method for analysis of hydrophobic drug molecules in matrices that contain Cremophor EL (CrEL) is presented. The method utilized a precipitation technique involving mercuric chloride in a reaction with CrEL to form an insoluble complex in an ethanol matrix. The hydrophobic drug molecule was then analyzed by HPLC-UV without interference from CrEL. Nuclear magnetic resonance and infrared spectroscopy indicated that the mechanism of precipitation involves the reaction of mercuric chloride with the ether bond of CrEL. Analysis by HPLC with UV detection of paclitaxel and related substances was used to verify that the reaction is specific toward CrEL.

New accessory for studies of isotopic 1H/2H exchange and biomolecular interactions using transmission infrared spectroscopy

We present here a new accessory for IR transmission measurements of 1H/2H exchange, as an ancillary tool for monitoring structural features of biomolecules in aqueous solution. This new accessory results from the combination of two dialysis membranes and a conventional liquid cell having two cylinders containing 2H2O buffer. When compared with conventional transmission measurements, carried out either after dissolving lyophilized biomolecules in 2H2O or after dialyzing the aqueous solution considered against 2H2O buffer, this accessory shows the following advantages: (1) controlled measurements over the initial steps of this isotopic exchange and absence of molecular aggregation, and (2) smaller sample amounts. This new Fourier transform IR cell can also be used to analyze ligand-biomolecule and drug-cell interactions.

Infrared up-converting phosphors for bioassays

The development of up-converting phosphor reporter particles has added a powerful tool to modern detection technologies. Carefully constructed phosphor reporters have core-shell structures with surface functional groups suitable for standard bio-conjugations. These reporters are chemically stable, possess the unique property of infrared up-conversion, and are readily detected. In contrast to conventional fluorescent reporters, up-converting phosphor particles do not bleach and allow permanent excitation with simultaneous signal integration. A large anti-Stokes shift (up to 500 nm) separates discrete emission peaks from the infrared excitation source. Along with the unmatched contrast in biological specimens due to the absence of autofluorescence upon infrared excitation, up-converting phosphor technology (UPT) has unique properties for highly-sensitive particle-based assays. The production and characteristics of UPT reporter particles as well as their application in various bioassays is reviewed.

Interfacing capillary gel microfluidic chips with infrared laser desorption mass spectrometry

We report on the fabrication and performance of a gel microfluidic chip interfaced to laser desorption/ionization (LDI) mass spectrometry with a time-of-flight mass analyzer. The chip was fabricated from poly(methylmethacrylate) with a poly(dimethyl siloxane) cover. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis was performed in the channel of the microfluidic chip. After electrophoresis, the cover was removed and either the PDMS chip or the PMMA cover was mounted in a modified MALDI ion source for analysis. Ions were formed by irradiating the channel with 2.95 microm radiation from a pulsed optical parametric oscillator (OPO), which is coincident with IR absorption by N-H and O-H stretch of the gel components. No matrix was added. The microfluidic chip design allowed a decrease in the volume of material required for analysis over conventional gel slabs, thus enabling improvement in the detection limit to a pmol level, a three orders of magnitude improvement over previous studies in which desorption was achieved from an excised section of a conventional gel.

Sampling soil-derived CO2 for analysis of isotopic composition: a comparison of different techniques

A new system for soil respiration measurement [P. Rochette, L.B. Flanagan, E.G. Gregorich. Separating soil respiration into plant and soil components using analyses of the natural abundance of carbon-13. Soil Sci. Soc. Am. J., 63, 1207-1213 (1999).] was modified in order to collect soil-derived CO2 for stable isotope analysis. The aim of this study was to assess the suitability of this modified soil respiration system to determine the isotopic composition (delta13C) of soil CO2 efflux and to measure, at the same time, the soil CO2 efflux rate, with the further advantage of collecting only one air sample. A comparison between different methods of air collection from the soil was carried out in a laboratory experiment. Our system, as well as the other dynamic chamber approach tested, appeared to sample the soil CO2, which is enriched with respect to the soil CO2 efflux, probably because of a mass dependent fractionation during diffusion and because of the atmospheric contribution in the upper soil layer. On the contrary, the static accumulation of CO2 into the chamber headspace allows sampling of delta13C-CO2 of soil CO2 efflux.