Near-infrared spectroscopy (NIRS) has been used for noninvasive monitoring of changes in tissue oxygen levels since the late 1970s and gathered a world-wide interest because of its wide-range applicability to both research and clinics.

Extension of optical monitoring of intact tissues from the visible and ultraviolet to the near-infrared (NIR) range (700–1300 nm) was first undertaken in 1977 for the purpose of monitoring the redox behavior of Cytochrome c oxidase (cyt c ox) in vivo. Soon it became evident that the much greater NIR translucency of skin and bone made it possible to reach brain and muscle tissue without surgical intervention. The presence of hemoglobin absorption led to complications forcing the construction of algorithms to separate the signals of the two molecular entities. It was also realized, however, that the hemoglobin signals provide information regarding the source of oxygen in the tissue, while the cyt c ox signals indicate the intracellular availability of oxygen for oxidative phosphorylation. This ability of recognizing the source/sink relationship greatly enhances the value of NIR spectrophotometry (NIRS) for research and clinical purposes.

Near infrared spectroscopy (NIRS) can monitor both the redox status of Cytochrome c oxidase located in the mitochondria within the cell and the oxygenation of the blood in the tissue being monitored. Since the enzyme catalyzes more than 90% of oxygen utilization, it is the sink for the oxygen while the hemoglobin in the capillaries is the oxygen source. In order to evaluate the oxidative metabolic status of a tissue the optical data obtained from both molecules are commonly interpreted on the basis of test tube experiments with purified preparations. We are concerned that the validity of this practice may not have been tested sufficiently and raise four basic questions that have not yet been answered. Citing some examples of in vitro versus in vivo differences we conclude that more effort should be expended on the in vivo testing of the range of the signals, their natural variability, and the physiological and pathological meaning of their deviations from norm.

Near-infrared spectroscopy is discussed from the viewpoint of human higher-order brain function analysis. Pioneering work in this field is reviewed; then we describe our concept of noninvasive trans-cranial dynamic optical topography and its instrumentation. Also, the validity of its functional images is assessed from both physical and physiological viewpoints. After confirming the validity of this method, we have applied it to a wide variety of fields such as clinical medicine, cognitive science, and linguistics in collaboration with researchers at several other institutes. Further application possibilities and the future of trans-cranial dynamic optical topography are also discussed.

We have developed a 24-channel simultaneous measurement system for optical topography that noninvasively obtains dynamic images of brain activity using near-infrared light. To evaluate the system performance, we utilized a dynamic phantom containing a rotating absorber in a cylindrical scattering medium. In this system, eight incident and eight detecting optical fibers are arranged alternately at square lattice points on the phantom. The phantom is illuminated with light of two wavelengths (780 and 830 nm) from each incident fiber. Reflected light is received by the detecting fibers, each of which is connected to an avalanche photodiode. Multiple light intensity modulation and lock-in detection are used to enable highly sensitive measurement with negligible cross talk for multichannel measurement. In the phantom measurement, we obtained topographic dynamic images of the absorber rotating in the medium with a temporal resolution of 0.5 s over a measurement area of 90 mmx90 mm.

The cerebral circulation and metabolism of ten preoperative cardiac surgery patients were assessed. Alterations in regional cerebral blood flow (rCBF), measured by 123I-N-isopropyl-p-iodo-amphetamine singlephoton emission computed tomography, and in cerebral oxygen metabolism, simultaneously detected by near-infrared spectroscopy (NIRS) before and after acetazolamide administration, were investigated. The rCBF (ml/min/100 g) increased significantly from 40.21±7.65 to 56.24±13.69 (p<0.001), and a significant increase in oxyhemoglobin (Oxy-Hb) of 13.9% (p=0.0022) and total hemoglobin (Total-Hb) of 5.7% (0.0047) along with a significant decrease in deoxyhemoglobin (Deoxy-Hb) of 8.9% (p=0.0414) were observed concomitantly. Thus, the Oxy-Hb/Total-Hb ratio (%Oxy-Hb) rose significantly from 67.26±9.82% to 72.98 ±8.09% (p=0.0022). Examination of the relationships between individual parameters showed that the percentage changes in rCBF and Oxy-Hb were significantly correlated (r=0.758, p=0.011). The percentage changes in rCBF and %Oxy-Hb were also correlated significantly (r=0.740, p=0.014). In conclusion, this evidence suggested that NIRS is able to detect relative changes in cerebral hemodynamics and reflect luxury perfusion induced by acetazolamide.

The severity of liver cirrhosis was related with the optical properties of liver tissue. Various grades of liver cirrhosis were produced in rats by intraperitoneal injection of thioacetamide (TAA) for different periods: 4 weeks, 8 weeks, 12 weeks, and 16 weeks. Optical properties of the liver, absorption coefficient (µa) and scattering coefficient (µs'), were measured by near-infrared time-resolved spectroscopy. Histological examination confirmed cirrhotic changes in the liver, which were more severe in rats with TAA administration for longer periods. The ma increased in 4- and 8-week rats, and then decreased in 12- and 16-week rats. The µa of blood-free liver decreased as liver cirrhosis progressed. The hemoglobin content in the liver calculated from the µa values increased in 4- and 8-week rats and decreased in 12- and 16-week rats. The µs' decreased in the cirrhotic liver, probably reflecting the decrease in the mitochondria content. It was shown that µa and µs' determination is useful to assess the severity of liver cirrhosis.

We describe a method to detect the presence of fluorophores in scattering media, including intralipid suspensions and chicken muscle covered with skin. The fluorophores were rhodamine 800 (Rh800) and indocyanine green (IcG), both of which can be excited at long wavelengths where there is minimal absorption by tissues. These fluorophores were dissolved in intralipid or in chicken muscle under skin. A method to approximate the fluorophore concentration in such samples was developed using a long lifetime reference fluorophore in a polymer film placed immediately on the illuminated surface of the sample. Because of the long lifetime of the reference film, the modulation of its emission at low frequencies near 2 MHz is near zero. Since the lifetime of Rh800 and IcG are below 2 ns the modulation of the combined emission is a measure of the intensity of the fluorophore (Rh800 or IcG) relative to the long lifetime reference. Using this method we were able to measure the concentration-dependent intensities of Rh800 and IcG in an intralipid suspension. Additionally, micromolar concentrations of these probes could be detected in chicken muscles, even when the muscle was covered with a layer of chicken skin. The presence of an India ink absorber in the intralipid had only a moderate effect on the modulation values. We suggest the use of this transdermal detection of longwavelength fluorophores as a noninvasive method to monitor patient compliance when taking medicines used for treatment of chronic diseases such as AIDS or tuberculosis.

We describe a new approach to glucose sensing using polarization measurements in the presence of a stretchoriented reference film. The method relies on measurement of the polarized emission from the reference film and of a fluorophore which changes intensity in response to glucose. A glucose-sensitive fluorescent signal was provided by the glucose/galactose binding protein from E. coli. This protein was labeled with an environmentally sensitive fluorophore at a single genetically inserted cysteine residue, and displayed decreased fluorescence upon glucose binding. Using the protein and the reference film we observed glucose-sensitive polarization values for micromolar glucose concentrations. This method of polarization-based sensing is generic and can be used for any sensing fluorophore which displays a change in intensity. In principle, one can construct simple and economical devices for this type of glucose measurement.

This paper presents a comparison of in vivo optical coherence tomography (OCT) captured cataract images to subsequent histopathological examination of the lenticular opacities. OCT imaging was performed on anesthetized Rhesus monkeys, known as the delayed effects colony (DEC), with documented cataracts. These monkeys were exposed to several types of radiation during the mid and late 1960s. The radiation and age related cataracts in these animals were closely monitored using a unique grading system developed specifically for the DEC. In addition to this system, a modified version of a common cataract grading scheme for use in humans was applied. Of the original 18 monkeys imaged, lenses were collected at necropsy from seven of these animals, processed, and compared to OCT images. Results showed a direct correlation between the vertical OCT images and the cataractous lesions seen on corresponding histopathologic sections of the lenses. Based on the images obtained and their corresponding documented comparison to histopathology, OCT showed tremendous potential to aid identification and characterization of cataracts. There can be artifactual problems with the images related to movement and shadows produced by opacities. However, with the advent of increased speed in imaging and multiplanar imaging, these disadvantages may easily be overcome.

A compact instrument is described that allows the measurement of the laser Doppler flow parameters, i.e., the velocity, the volume, and flow of blood in the foveal region of the human choroidal vascular system. This new device uses the optical principle of confocality for the delivery of the laser light to the site of measurement and heterodyne detection of the Doppler frequency shifted scattered light. Power of the incident light (785 nm) at the cornea is 90 µW. Measurements were obtained in both eyes of a group of 21 normal volunteers without pupil dilatation. We determined the intrasubject reproducibility and the minimum statistically significant detectable changes in the flow parameters for a group of 21 eyes (one in each subject). Linear correlations were also established between the flow parameters in the right and left eyes.

Photospallation is proposed as the primary mechanism behind our recent animal studies involving corneal ablation by nanosecond-pulse mid-IR laser beams. Following a brief summary of earlier work directed to refractive procedures in the mid-IR, a preliminary analysis is presented, based on simple one-dimensional models of thermoelastic expansion developed previously. The results of the analysis indicate that front surface spallation is consistent with the striking tissue ablation characteristics observed in our recent in vivo work with short pulse beams, including very small ablation rates and submicron thermal damage zones. This is attributed to the fact that spallation is a mechanical—rather than a thermal—mechanism, which allows tissue to be removed in small layers at fluences far lower than those used in the earlier corneal studies with mid-IR beams, typically under 200 mJ/cm2, resulting in minimal heating of tissue. Unlike prior work in the area of photospallation, we also suggest that the existing theoretical basis supports the use of nanosecond pulses as an effective approach to achieving controlled ablation in the presence of very high absorption. We further suggest that such domain of operation may be preferred over shorter pulses, both from a practical standpoint and to mitigate against potential damage from shock waves.

Visible-near infrared multispectral reflectance image sets were acquired from the dorsal surface of rats both before and after elevation of reversed McFarlane skin flaps. Raw images were dominated by uneven surface illumination and shadowing along with the variation associated with instrument response. These interfering features obscured variation associated with a change in tissue reflectance, which is related to the degree of flap perfusion. Logarithmic residual preprocessing followed by principal component analysis of multispectral images could clearly detect a difference in the optical properties between the base and distal section of the flap. The difference in the reflectance properties correlates with the varying degree of tissue perfusion. Principal component analysis detected this optical difference between the well-perfused base of the skin flap and the compromised distal section of the flap immediately following surgery. The first visual signs of compromised tissue perfusion appeared only 6 or more hours after surgery. The results from this study indicate that the application of principal component analysis to discrete wavelength near infrared multispectral reflectance images of skin flaps can effectively distinguish reflectance changes related to the degree of tissue perfusion immediately following surgical elevation of the reversed McFarlane skin flap.

Advances in optical coherence tomography (OCT) rely on the availability of broadband light that is spatially coherent. We present a technique to characterize coherence properties of broadband light using optical phasespace contours in transverse momentum and position. We demonstrate that these contour plots can be directly measured by a simple heterodyne imaging scheme possessing high dynamic range (130 dB) and 0.1 fW sensitivity (for mW input beams). These phase space distributions are shown to yield quantitative information on the longitudinal and transverse coherence and the wavefront curvature of the light beam. We apply this technique to characterize the light emitted by a novel high-power extended-bandwidth superluminescent diode (SLD) recently developed at the David Sarnoff Research Center. Its performance is compared to that of standard commercially available SLDs.

The obtainment of stable solutions of inverse problems for studying the disperse composition of suspensions using effects of elastic light scattering was discussed. Versions of a regularization of solving the inverse problems of the spectroturbidimetric method were considered, taking into account unavoidable restrictions on the scope of the necessary prior data for particles and on the width of the spectral interval for real biological disperse systems. Possibilities for increasing the number of particle parameters determined in a single optical experiment were analyzed. They were shown to be provided by the use of effects of the orientation ordering of a system on combination of capabilities of the methods of spectroturbidimetry and electro-optics, using bacterial cell suspensions as an example.