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We propose a non-invasive and non-contact imaging technique based on the diffuse reflectance spectroscopy to evaluate the concentrations of methemoglobin (metHb), oxygenated hemoglobin (HbO), and deoxygenated hemoglobin (HbR) simultaneously. Experiments with in vivo rat exposed to sodium nitrite (NaNO2) shows the feasibility of the method for monitoring changes in metHb, HbO, and HbR concentrations during methemoglobinemia.

Tissue survival depends on the hemoglobin in blood to deliver oxygen to support cellular respiration. Without oxygenated blood surrounding tissue dies. Oxygenated hemoglobin has a distinct visible – near infrared absorption spectrum compared to deoxygenated hemoglobin. A hand-held multispectral reflectance imaging device, SnapshotNIR, was designed to provide a measure of the relative attenuation of reflected light from oxygenated and deoxygenated hemoglobin. The device combines the use of near real-time image classification / segmentation and regression to effectively determine the ratio of oxygenated to deoxygenated hemoglobin in the superficial vascular bed being imaged. This measurement of hemoglobin oxygen saturation can help detect local deficits in oxygen delivery to tissue. This simple, hand-held, battery-powered imaging device can quickly survey large areas of tissue in a complete non-invasive fashion. It is easy to use and suitable for use in the operating room or clinics that have limited infrastructure. Examples are given on the performance of the device in general surgery and in the assessment of peripheral circulation.

We investigated a rapid imaging method to monitor the spatial distribution of total hemoglobin concentration (CHbT), the tissue oxygen saturation (StO2), and the scattering power b in the expression of musp=a(lambda)^-b as the scattering parameters in cerebral cortex using a digital red-green-blue camera. In the method, Monte Carlo simulation (MCS) for light transport in brain tissue is used to specify a relation among the RGB-values and the concentration of oxygenated hemoglobin (CHbO), that of deoxygenated hemoglobin (CHbR), and the scattering power b. The results in this study indicate potential of the method to evaluate the physiology and loss of tissue viability in biological tissues.

Two-thirds of out-of-hospital cardiac arrest patients, who survive to hospital admission, die in the hospital from neurological injuries related to cerebral hypoperfusion. Hyperspectral near infrared spectroscopy (hNIRS) is a noninvasive technique that measures the major chromophores in the brain, such as oxygenated hemoglobin, deoxygenated hemoglobin and cytochrome C oxidase ([CCO]), an intracellular marker of oxygen consumption. We have demonstrated that hNIRS is feasible and can detect changes in cerebral oxygenation and metabolism in patients undergoing transcatheter aortic valve insertion (TAVI) – a procedure that temporarily induces sudden hypotension and hypoperfusion that mimics cardiac arrest. Using multi-distance hNIRS, we found that while measured regional oxygen saturation (rSO2) changes resulted mainly from the extra-cerebral tissues, [CCO] changes during cardiac arrests occurred mainly in the brains of patients. We also applied the hNIRS algorithm based on the “2-layer model” to the data to measure cerebral oxygen saturation and [CCO] in patients during the procedure.

We investigated a rapid imaging method to monitor the spatial distribution of total hemoglobin concentration (CHbT), the tissue oxygen saturation (StO2), and the scattering power b in the expression of musp=a(lambda)^-b as the scattering parameters in cerebral cortex using a digital red-green-blue camera. In the method, Monte Carlo simulation (MCS) for light transport in brain tissue is used to specify a relation among the RGB-values and the concentration of oxygenated hemoglobin (CHbO), that of deoxygenated hemoglobin (CHbR), and the scattering power b. In the present study, we performed sequential recordings of RGB images of in vivo exposed brain of rats while changing the fraction of inspired oxygen (FiO2), using a surgical microscope camera system. The time courses of CHbO, CHbR, CHbT, and StO2 indicated the well-known physiological responses in cerebral cortex. On the other hand, a fast decrease in the scattering power b was observed immediately after the respiratory arrest, which is similar to the negative deflection of the extracellular DC potential so-called anoxic depolarization. It is said that the DC shift coincident with a rise in extracellular potassium and can evoke cell deformation generated by water movement between intracellular and extracellular compartments, and hence the light scattering by tissue. Therefore, the decrease in the scattering power b after the respiratory arrest is indicative of changes in light scattering by tissue. The results in this study indicate potential of the method to evaluate the pathophysiological conditions and loss of tissue viability in brain tissue.

Low level light/laser therapy (LLLT) is considered as a novel, non-invasive, and potential therapy in a variety of psychological and physical conditions, due to its effective intricate photobiomodulation. The mechanism of LLLT is that when cells are stimulated by photons, mitochondria produce a large quantity of ATP, which accelerates biochemical responses in the cell. It is of great significance to gain a clear insight into the change or interplay of various physiological parameters. In this study, we used functional near-infrared spectroscopy (fNIRS) and venous-occlusion plethysmography to measure the LLLT-induced changes in blood flow, oxygenation, and oxygen consumption in human forearms in vivo. Six healthy human participants (4 males and 2 females) were administered with 810-nm light emitted by LED array in ten minutes and blood flow, oxygenation and oxygen consumption were detected in the entire experiment. We found that LLLT induced an increase of blood flow and oxygen consumption on the treated site. Meanwhile, LLLT took a good role in promoting oxygenation of regional tissue, which was indicated by a significant increase of oxygenated hemoglobin concentration (Δ[HbO2]), a nearly invariable deoxygenated hemoglobin concentration (Δ[Hb]) and a increase of differential hemoglobin concentration (Δ[HbD] = Δ[HbO2] - Δ[Hb]). These results not only demonstrate enormous potential of LLLT, but help to figure out mechanisms of photobiomodulation.

To reveal hemolysis phenomena induced by a photosensitization reaction with its environment, we measured absorption spectrum of a blood sample to analyze hemoglobin oxidation and resolved oxygen desorption dynamics. The quartz glass cell with 1 mm optical path length was used as a cuvette. Red blood cell suspension medium of 0.625 hematocrit with 30 μg/ml talaporfin sodium was used as a sample. A red diode laser of 664 nm wavelength was emitted to the cuvette with 120 mW/cm2 in irradiance for 40 J/cm2. Absorption spectra of the sample were obtained before and after the photosensitization reaction by a spectrophotometer. Multiple regression analysis was employed to obtain concentrations of various hemoglobin species from measured absorption spectrum. Comparing to 0 and 40 J/cm2, methemoglobin and deoxygenated hemoglobin concentrations increased 0.19 g/dL and 0.02 g/dL, respectively. Oxygenated hemoglobin concentration decreased 0.17 g/dL. Oxygen environment could also be presented by oxygen pressure calculated from the concentrations of oxygenated hemoglobin and deoxygenated hemoglobin. These obtained hemoglobin concentration changes might indicate hemolysis progress and oxygen environment. We think this simple optical measurement could reveal both the hemolysis and oxygen environment.

We propose a rapid imaging method to monitor the spatial distribution of total hemoglobin concentration (CHbT), the tissue oxygen saturation, and the scattering power b in the expression of μs’=aλ-b as the scattering parameters in cerebral cortex using a digital red-green-blue camera. In the method, the RGB-values are converted into the tristimulus values in CIEXYZ color space which is compatible with the common RGB working spaces. Monte Carlo simulation (MCS) for light transport in tissue is used to specify a relation among the tristimulus XYZ-values and the concentration of oxygenated hemoglobin, that of deoxygenated hemoglobin, and the scattering power b. In the present study, we performed sequential recordings of RGB images of in vivo exposed rat brain during the cortical spreading depolarization evoked by the topical application of KCl. Changes in the total hemoglobin concentration and the tissue oxygen saturation imply the temporary change in cerebral blood flow during CSD. Decrease in the scattering power b was observed before the profound increase in the total hemoglobin concentration, which is indicative of the reversible morphological changes in brain tissue during CSD. The results in this study indicate potential of the method to evaluate the pathophysiological conditions in brain tissue with a digital red-green-blue camera.

Using near-infrared time-resolved spectroscopy (TRS), we measured the human head in transmittance mode to obtain the optical properties, tissue oxygenation, and hemodynamics of deep brain tissues in 50 healthy adult volunteers. The right ear canal was irradiated with 3-wavelengths of pulsed light (760, 795, and 835nm), and the photons passing through the human head were collected at the left ear canal. Optical signals with sufficient intensity could be obtained from 46 of the 50 volunteers. By analyzing the temporal profiles based on the photon diffusion theory, we successfully obtained absorption coefficients for each wavelength. The levels of oxygenated hemoglobin (HbO 2 ), deoxygenated hemoglobin (Hb), total hemoglobin (tHb), and tissue oxygen saturation (SO 2 ) were then determined by referring to the hemoglobin spectroscopic data. Compared with the SO 2 values for the forehead measurements in reflectance mode, the SO 2 values of the transmittance measurements of the human head were approximately 10% lower, and tHb values of the transmittance measurements were always lower than those of the forehead reflectance measurements. Moreover, the level of hemoglobin and the SO 2 were strongly correlated between the human head measurements in transmittance mode and the forehead measurements in the reflectance mode, respectively. These results demonstrated a potential application of this TRS system in examining deep brain tissues of humans.

Optical fiber spectroscopy is a versatile tool for measuring diffuse reflectance and extracting absorption information that can noninvasively quantify the presence of chromophores such as oxyhemoglobin and deoxy-hemoglobin in tissues. Cerebrovascular abnormalities were widely recognized in Alzheimer’s disease (AD) patients. We analyzed blood volume fraction and level of oxygenated hemoglobin in Tg6799 mice, which are transgenic mice expressing five different familial Alzheimer disease-associated mutations in the human amyloid precursor protein and presenilin-1 genes. Diffuse reflectance spectra were iteratively fit as weighted sums of oxy- and deoxy-hemoglobin. Our observations showed slightly hypoxic conditions and significantly increased blood volume in the Alzheimer’s mice versus wild type. These results suggest that hyperperfusion of our AD mice may be a compensating mechanism for impaired cerebral vascular function and somehow relevant with early stage of AD patients. Ongoing work focuses on developing a cannula fixture that allows measurement in awake, behaving animals.

The challenge of correctly identifying malaria infection continues to impede our efforts to control this disease. Recent studies report highly specific retinal changes in severe malaria patients; these retinal changes may represent a very useful diagnostic indicator for this disease. To further explore the ocular manifestations of malaria, we used hyperspectral imaging to study retinal changes caused by Plasmodium berghei ANKA parasitization in a mouse model. We collected the spectral reflectance of the ocular fundus from hyperspectral images of the mouse eye. The blood oxygen sensitive spectral region was normalized for variances in illumination, and used to calculate relative values that correspond to oxygenated hemoglobin levels. Oxygen hemoglobin levels are markedly lower in parasitized mice, indicating that hemoglobin digestion by P. berghei may be detected using spectral reflectance. Furthermore, the ocular reflectance of parasitized mice was abnormally elevated between 660nm and 750nm, suggesting fluorescence in this region. While the source of this fluorescence is not yet clear, its presence correlates strongly with P. Berghei parasitization, and may indicate the presence of hemozoin deposits in the retinal vasculature. The pathology of severe malaria still presents many questions for clinicians and scientists, and our understanding of cerebral malaria has been generally confined to clinical observation and postmortem examination. As the retina represents a portion of the central nervous system that can be easily examined noninvasively, our technique may provide the basis for an automated tool to detect and examine severe malaria via retinal changes.

We present a photoacoustic measurement system based on semiconductor lasers for blood oxygenation measurements. It permits to use four different optical wavelengths (650nm, 808nm, 850nm, 905nm) to generate photoacoustic signals. As the optical extinction coefficient of oxygenated hemoglobin and deoxygenated hemoglobin is different at specific wavelengths, a blood oxygenation measurement by a multi-wavelength photoacoustic laser system is feasible. Especially at 650nm, the clear difference between the extinction coefficients of the two hemoglobin derivates permits to determine the blood oxygenation in combination with other near infrared wavelengths. A linear model based on tabulated values of extinction coefficients for fully oxygenated and fully deoxygenated hemoglobin is presented. We used heparin stabilized whole porcine blood samples to model the optical behavior of human blood, as the optical absorption behavior of porcine hemoglobin does not differ significantly from human hemoglobin. To determine the real oxygen saturation values of the blood samples, we measured the partial oxygen pressure with an IRMA Trupoint Blood Analysis System. The oxygen saturation values were calculated from a dissociation curve for porcine blood. The results of the photoacoustic measurement are in qualitatively good agreement with the predicted linear model. Further, we analyze the abilities and the limitations of quantitative oxygenation measurements.

Diffuse Optical Tomography (DOT) is based on acquiring information from multiply scattered light which penetrates into the tissue up to depths of several centimeters. This technique allows for imaging of absorbing and scattering inclusions inside tissue and distinguishing between them after computer processing of an image. An experimental setup for multicolor frequency-domain diffuse optical tomography (FD DOT) to visualize neoplasia of breast tissue and to estimate its size has been created. A breast is scanned in the transilluminative configuration by a single source and detector pair. Illumination at three wavelengths (684 nm, 794 nm, and 850 nm) which correspond to different parts of the absorption spectrum provides information about concentration of the main absorbers (oxygenated hemoglobin, deoxygenated hemoglobin, and fat/water). Source amplitude modulation at 140 MHz increases spatial resolution and provides separate reconstruction of scattering and absorption coefficients. In vivo study of breast carcinoma has been performed. Maps of 2D distributions of reconstructed absorption and scattering coefficients and concentration of hemoglobin have been obtained. An increase of absorption and scattering coefficient, total hemoglobin concentration and decrease of blood oxygen saturation is observed in the tumor area in comparison with the surrounding tissue. We can conclude that FD DOT technique confirms a possibility of detecting neoplastic changes.

Near-infrared spectroscopy (NIRS) calculates hemoglobin parameters, such as oxygenated hemoglobin (oxyHb) and deoxygenated hemoglobin (deoxyHb) using the near-infrared light around the wavelength of 800nm. This is based on the modified-Lambert-Beer's law that changes in absorbance are proportional to changes in hemoglobin parameters. Many conventional measurement methods uses only a few wavelengths, however, in this research, basic examination of NIRS measurement was approached by acquiring wide range of wavelength information. Venous occlusion test was performed by using the blood pressure cuff around the upper arm. Pressure of 100mmHg was then applied for about 3 minutes. During the venous occlusion, the spectrum of the lower arm muscles was measured every 15 seconds, within the range of 600 to 1100nm. It was found that other wavelength bands hold information correlating to this venous occlusion task. Technique of improving the performance of NIRS measurement using the Spectroscopic Method is very important for Brain science.

Radio surgical interventions such as Gamma Knife and Cyberknife have become attractive as therapeutic interventions. However, one of the drawbacks of cyberknife is radionecrosis, which is caused by excessive radiation to surrounding normal tissues. Radionecrosis occurs in about 10-15% of cases and could have adverse effects leading to death. Currently available imaging techniques have failed to reliably distinguish radionecrosis from tumor growth. Development of imaging techniques that could provide distinction between tumor growth and radionecrosis would give us ability to monitor effects of radiation therapy non-invasively. This paper investigates the use of near infrared spectroscopy (NIRS) as a new technique to monitor the growth of brain tumors. Brain tumors (9L glioma cell line) were implanted in right caudate nucleus of rats (250-300 gms, Male Fisher C) through a guide screw. A new algorithm was developed, which used broadband steady-state reflectance measurements made using a single source-detector pair, to quantify absolute concentrations of hemoglobin derivatives and reduced scattering coefficients. Preliminary results from the brain tumors indicated decreases in oxygen saturation, oxygenated hemoglobin concentrations and increases in deoxygenated hemoglobin concentrations with tumor growth. The study demonstrates that NIRS technology could provide an efficient, noninvasive means of monitoring vascular oxygenation dynamics of brain tumors and further facilitate investigations of efficacy of tumor treatments.

Motivation: Early diagnosis of Alzheimer's disease (AD) is crucial because symptoms respond best to available treatments in the initial stages of the disease. Recent studies have shown that marked changes in brain oxygenation during mental and physical tasks can be used for noninvasive functional brain imaging to detect Alzheimer’s disease. The goal of our study is to explore the possibility of using near infrared spectroscopy (NIRS) and mapping (NIRM) as a diagnostic tool for AD before the onset of significant morphological changes in the brain. Methods: A 16-channel NIRS brain imager was used to noninvasively measure spatial and temporal changes in cerebral hemodynamics induced during verbal fluency task and physical activity. The experiments involved healthy subjects (n = 10) in the age range of 25±5 years. The NIRS signals were taken from the subjects' prefrontal cortex during the activities. Results and Conclusion: Trends of oxygenated and deoxygenated hemoglobin in the prefrontal cortex of the brain were observed. During the mental stimulation, the subjects showed significant increase in oxygenated hemoglobin [HbO 2 ] with a simultaneous decrease in deoxygenated hemoglobin [Hb]. However, physical exercise caused a rise in levels of HbO 2 with small variations in Hb. This study basically demonstrates that NIRM taken from the prefrontal cortex of the human brain is sensitive to both mental and physical tasks and holds potential to serve as a diagnostic means for early detection of Alzheimer's disease.

The goal of this study is to evaluate the feasibility of Near Infrared Spectroscopy (NIRS) as an in vivo monitoring tool for rat breast tumor oxygenation and vascular blood volume by comparison with the established modalities, magnetic resonance imaging/spectroscopy (MRI/MRS). The changes in oxygenated hemoglobin concentration and total hemoglobin concentration (Δ[HbO 2 ], Δ[Hb] total ) with respect to hyperoxic gas interventions were monitored by NIRS. Changes in deoxygenated hemoglobin, a blood oxygenation level dependent (BOLD) contrast, and blood volume on breast tumors were monitored by BOLD MRI and 19 F MRS of PFOB, respectively. Results showed strong consistency among the two pairs: Δ[HbO 2 ] versus BOLD signal, Δ[Hb] total versus tumor blood volume. These consistent results demonstrated the ability of NIRS as a valid in-vivo real time monitoring tool for studying the dynamic responses of Δ[HbO 2 ] and Δ[Hb] total to therapeutic interventions applied to rat breast tumors. Furthermore, the results suggested that NIRS and MRS are complimentary with each other in terms of temporal and spatial resolutions.

Three oxygen-sensitive parameters (arterial hemoglobin oxygen saturation SaO 2 , tumor vascular oxygenated hemoglobin concentration [HbO 2 ], and tumor oxygen tension pO 2 ) were measured simultaneously by three different optical techniques (pulse oximeter, near infrared spectroscopy, and FOXY) to evaluate dynamic responses of breast tumors to carbogen (5% CO 2 and 95% O 2 ) intervention. All three parameters displayed similar trends in dynamic response to carbogen challenge, but with different response times. These response times were quantified by the time constants of the exponential fitting curves, revealing the immediate and the fastest response from the arterial SaO 2 , followed by changes in global tumor vascular [HbO 2 ], and delayed responses for pO 2 . The consistency of the three oxygen-sensitive parameters demonstrated the ability of NIRS to monitor therapeutic interventions for rat breast tumors in-vivo in real time.

In a previous report, we showed that there was distinct heterogeneity in tumor oxygenation dynamics even within the same tumor monitored by F MR pO 2 mapping and near-infrared spectroscopy (NIRS). In this study, we applied a three-channel NIRS system to reveal dynamic heterogeneity of tumor vascular oxygenation during respiratory challenges. When the implanted tumors reached ~3 cm, three photo detectors were attached to the surface of the tumor in three different positions to measure changes in oxygenated hemoglobin concentration ([HbO 2 ]), while the inhaled gas was alternated between air and carbogen (95% O 2 + 5% CO 2 ). Significant changes in tumor oxygenation were observed at all three detectors accompanying the respiratory challenge, and these changes could be modeled with two exponential components with fast and slow time constants. Time constant, amplitude, vascular coefficient, and the ratio of perfusion rate were obtained from fitting curve of Δ[HbO 2 ]. These parameters revealed that there were indeed different responses of tumor vascular oxygenation during carbogen inhalation at the three different locations. These results clearly reveal that tumors are highly heterogeneous during vascular oxygenation in response to carbogen inhalation. This study demonstrates that the NIR technology can provide an efficient, real-time, non-invasive way for monitoring tumor physiology, and it may have prognostic value, providing insight into tumor vascular development and angiogenesis.

The effective measurement of dynamic changes of blood and tissue oxygenation of tumors could be valuable for optimizing tumor treatment plans. For this study, a near- infrared spectroscopy system and pO2 needle electrode were used to measure simultaneously changes in total hemoglobin concentration ([Hb]total), oxygenated hemoglobin concentration ([HbO2[) and local oxygen tension (pO2) in the vascular bed of prostate tumors implanted in rats in response to respiratory challenge. The inhaled gas was alternated between air and carbogen (95% oxygen, 5% CO2). Significant changes in tumor vascular oxygenation were observed with an apparent threshold for variation in [HbO2]/[HbO2]max. For comparison, a phantom study was undertaken with 1% intralipid solution and blood. The slope of [HbO2]/[HbO2[max vs. pO2 in the phantom was ten times larger than in the tumor indicating that tumor cells are relatively resistant to oxygenation. This study demonstrates that the NIR technology can provide an efficient, real-time, non-invasive approach to monitoring tumor physiology and is compatible with additional techniques.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Near infrared spectroscopy has recently been used to measure changes of optical parameters (i.e., light absorption or scattering) of brain tissue. The fact that the equipment is generally compact, portable, noninvasive, and reasonably prices makes it ideal for clinical and nonclinical evaluation and monitoring of brain function. Clinical and nonclinical studies evaluating changes related to light absorption are discussed, with an emphasis on cerebral blood oxygenation (CBO) changes and hemodynamic responses while performing cognitive tasks. With respect to the clinical studies, the focus is on variations in patterns of oxygenated hemoglobin (Oxy-Hb), deoxygentated hemoglobin (Deoxy-Hb) and Total-Hb (sum of Oxy-Hb and Deoxy-Hb). The studies about clinical applications includes research we have conducted with older adults and aphasics. Implications regarding the use of NIRS for clincal purposes are considered.

We report on an analysis of diffuse reflectance spectra measured in conjunction with the fluorescence from normal human breast tissues and malignant breast tumors. The diffuse reflectance spectra from excised, air-equilibrated, human breast tissue samples show lower fractions of oxygenated hemoglobin and higher content of ferric (Fe3+) heme in malignant breast tumor samples than in normal breast tissues. Normal tissues are found to be easily deoxygenated and reoxygenated, but malignant tumors usually do not change their state as much. An analysis of tissue oxygenation parameters is discussed with respect to an enhancement of predictive power of fluorescence diagnostic method. The oxygenation state of tissues may be used as an additional marker in cancer diagnostics.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Near infrared spectroscopy was used to monitor changes in hemoglobin concentration in the kidney of 9 anesthetized rabbits during ischemic conditions. Laser diodes (775, 810, 862, and 904 nm) were placed on the surface of the kidney, the transmitted light was detected by a photomultiplier. Three stages of ischemia were induced in the kidney by occluding the abdominal aorta to approximately 1/3, 2/3 and total occlusion. Hemoglobin content was monitored during 10 minutes of occlusion followed by 10 minutes of recovery. The occlusions resulted in an average decrease in blood pressure of 36%, 53% and 100% in the abdominal aorta. The partial occlusions resulted in an increasing trend in total hemoglobin content during the 10 minutes occlusion followed by a return to baseline hemoglobin concentration. Within the first 30 seconds of the total occlusion maneuver, an immediate decrease in hemoglobin content was observed. During the occlusion the hemoglobin content increases slowly, corresponding to an increase in reduced hemoglobin content. Oxygenated hemoglobin content remained constant during the occlusion and increased immediately when the occlusion was released.

Near-IR spectroscopy was used to quantify blood content and oxygenation dynamics in abdominal organs and skeletal muscle of 18 anesthetized rabbits during hypoxic hypoxia. Liver, kidney, and hindlimb muscle were exposed surgically. Laser diode pulses transmitted across the tissues were detected by means of a photomultiplier. The amount and redox level of tissue hemoglobin were estimated from the near-IR signals and monitored during 5- min-long hypoxic challenges and subsequent recovery periods. In the kidney, exposure to 10% FiO2 resulted in rapid and symmetrical changes in oxygenated and reduced hemoglobin with 50% of the variations occurring within 1 min and a plateau after 3 min. Total hemoglobin did not change and hemoglobin oxygenation returned to baseline within 1 min of hypoxia cessation. Exposure to 6% FiO2 doubled the decrease in oxygenated hemoglobin and induced a sustained vasoconstriction which decreased total hemoglobin content 2 min after initiation of hypoxia. Comparable patterns were observed in the liver and skeletal muscle with the following exceptions: local vasoconstriction was generally not observed at 6% FiO2, return to baseline oxygen availability was much slower in skeletal muscle than in the other organs.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.