Comparison of time-resolved and -unresolved measurements of deoxyhemoglobin in brain

Characterization of absorption and scattering properties for various yeast strains by time-resolved spectroscopy

An understanding of the optical properties of biological media and cells is essential to the development of noninvasive optical studies of tissues. Unicellular organisms offer a unique opportunity to investigate the factors affecting light propagation, since they can be manipulated in ways impossible for more complex biological samples. In this study, we examined optical absorption and scattering properties of strongly multiple scattering yeast suspensions by means of near-infrared (NIR) time-resolved spectroscopy (TRS) and a sample substitution method. We determined the critical parameters for photon migration by varying the cell organelle content, the cell ploidy, the cell size, and the concentration of suspended cells. The results indicate that the photon absorption is insensitive to cell differentiation and that the cell volume is the primary factor determining light-scattering property.

A comparison of two systems for assessing cerebral venous oxyhaemoglobin saturation during cardiopulmonary bypass in humans

The Somanetics Invos 3,100 cerebral oximeter is a new noninvasive device which measures the regional oxygen saturation of haemoglobin within the cerebral microvasculature by infrared spectroscopy. It was used in nine patients undergoing elective cardiac surgery and its results were compared with measurements of jugular venous bulb saturations obtained by the Oximetrix Opticath oximetry system. The index value of jugular venous bulb saturation was obtained by analysis of jugular bulb blood in an Il282 cooximeter. The cerebral oximeter was less accurate and precise (standard deviation of difference 14.1%) than the Oximetrix system (standard deviation of the difference 2.65%) and also demonstrated a systematic error in bias unrelated to cerebral perfusion pressure. The cerebral oximeter may therefore be less useful than jugular venous bulb saturation in the clinical management of patients undergoing cardiopulmonary bypass.

Near-infrared monitoring of the cerebral circulation

Near-infrared spectroscopy is a noninvasive bedside technique for monitoring hemoglobin saturation (HbO2%) in brain vasculature. The method linearly relates the optical signal detected from the surface of the head to HbO2%. To do so, the method relies on constant transcranial optical pathlength and light scattering as well as minimal interference by tissues overlying the brain. This study examined these premises. Optical signals from a dual-wavelength, near-infrared spectrometer were correlated with sagittal sinus HbO2% in 7 anesthetized piglets subjected to 7 different physiological conditions: normoxia, moderate and severe hypoxia, hyperoxia, hypocapnia, hypercapnic hyperoxia, and hypotension. These conditions were induced by varying the inspired O2 concentration (7-100%), ventilatory rate (5-35 breaths/min), and blood pressure (phlebotomy 20 ml/kg) to force HbO2% over a wide range (5-93%). To evaluate interference by tissues overlying the brain, correlations were repeated after the scalp and skull were rendered ischemic. Transcranial optical pathlength was measured by phase-modulated spectroscopy. Linear relationships between optical signals and sagittal sinus HbO2% were found with correlation coefficients ranging from -0.89 to -0.99 (p < 0.05) among animals; however, slope and intercept had coefficients of variability of approximately 15 and 333%, respectively. Almost identical linear expressions were observed whether scalp and skull were ischemic or perfused. Transcranial optical pathlength was constant in each animal, but ranged from 10 to 18 cm among animals. The data indicate that the assumptions underlying near infrared spectroscopy are reasonably accurate in a given animal, but that the constants for transcranial optical pathlength and light scattering are not the same in all animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Near-infrared spectroscopic localization of intracranial hematomas

Near-infrared spectroscopy (NIRS) of the cerebral hemispheres, applied transcranially through the intact scalp and skull, was evaluated for its ability to detect the presence of an intracranial hematoma in 46 head-injured patients. In 40 patients intracranial hematomas (22 subdural, 10 epidural, eight intracerebral) were identified on computerized tomography (CT); in all 40 cases, NIRS demonstrated greater absorption of light at 760 nm on the side of the hematoma. The mean difference in optical density (OD) between the hemisphere with the hematoma and the normal hemisphere was 0.99 +/- 0.30 for epidural hematomas, 0.87 +/- 0.31 for subdural hematomas, but only 0.41 +/- 0.11 for intracerebral hematomas. In 36 patients, the asymmetry in OD resolved after surgical evacuation of the hematoma or with spontaneous resorption of the hematoma. Four patients who developed postoperative or delayed hematomas exhibited persistence of the asymmetry in OD. Six patients had only diffuse injuries and exhibited only minor differences in OD between the hemispheres, similar to 10 patients in the control group with no head injury. It appears that NIRS is useful in the initial examination of the head-injured patient, as an adjunct to CT, and in following patients postoperatively in the intensive care unit.

Cognition-activated low-frequency modulation of light absorption in human brain

Animal model studies indicate light-absorption changes of the exposed animal brain in response to visual stimulation. Here we report observations of red-light absorbance changes, attributable to repetitive blood concentration changes in response to stimulation in the human brain frontal region by a cognitive process. These responses are observed as low-frequency recurrence of changes by Fourier transform analysis and are attributed to blood concentration change stimulated by the increased metabolic rate of brain tissue in cognitive function. A simple, portable dual wavelength spectrophotometer was attached noninvasively to the human forehead to measure the low frequency and power spectra of fluctuations of absorbances attributed to variations of brain blood concentration in the frontal region. The responses are associated with brain activity in responses to problem solving of analogies presented visually that require an associative function in the frontal region. The method of subtraction of test -rest Fourier transforms minimizes the arterial pulse frequency contributions and identifies specific frequencies--for example, 0.8, 1.6, 1.8 Hz in 24 of 28 tests of nine individuals (85%). Tests in which no increased brain activity was elicited (rest-rest) showed small differences. It is concluded that low-frequency recurrences of brain activity linked to blood concentration increases can be detected in human subjects with an optical device of potentially for simplified tests of cognitive function in the 0- to 3-Hz region and with modifications for wider band recordings in localized tissue volumes by time-resolved spectroscopy.

Rapid screening of cytochromes of respiratory mutants of Saccharomyces cerevisiae. Application to the selection of strains containing novel forms of cytochrome-c oxidase

A technique has been developed for the direct analysis by visible spectrophotometry of yeast spots growing on agar plates. This allows rapid semi-quantitative estimations of cytochromes c, b and oxidase and permits the identification of strains with impaired respiratory electron flow. Results of screening of 105 mutants are presented. There appears to be a correlation between the exonic location of the mutation in COX1 of oxidase and the level of optically detectable enzyme. Mutations in cytochrome b of the bc1 complex also affect the level of expression of cytochrome oxidase and can cause either an increased or decreased level of expression of oxidase relative to the wild-type strain. Twelve strains selected by the rapid level-1 screening were grown as lawns on sections of an agar plate and resuspended for a second level of screening. Quantitative estimates have been made of the concentrations of cytochromes, the turnover number of cytochrome oxidase and the kinetics of recombination of carbon monoxide with oxidase after flash photolysis. This confirmed the validity of the rapid screening procedure, and we have identified several strains which contain high levels of a mutant form of cytochrome oxidase with properties worthy of further investigation.

Optical time-of-flight and absorbance imaging of biologic media

Imaging the interior of living bodies with light may assist in the diagnosis and treatment of a number of clinical problems, which include the early detection of tumors and hypoxic cerebral injury. An existing picosecond time-of-flight and absorbance (TOFA) optical system has been used to image a model biologic system and a rat. Model measurements confirmed TOFA principles in systems with a high degree of photon scattering; rat images, which were constructed from the variable time delays experienced by a fixed fraction of early-arriving transmitted photons, revealed identifiable internal structure. A combination of light-based quantitative measurement and TOFA localization may have applications in continuous, noninvasive monitoring for structural imaging and spatial chemometric analysis in humans.

Laser tomography of heterogeneous scattering media using spatial and temporal resolution

Time-resolved tomography is performed in transillumination by using 527 nm picosecond pulses from a passively mode-locked doubled Nd/glass laser and a streak camera to select photons according to their flight time. This work reports on the increase in contrast of a time-resolved profile of a 2 mm radius opaque object embedded in a scattering medium, constituted of diluted milk in a 30 mm thick cell. For spatial analysis, the emerging photons are detected through a 6 mm slit at the outlet face of the cell. Transmission profiles obtained as a function of time show that the contrast is enhanced for the shortest flight times, while the 'shadow' of the object is no longer detected after about 100 ps. Moreover, improvements in contrast are studied for different configurations of the model, to analyse separately the role of collimated and scattered photons. It is expected that such a tomographic method based on time-resolved absorption could be applied to imaging for more complex biological structures in the red and near-infra-red range.

Spectral dependence of temporal point spread functions in human tissues

We have determined the spectral dependence of the temporal point spread functions of human tissues experimentally between 740 and 840 nm in transmittance measurements on the adult head, forearm, and calf (in vivo) and the infant head (post mortem) by using picosecond laser pulses and a streak camera detector. Two parameters are extracted from the temporal point spread function; the differential path-length factor (DPF), calculated from the mean time, and the slope of the logarithmic intensity decay. In all tissues the DPF and the logarithmic slope show a reciprocal relationship and exhibit characteristics of the absorption spectra of hemoglobin. The DPF falls with increasing wavelength, the variation being typically 12%, while the logarithmic slope increases with wavelength. A quantitative analysis of the logarithmic slope spectrum significantly underestimated expected tissue chromophore concentrations. The absolute magnitudes of the DPF showed considerable intersubject variation, but the variation with wavelength was consistent and thus may be used in the correction of tissue attenuation spectra.

Numerical method for studying the detectability of inclusions hidden in optically turbid tissue

We introduce an efficient numerical method for studying the detectability of absorptive inclusions in a multiple-scattering optical medium. Use of the method is demonstrated by the forward calculation of integrated and time-gated photon intensities. Schemes for positioning the light source above an inclusion and otherwise determining the location of a hidden object, involving either reflected or transmitted reemissions, are discussed as examples. They are investigated for several illustrative models, and images are calculated as a function of the size and shape of the inclusion.

Photon hitting density

Optical and near-IR spectroscopy and imaging of highly scattering tissues require information about the distribution of photon-migration paths. We introduce the concept of the photon hitting density, which describes the expected local time spent by photons traveling between a source and a detector. For systems in which photon transport is diffusive we show that the hitting density can be calculated in terms of diffusion Green's functions. We report calculations of the hitting density in model systems.

Spatial localization of absorbing bodies by interfering diffusive photon-density waves

The use of the destructive interference of diffusive photon-density waves for the localization of an absorbing (and scattering) body in a scattering medium was studied. The objectives of this approach in the reflectance mode were as follows: first, to reduce sensitivity to absorption features occurring in superficial layers while sensitivity to bodies lying deeper is maintained; second, to establish a confined depth region of maximum sensitivity in which the distance of an absorbing body could be determined through phase measurement. Intensity and phase data were acquired with a modified frequency-domain spectrometer at modulation frequencies up to 600 MHz as an absorbing body was moved in three dimensions. The experimental results are compared with simulations based on a numerical solution of a time-dependent photon-diffusion equation.

Double-stage picosecond Kerr gate for ballistic time-gated optical imaging in turbid media

We demonstrate that use of a picosecond double-stage Kerr gate system results in a 3-orders-of magnitude improvement in signal-to-noise ratio and a threefold improvement in shutter speed comparedwith those of a single-stage Kerr gate.

Scaling relationships for theories of anisotropic random walks applied to tissue optics

Monte Carlo simulations are used to discern scaling relationships for photon migration occurring within homogeneous, anisotropic scattering media of semi-infinite extent. Special attention is given to events associated with short path lengths. Empirical scaling relationships for path lengths and surface intensities are shown to agree with a consistency equation derived in an earlier study of anisotropic random walks. They are augmented here by a procedure that accounts for concomitant scaling of optical absorption coefficients. Results then are used to transform expressions that were obtained previously by analytical random-walk theory developed for an isotropic scattering model of photon migration. Quantities that are studied include the diffuse surface reflectance, the depth distribution of the fluence, and the time-resolved intensity of backreflected photons.

Properties of photon density waves in multiple-scattering media

Amplitude-modulated light launched into multiple-scattering media, e.g., tissue, results in the propagation of density waves of diffuse photons. Photon density wave characteristics in turn depend on modulation frequency (omega) and media optical properties. The damped spherical wave solutions to the homogeneous form of the diffusion equation suggest two distinct regimes of behavior: (1) a high-frequency dispersion regime where density wave phase velocity V(p) has a radicalomega dependence and (2) a low-frequency domain where V(p), is frequency independent. Optical properties are determined for various tissue phantoms by fitting the recorded phase (?) and modulation (m) response to simple relations for theappropriate regime. Our results indicate that reliable estimates of tissue like optical properties can be obtained, particularly when multiple modulation frequencies are employed.

Optical CT imaging of hemoglobin oxygen-saturation using dual-wavelength time gate technique

Our results may be summarized as follows: 1. In-vivo hemoglobin oxygen-saturation images of rat brain can be obtained using near-infrared light. 2. The transmitted light has multiple scattered components, which degrade spatial resolution, though most of the scattering can be excluded by a suitable time-gating technique. 3. The hemoglobin oxygen-saturation of the cerebellum is higher than that of the cerebrum in untreated anesthetized rats. Although further improvements in the technique should result in better resolution, we have demonstrated the feasibility of using near-infrared light to obtain in-vivo hemoglobin oxygen-saturation images.

Frequency domain imaging of absorbers obscured by scattering

Multiple pixel, frequency domain measurements of phase shift, theta, and modulation, m, in a phantom containing an absorber obscured by a relatively non-absorbing scattering solution are presented in combination with a theory of photon migration imaging. Results employing a single point source show that two dimensional theta measurements made in the presence (theta presence) and in the absence (theta absence) of an absorber can be used to create delta theta images. delta theta (theta absence-theta presence) images can be used to detect as well as locate the three dimensional position of the absorber. Images of mpresence measured in the presence of the absorber normalized by mabsence also provided detection and two dimensional location of its position. Images of % mpresence/mabsence at higher modulation frequencies provided greater resolution as predicted by photon migration theory. Neither theta nor m images alone could be used to detect or locate the presence of the absorber.

Use of polarized light to discriminate short-path photons in a multiply scattering medium

<p>We describe a method for discriminating short- and long-path photons transmitted through a multiply scattering medium that is based on the relationship between the polarization states of the incident and forward-scattered light. Results of Monte Carlo simulations and experiments show that if the scattering anisotropy of the scatterers is sufficiently small, absorbing barriers embedded in optically dense suspensions of polystyrene spheres can be resolved with good contrast by selectively detecting a component of the scattered-light intensity that has preserved its incident circular polarization state.</p><p>The principles of operation of a polarization-modulation system capable of measuring small polarization fractions are explained. Using this system we were able to measure polarized light in a depolarized background over 1000 times as large.</p>

Long-time behavior of photon diffusion in an absorbing medium: application to time-resolved spectroscopy

We have investigated the motion of photons after the injection of a light pulse into a highly scattering, inhomogeneous absorbing medium. We show that the terminal slope of the logarithm of the transmittance curve does not depend on the positions of the source or of the detector. We use numerical calculations to follow the motion of photons in a model system to further understand the physical process that gives rise to this result.

Fluctuations in cerebral oxygenation and blood volume during endotracheal suctioning in premature infants

To investigate the effect that suctioning of the endotracheal tube has on the cerebral circulation, we monitored brain intravascular hemoglobin saturation (tHbo2%), cerebral blood volume (CBV), and arterial hemoglobin saturation (Spo2) during suctioning in 12 infants (24 to 33 weeks of gestational age) with respiratory distress syndrome treated with mechanical ventilation. The tHbo2% and CBV values were monitored over the forebrain by dual-wavelength near-infrared spectroscopy, and Spo2 was monitored by pulse oximetry of a finger. The monitored variables were stable during the baseline period. With suctioning, Spo2 decreased from 94% +/- 1% to 84% +/- 1%, tHbo2% decreased, and CBV increased (p less than 0.05). Desaturation in the arterial and cerebral circulations began within 5 seconds of the onset of suctioning. Arterial reoxygenation began with the onset of reventilation, whereas reoxygenation in the brain was delayed by 15 seconds. The Spo2, tHbo2%, and CBV values returned to baseline within 1 minute of reventilation. Studies were repeated in six of the infants after the fraction of inspired oxygen was increased to attain a baseline Spo2 of 100%. In the preoxygenated infants, tHbo2%, CBV, and Spo2 remained constant during suctioning. These studies confirm that endotracheal suctioning results in transient hypoxemia, and demonstrate that this is reflected in the brain by vasodilation and deoxygenation. These effects are preventable by preoxygenation before suctioning.

The relationship of surface reflectance measurements to optical properties of layered biological media

Reflectance from a turbid biological tissue is discussed for a diffusive light source illuminating the surface of the medium, and is related to the optical property distribution within the medium and to photon propagation through the medium. A three-dimensional photon diffusion model with closed form is developed to describe the photon diffuse intensity in a homogeneous medium. The solution is extended by numerical methods to the medium with layered structure. The concepts of photon flux paths and of reflectance indexes are utilized, together with reflectance data, to extract information about the internal optical properties of a medium. The flux path concept was corroborated by successfully detecting in vivo and ex vivo layered differences in optical properties within the biological medium. These studies suggest that the optical properties of subdermal tissue can be measured from light reflectance and that the effect of the upper skin layers can be eliminated.

Intracerebral penetration of infrared light. Technical note

Near infrared transmission spectroscopy of the human cerebrum may allow noninvasive evaluation of cerebral hemoglobin saturation in humans. The emerging spectroscopy configuration for this application is a side-by-side source-receiver construct. The ability of this spectroscopy paradigm to detect changes in intracerebral attenuation by selective injection of the infrared tracer indocyanine green into the internal and external carotid arteries during endarterectomy is evaluated in five adult patients. In all five, simultaneous two-channel infrared transmission spectroscopy over the ipsilateral hemisphere documented tracer bolus transit with a signal-to-noise ratio greater than 100:1. In addition, the two channels could be configured to achieve depth resolution of the collected spectra.

Quantitation of tissue optical characteristics and hemoglobin desaturation by time- and frequency-resolved multi-wavelength spectrophotometry

Photon migration in highly scattering tissues such as muscle and brain gives optical pathlengths that are dependent upon absorption and scattering parameters, mu a, mu s'. Determination of these parameters gives the correct concentration of principal absorber such as hemoglobin in the red region of the spectrum. Determinations of scatter factor in functioning and pathological tissues are made.

Near-infrared imaging in vivo (I): Image restoration technique applicable to the NIR projection images

To enhance spatial resolution of NIR projection images in vivo, we performed refocusing of NIR projection images of human forearm of about 50 mm thickness. A volunteer's forearm was illuminated by parallel light beam-flux, and then projection images at 750 nm was measured with a Pertier-cooled CCD video camera and digitized. For refocusing computation, PSF of the light transmitted through the tissue was calculated by general equation proposed by van der Zee and Delpy (1988). Simple inverse, constrained least squares, and Wiener filters were tested as refocusing algorithms. Wiener filter gave the best result in terms of image quality and computation time. By applying Wiener filter to the image refocusing of NIR projection images of human forearm, we obtained enhanced spatial resolution.

Experimentally measured optical pathlengths for the adult head, calf and forearm and the head of the newborn infant as a function of inter optode spacing

The Differential Pathlength Factor (DPF) has been measured for several different tissues. The results showed that the DPF varied with the type of tissue studied, and in the case of the adult calf with sex. However, the DPF for all tissues studied was constant once the inter optode spacing exceeded 2.5 cm. Thus, measurements can be made by NIR spectroscopy at a range of inter optode spacings, and a single DPF used in the calculation of chromophore concentration. The results also showed that the major source of error in the DPF lay in the measurement of the inter optode spacing. To improve accuracy, two options are possible. Firstly, some means of continuous measurement of inter optode spacing could be incorporated in the NIR instrumentation. The better alternative would be an instrument incorporating a method of directly measuring the optical pathlength at each wavelength. This could be done either by time of flight measurement, or if it can be validated, by phase shift measurement.

Optical coherence tomography

A technique called optical coherence tomography (OCT) has been developed for noninvasive cross-sectional imaging in biological systems. OCT uses low-coherence interferometry to produce a two-dimensional image of optical scattering from internal tissue microstructures in a way that is analogous to ultrasonic pulse-echo imaging. OCT has longitudinal and lateral spatial resolutions of a few micrometers and can detect reflected signals as small as approximately 10(-10) of the incident optical power. Tomographic imaging is demonstrated in vitro in the peripapillary area of the retina and in the coronary artery, two clinically relevant examples that are representative of transparent and turbid media, respectively.

Monte Carlo simulation of light transmission through living tissues

To analyze the fundamental characteristics of light transmitted through living tissues, we used the Monte Carlo method to trace the paths of the rays incident upon slabs of particles. The slabs contained either (i) two types of scattering particles in a solution or (ii) one type of particle with pigment added to the solution. Temporal analyses of the transmittance have illustrated that the differences in the optical density among the slabs having different absorption coefficients with the same scattering coefficient vary linearly with time. Also, their gradients have been shown to be proportional to the differences in the absorption coefficients, thus verifying the microscopic Beer-Lambert law in highly scattering media when temporally resolved measurement is used.

Oxygen in mammalian tissue: methods of measurement and affinities of various reactions

Oxygen is the primary oxidant in energy-producing biological reactions and is also involved in the synthesis and degradation of many structural and regulatory molecules of physiological importance. This review discusses the advantages and limitations of the currently available methods for measuring oxygen in mammalian tissue and bodily fluids. These include 1) the effects of O2 on the relaxation time of molecules excited by electromagnetic radiation and observed by optical (fluorescence and phosphorescence) and magnetic (nuclear magnetic resonance and electron paramagnetic resonance) techniques, 2) the polarographic and galvanic reduction of oxygen at metal surfaces, 3) in vivo spectrophotometry of intrinsic redox systems, 4) manometry and tonometry, and 5) mass spectroscopy. The values of tissue oxygenation obtained with these techniques are compared with the Michaelis constant values for oxygen of almost 60 oxygen-consuming enzymes involved in mammalian tissue metabolism.

Regional cerebrovascular oxygen saturation measured by optical spectroscopy in humans

Regional cerebrovascular oxygen saturation, a quantitative measure of hemoglobin saturation in the combined arterial, venous, and microcirculatory compartments of the brain, can be measured noninvasively with near infrared spectroscopy. We assessed the sensitivity of this aggregate saturation to cerebral hypoxia during transient cerebral hypoxic hypoxia in seven human subjects. Regional cerebrovascular oxygen saturation measured over the middle frontal gyrus and analog electroencephalogram were recorded. We compared the time to achieve two end points: the earliest paroxysmal burst of theta-delta background slowing and a cerebrovascular oxygen saturation of less than 55%. Saturation fell below 55% prior to the electroencephalographic change (p less than 0.05). In a related effort, we also compared spectroscopically measured regional cerebrovascular oxygen saturation with an estimate of this value calculated from arterial and cerebral mixed venous saturation in nine patients. A positive linear relation (n = 68, R2 = 0.55, s = 4.2) was noted.

Defining the resistance to oxygen transfer in tissue hypoxia

Studies of O2 supply in freshly isolated adult mammalian cells provide new insight into the factors that limit mitochondrial oxygenation in vivo. Of particular importance, mitochondria are present at high densities and often in apparent clusters, both of which contribute to local O2 gradients under hypoxic conditions. Current evidence indicates that the mitochondrial distribution is a component of the differentiated phenotype of adult mammalian cells and that specific motors and anchoring mechanisms are present to allow redistribution in response to developmental, physiological and pathological challenges. To compare the importance of resistance to O2 transfer under different conditions and at different sites along the supply path in vivo, a simple mathematical expression of relative resistance to O2 supply is introduced. Under various pathophysiological conditions, this resistance increases in specific regions of the pulmonary, circulatory or cellular supply path and results in O2 deficiency in the mitochondria. Regardless of cause, the relative resistance increases dramatically in the vicinity of mitochondrial clusters during hypoxia.

Pathobiochemistry of CO poisoning

A hypothesis for the pathobiochemical mechanism of CO poisoning, amenable to in vivo testing with optical reflectance spectrophotometry, is presented. It differs from the classical formulation in which loss of cytochrome c oxidase function is attributed entirely to O2 depletion, by including ligation and oxygenation of CO as essential components of the inhibitory process.

Cortical functional architecture and local coupling between neuronal activity and the microcirculation revealed by in vivo high-resolution optical imaging of intrinsic signals

We have shown previously the existence of small, activity-dependent changes in intrinsic optical properties of cortex that are useful for optical imaging of cortical functional architecture. In this study we introduce a higher resolution optical imaging system that offers spatial and temporal resolution exceeding that achieved by most alternative imaging techniques for imaging cortical functional architecture or for monitoring local changes in cerebral blood volume or oxygen saturation. In addition, we investigated the mechanisms responsible for the activity-dependent intrinsic signals evoked by sensory stimuli, and studied their origins and wavelength dependence. These studies enabled high-resolution visualization of cortical functional architecture at wavelengths ranging from 480 to 940 nm. With the use of near-infrared illumination it was possible to image cortical functional architecture through the intact dura or even through a thinned skull. In addition, the same imaging technique proved useful for imaging and discriminating sensory-evoked, activity-dependent changes in local blood volume and oxygen saturation (oxygen delivery). Illumination at 570 nm allowed imaging of activity-dependent blood volume increases, whereas at 600-630 nm, the predominant signal probably originated from activity-dependent oxygen delivery from capillaries. The onset of oxygen delivery started prior to the blood volume increase. Thus, optical imaging based on intrinsic signals is a minimally invasive procedure for monitoring short- and long-term changes in cerebral activity.

Time-resolved reflectance spectroscopy in turbid tissues

Monte Carlo simulations illustrate how various absorption mu a and scattering mu s coefficients influence time-dependent reflectance R (t) from a semi-infinite homogeneous turbid tissue following an impulse of narrow-beam irradiation. The tissue absorption coefficient mu a in cm -1 can be obtained from measurements of R (t) after the first 20-200 ps (depends on mu s) following an impulse by the expression: mu a = -(n/c) d In [R(t)]/dt - 3n/2ct where n is the tissue-refractive index and c is the in vacuo speed of light. Early data in the first 20-200 ps do not conform to this expression or to diffusion theory. Monte Carlo simulations allow study of the early R(t) behavior. The volume of tissue involved in a measurement is specified by a volume radius r that approximately equals (6Dtc/n)1/2 where t is the time of measurement and D is the optical diffusion constant D = (3 mu s (1 - g]-1. At 50 ps and typical values of mu s = 100 cm-1 and anisotropy equal to 0.9, r equals 5 mm. The upper limit for measurable mu a values is limited by how quickly the reflectance signal is attenuated, and is estimated for current streak camera technology to be mu a less than or equal to 21 cm-1, assuming several measurements are taken over a dynamic range of two orders of magnitude within a 10 ps period.

Influence of path length on remote optical sensing of properties of biological tissue

A photon migration model is applied to the analysis of remote-sensing techniques that use light to probe the properties of biological tissues. Specific consideration is given to the effect of photon path length on the signals obtained from laser Doppler detection of blood flow in tissue microvasculature. We show how changes in blood volume or variation in spacing between delivery and detecting fibers might affect the measurements. We also examine how changes in path length can affect parameters determined by time resolved optical spectroscopy of blood oxygenation. Anisotropies in the angular distribution of scattered photons are taken into account.

Time resolved spectroscopic (TRS) and continuous wave spectroscopic (CWS) studies of photon migration in human arms and limbs

The combination of continuous light spectrophotometry (CWS) and time resolved spectrophotometry (TRS) afford for the first time a quantitation of the optical path and the concentration changes detected by the CWS instrument. The application of these two techniques and magnetic resonance spectroscopy (MRS) to muscle during exercise affords a correlation of the biochemical activation and the response of the peripheral circulation (NRS) to the exercise stress (MRS). In preliminary experiments, the well-trained endurance performance limb shows a near perfect homeostasis to exercise stress while ischemia will cause a significant deoxygenation and an impairment of the work output. The use of this device in evaluation of peripheral vascular disease is obvious and hemoglobin deoxygenation may well occur at work levels less than those at which the diseased limb is capable.

Optical and nuclear magnetic resonance studies of hypoxia in human tissue and tumors

Correlations of energy state with response to therapy are more difficult to analyze because of the large effect of tumor clearing and oxygenation upon the tumor energy state as detected by PMRS alone. The combination of time-resolved hemoglobinometry using picosecond laser technology and localized PMRS seems appropriate to unravel the complexities of therapeutic intervention, tumor energetics, and oxygenation.

Mitochondrial function in normal and genetically altered cells and tissues

The impact upon oxidative metabolism of normal and pathological variations of oxidative capability is just beginning to be understood, based upon the few examples of human and animal subject survivals and the relatively few cell systems in which the impact of molecular pathologies on function has been studied. On the one hand, difficulties of isolation of systems containing altered oxidases are significant because of ineffective assembly or small amounts of surviving isoenzymes, and on the other hand, unexpected fragilities of the oxidase system may lead to low yields when subjected to the preparative stresses appropriate to the wild types. To circumvent these problems, this paper describes the application, in vivo, of noninvasive, nondestructive techniques to study the function of cytochrome oxidase and other components of the respiratory chain, particularly cytochromes b-c1 in human subjects on the one hand, and in isolated cells on the other, principally mutants of Saccharomyces cerevisiae in which the subunit content is varied. Two principal spectroscopic approaches are employed: optical and phosphorus magnetic resonance spectroscopy (P MRS). Optical spectroscopy of the near red region of the spectrum provides effective analysis of brain and muscle, as does the surface coil of space-resolved phosphorus magnetic resonance. Both techniques are applicable to suspensions of single cells such as yeast. The optical method yields essential information on oxygen delivery to tissues by hemoglobin and myoglobin and oxygen utilization by cytochrome oxidase. P MRS affords essential information on the efficiency of ATP generation and the extent to which oxidative metabolism meets the needs of cell function in terms of the ratio of phosphocreatine to inorganic phosphate (PCr/Pi). This in turn enables the calculation of the velocity of oxidative metabolism, V, in relation to its maximum capability, Vm, according to a Michaelis-Menten relationship that involves control not only by ADP (Pi/PCr) and Pi, but also by oxygen and substrate deliveries. Thus, an overview of the functionality of mitochondria in cells and tissues is uniquely provided by this combined approach and thereby deficiencies of components of the respiratory chain are quantified.