Michael B. Maris

Quantitation of time- and frequency-resolved optical spectra for the determination of tissue oxygenation

The recent development of near-infrared time- and frequency-resolved tissue spectroscopy techniques to probe tissue oxygenation and tissue oxygenation kinetics has led to the need for further quantitation of spectroscopic signals. In this paper, we briefly review the theory of light transport in strongly scattering media as monitored in the time and frequency domains, and use this theory to develop algorithms for quantitation of hemoglobin saturation from the photon decay rate (delta log R/delta t) obtained using time-resolved spectroscopy, and from the phase-shift (theta) obtained from frequency-resolved, phase-modulated spectroscopy. To test the relationship of these optical parameters, we studied the behavior of delta log R/delta t and theta as a function of oxygenation in model systems which mimicked the optical properties of tissue. Our results show that deoxygenation at varying hemoglobin concentrations can be monitored with the change in the photon decay kinetics, delta delta log R/delta t in the time-resolved measurements, and with the change in phase-shift, delta theta, in the frequency-resolved technique. Optical spectra of the adult human brain obtained with these two techniques show similar characteristics identified from the model systems.

Reperfusion Hyperoxia in Brain after Circulatory Arrest in Humans

Changes in the electroencephalogram (EEG), mean arterial blood pressure (MABP), and hemoglobin saturation in brain vasculature of lightly anesthetized normothermic humans undergoing induced circulatory arrest for implantation of an automatic internal cardioverting defibrillator were studied. EEG was measured using a four-channel bipolar montage and hemoglobin saturation was measured transcranially using reflectance spectroscopy at 760 nm with an isosbestic reference at 800 nm. Hemoglobin saturation of blood in the quadriceps muscle was also measured. Thirty-two episodes of hypotension due to ventricular fibrillation were studied along with 31 episodes of hypotension related to ventricular tachycardia and rapid ventricular pacing. In a typical fibrillatory event there was a decrease in MABP followed almost immediately by changes in hemoglobin saturation of blood in the brain vasculature. The first changes in EEG were detected an average of 6.5 s (P less than 0.001, paired t test) after the beginning of change of brain vascular hemoglobin. In some cases changes in hemoglobin saturation could be detected without changes in EEG. Desaturation curves from muscle and brain were significantly different, suggesting that the brain probe was measuring hemoglobin change in a rapidly metabolizing volume of tissue that was dissimilar to the skin, muscle, and bone monitored by the probe over the quadriceps muscle. Examination of the 32 episodes of circulatory arrest revealed a marked response that began immediately with recirculation characterized by an increase of the hemoglobin saturation signal from brain vasculature to above baseline as the duration of circulatory arrest exceeded 37 s, this response is termed reperfusion hyperoxia.(ABSTRACT TRUNCATED AT 250 WORDS)

Phase modulation system for dual wavelength difference spectroscopy of hemoglobin deoxygenation in tissues

Time resolved spectroscopy of tissue makes it possible to quantify tissue hemoglobin concentrations because of the direct measurement of the optical path length for photon migration. However, the laser system is bulky and unwieldy and impractical for clinical studies. Thus, the application of the more compact and efficient phase modulation technology well known for fluorescence lifetime studies to time resolved spectroscopy of tissue offers opportunities to simplify the methodology and in addition to afford continuous readout of tissue photon propagation. This paper describes single and dual wavelength systems operating at two wavelengths in the deep red region based upon a time-sharing system. These devices have noise levels in a 2 Hz bandwidth of less than 2 ps and drifts of < 1ps/min. Applications of the noninvasive devices include measurement of hemoglobin deoxygenation in brain and hemoglobin and myoglobin deoxygenation in human skeletal muscle and animal models. Numerous applications to medical and biological problems now become available.

Functional near-infrared imaging of deoxygenated hemoglobin during exercise of the finger extensor muscles using the frequency-domain technique

A functional image of the extensor digitorum muscle was obtained using the frequency domain spectroscopic technique with near-infrared light. The muscle was imaged by measuring the change in phase angle of modulated waves of near-infrared light in response to increased absorption of deoxygenated hemoglobin, which resulted from increased O2 metabolism during arm exercise. The image was compiled from a raster scan of flexion and relaxation exercises at 95 separate points on the dorsal surface of the right forearm. Each point on the arm consisted of a complete experiment that included continuous frequency-domain measurements of phase and intensity during rest, isotonic exercise, and recovery from exercise. Analysis consisted of a simple subtraction of the average phase angle baseline at rest from the average phase reading during steady-state contraction of the extensor digitorum muscle. A difference array of data was then compiled from these 95 experiments to give a 2-D density-plot image. The image clearly demonstrates the extensor digitorum muscle in its oblique position within the density-plot array. This experiment shows the in vivo functional imaging capability of the frequency-resolved technique and indicates that a high degree of resolution is obtainable.

Photon Migration in Muscle and Brain

The initial studies of highly scattering biological materials by optical means were made by David Keilin and E.F. Hartree1,2. Using the microspectroscope they observed that the cytochrome absorption bands from various microorganisms, which had been frozen in water or glycerol, were greatly enhanced by micro-recrystallization. Their samples were rapidly frozen in liquid nitrogen, and then rewarmed to a temperature at which a “red halo” was visible in the transmitted light. This produced a better definition of cytochrome absorption bands because of increased light scattering. Furthermore, Keilin showed that the low dispersion microspectroscope gave better visualization of cell and tissue absorption bands than did a higher resolution spectrograph.

Monte Carlo and diffusion calculations of photon migration in noninfinite highly scattering media

We have investigated the effect of an absorbing object on the time- course and the migration paths of photons within a highly scattering cylindrical phantom. Experimentally, we injected photons into the phantom at one point on the circumference, and recorded the time course of the photons arriving at various detection positions round the cylinder. The simulations used both a Monte-Carlo approach and a diffusion approach to calculate the photon migration. The two computational approaches are similar. The calculated time-course signals agree well with the experimentally observed signals. Moreover, we are able to use a diffusion approximation to calculate the probable paths taken for photons which take a defined time to travel from source to detector.

Analysis of absorption, scattering, and hemoglobin saturation using phase-modulation spectroscopy

Previously, Chance and coworkers have demonstrated the use of time- resolved spectroscopy to detect changes in deoxy- and oxy-hemoglobin concentrations in brain, muscle, and tumors. In this study, we examine the potential to quantitate hemoglobin saturation and tissue oxygenation from steady-state, dual-wavelength measurements in the frequency domain. Frequency-resolved spectroscopy depends upon monitoring light which emerges a known distance away from an incident light beam whose intensity is sinusoidally modulated. The phase-shift, (theta), of the emergent light with respect to the incident light is related to the light pathlengths traveled due to the scattering and absorption properties of the homogeneous medium. Using the diffusion approximation to describe the transport of photons through a highly scattering medium, we demonstrate the ability to detect changes in absorption and scattering properties from measurements of (theta) in model system if Intralipid/India ink and polystyrene microsphere suspensions using phase modulation spectroscopy. In addition, from measurements of (theta) at wavelengths that straddle the isosbestic point of hemoglobin, we demonstrate the ability to quantitatively follow changes in of absorption properties due to oxy- and deoxy- hemoglobin in an Intralipid/hemoglobin model. The advantages of using phase-modulation spectroscopy as an analytical tool in the clinic as well as the remaining problems associated with its use are discussed.

Frequency-domain measurements of changes of optical pathlength during spreading depression in a rodent brain model

Previously, we have shown that time-resolved spectroscopy can monitor changes in the distribution of photon migration pathlengths which are reflective of the changes in the tissue absorption due primarily to oxygenated or deoxygenated hemoglobin. In this study, we have monitored mean photon migration pathlengths in the frequency domain in the rodent brain insulted by hypoxia, ischemia and spreading depression (SD) using phase modulated spectroscopy (PMS). This technique consisted of monitoring light which emerged from the exposed rodent skull at 8 mm form an incident light source of 754 nm and 816 nm whose intensity was modulated at 220 MHz. The changes in phase-shift, (theta), of the emergent light with respect to the incident light are reflective of the photon pathlengths and hemoglobin absorbance. A multiprobe assembly holding PMS source fiber, nicotinamide dinucleotide (NADH) fluorometric probe, electrocortigraph (ECoG) electrodes, and doppler blood flow probe was placed on the rodent brain to simultaneously monitor brain metabolism, electrical cortical activity (ECoG) and blood flow. The PMS detector fiber was placed 8 mm posterior to the multiprobe assembly. Correlations between changes in intracellular deoxygenation (NADH) and hemoglobin deoxygenation as measured by PMS changes at 754 nm and 816 nm during hypoxia, and ischemia were found. The depolarization phase of spreading depression resulted in a similar increase at both 754 nm and 816 nm. We attribute this result to vasoconstriction and/or the decrease of extracellular space due to water shift in the rodent brain.

A Time Resolved Spectroscopic (TRS) Study of Migration of Visual to Infrared Waves in Brain Tissue in Relation to Absorption of Hemoproteins

Previous optical spectroscopic studies of brain tissue revealed that only short photon migration distributions yeilds absorption spectra in the visual wavelength range (Heinrich et al. 1985), while longer wavelengths penetrated over greater distances (Brazy et al., 1985). The alpha and beta bands of hemoproteins in brain tissue have extremely high absorption coefficients which reduce the length of light migration distributions. This phenomena can be explained by photon migration theory. As shown in Figure 1A, in scattering media such as a brain tissue, light path lengths have a distribution function. In this figure, the Y-axis represents the log of photon intensity (I) or number of photons, and the X-axis represents time in nanoseconds. Migrating light path lengths (L), can be represented by use of the conversion factor: 1 ns = 23 cm (light path length in water).