1. Field of The Invention
The present invention relates to a method and apparatus for measuring the concentration of an absorptive constituent (e.g., hemoglobin) present in a scattering medium (e.g., a living tissue).
2. Related Background Art
When light having a wavelength near a near-infrared range is incident on living tissue, absorption of the incident light by the living tissue is greatly influenced by hemoglobin in blood. The influence of oxyhemoglobin (HbO.sub.2) is different from that of deoxygenated hemoglobin (Hb). The absorption spectra of the living tissue changes in accordance with the states of hemoglobin, as shown in the graph in FIG. 1. The wavelength [nm] of incident light is plotted along the abscissa in FIG. 1, and the absorbance [mM.sup.-1 cm.sup.-1 ] is plotted along the ordinate. As shown in this graph, the absorbance caused by deoxygenated hemoglobin (Hb) is higher than that by oxyhemoglobin (HbO.sub.2) when the wavelength is shorter than 800 nm. When the wavelength is longer than 800 nm, the absorbance caused by oxyhemoglobin (HbO.sub.2) is higher than that by deoxygenated hemoglobin (Hb) . That is, the optical absorption in living tissue changes in accordance with the metabolic conditions of oxygen in the living tissue.
Conventionally, a monitor for oxygen metabolism in living tissue has been developed using this principle. Such a monitor has been put to practical use. In this oxygen metabolism monitor, however, the relative change since the start of measurement can only be monitored. For this reason, the degree (SO.sub.2 value) of saturation of oxygen in blood in living tissue, which is a ratio of an oxyhemoglobin (HbO.sub.2) concentration to a total hemoglobin concentration, cannot be measured. Therefore, the SO.sub.2 value as the index of oxygen metabolism in the living tissue cannot be known. Therefore, an attempt has been made using the following method and apparatus to measure this SO.sub.2 value.
First, a measuring apparatus (tradename: Cerebral Oximeter (invos 3100)) available from Somanetics is used as an apparatus for measuring an SO.sub.2 value in blood in living tissue. This apparatus measures the concentration of an absorptive constituent (hemoglobin) in the scattering medium (living tissue) using the principle shown in FIG. 2. More specifically, continuous (CW) light having different wavelengths in the near-infrared range is incident on living tissue 1, and attenuation amounts of the incident light are detected at detection positions spaced apart from the light incident position by distances r1 and r2. The Hb and HbO.sub.2 concentrations are obtained in accordance with the correlations between the attenuation amounts of incident light components having different wavelengths and the distances r1 and r2 between the light incident position and the detection positions, because the absorption profiles of incident light components in the living tissue is different as described above, thereby measuring the SO.sub.2 value in blood.
Second, a spectroscopic technique for an absorptive constituent in a scattering medium using a time-resolved spectroscopy is available, as disclosed in U.S. Pat. No. 5,119,815 by B. Chance et al. This reference describes that the spectroscopic technique is applied to an SO.sub.2 value measurement in blood of a living tissue. More specifically, an optical pulse is incident on the living tissue, and an optical pulse profile spreading as a function of time due to light scattering is time-resolved measured to obtain a profile representing a change in light intensity as a function of a change in time. Light absorption in the living tissue is measured such that the light intensity of the resultant profile is logarithmically calculated, and a gradient of the change in light intensity as a function of time is obtained. When time-resolved measurement is performed upon incidence of two optical pulses having different wavelengths on the living tissue, the light absorption for each optical pulse of each wavelength is measured, thereby the SO.sub.2 value in the blood can be calculated, because the light absorption profiles of Hb and HbO.sub.2 are different from each other.
Third, a spectroscopic technique for an absorptive constituent in a scattering medium, using phase modulation spectroscopy, is available, as disclosed in U.S. Pat. No. 4,972,331 by B. Chance et al. This reference describes that this spectroscopic technique can be applied to an SO., value measurement in blood. More specifically, modulated light is incident on the living tissue, and light absorption of the incident light is detected on the basis of a change in phase caused by propagation of the modulated light through the living tissue. When modulated light components having different wavelengths are incident on the living tissue, the concentration ratios of HbO.sub.2 and Hb are detected to calculate the SO.sub.2 value in the blood because the change in phase varies depending on the type of absorptive constituent in the living tissue and the wavelength of the incident modulated light.