JP2009136434A - Optical meter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To pick up the functional signal of brain activities from the measured data by the control of endogenous variations caused by autonomic nerve activities, heartbeats or breathing, various forms of changes arising from the bodily movements or physical conditions of a subject, which are known to be included besides the functional signal of brain activities actually measured by fNIRS. <P>SOLUTION: The Reference Subtraction technique (RS method) has been introduced, which newly uses reference probes in the vicinity of light receiving probes for calculating the variations of each hemoglobin (Hb) from measured absorbance differences between these two points. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a biological light measurement technique using near-infrared light, and particularly to a brain function measurement technique or a blood oxygen concentration measurement technique.

  Near-infrared brain function measurement method (fNIRS) is a technique that can monitor changes in cerebral blood flow and oxygen metabolism accompanying brain activity with a simpler device, similar to fMRI.

  In particular, a measurement method (hereinafter referred to as “MBL method”) that combines a device using a stationary light source and an algorithm based on Modified Beer Lambert low has been proposed the earliest and has been widely used (see Non-Patent Document 1). ).

  In this measurement method, near-infrared light incident from the scalp passes through tissues with various scattering coefficients and absorption coefficients, such as skin / muscle, skull, cerebrospinal fluid, membrane, cortical gray matter, and white matter. In order to measure the amount of light reaching the light receiving probe placed at a certain distance from the irradiation position, the amount of measurement includes, in principle, a change in scattering absorption in all the tissues that have passed through.

Among these, what is important in brain function measurement is mainly oxygenated hemoglobin and deoxygenated hemoglobin (oxy-, deoxy-Hb) in cortical gray matter accompanying local blood flow changes caused by neuronal cell activity (see Non-Patent Document 2). ).
Obrig H, Villringer A: Beyond the visible-Imaging the human brain with light.Journal of cerebral blood flow and metabolism 23, 1-18 (2003) Roy CS, Sherrington CS: On the regulation of the blood supply of the brain.J. Physiol., 11, 85-108 (1890) Boas DA, Dale AM, and Franceschini MA, Diffuse optical inaging of brain activation: approaches to optimizing image sensitivity, resolution, and accuracy., Neuro Image, 23, S275-S288, (2004) Okada E, Delpy DT: Near-infrared light propagation in an adult head model.I. Modeling of low-level scattering in the cerebrospinal fluid layer.APPLIED OPTICS 42, 2906-2914, (2003) Matcher SJ, Elwell CE, Cooper CE, Cope M, Delpy DT, Performance Comparison of Several Published Tissue Near-Infrared Spectroscopy Algorithms, Anal. Biochem. 227 (1): 54-68 (1995)

  However, signals actually measured by fNIRS include intrinsic fluctuations associated with heartbeat, respiration, and autonomic nerve activity in addition to brain activity (see Non-Patent Document 3). It is known that fluctuations associated with body movement and posture change occur in various ways. It is a technical problem in fNIRS to suppress these fluctuations and extract this functional signal from the measurement data.

  Although many theoretical studies and experimental studies using phantoms have been made on the state of light propagation in the head, analytically only diffusion propagation in a semi-infinite homogeneous medium has been solved. It is.

  On the other hand, in recent years, an attempt has been made to construct a three-dimensional head structure computationally according to the anatomical knowledge obtained by MRI, and to verify the light propagation here by Monte Carlo simulation (non-patent literature). 4).

  According to this, unlike the so-called banana-like optical path distribution in a homogeneous medium, the light incident on the head diffuses considerably in the process of reaching the cerebrospinal fluid from the scalp (and vice versa). After reaching this point, it is shown that it is connected between the cerebrospinal fluid layer and the surface layer of the brain cortex, almost directly under each probe.

  The scalp, subcutaneous muscle tissue, and bone tissue do not have a distinct topological structure compared to the functional localization of the cerebral cortex.

  Even if a scattering / absorption change occurs there, if the distribution diameter of the light diffused in the process of passing through these layers is 1 cm inside or outside, these tissues are almost in the vicinity of about 1 cm of the light receiving probe. It can be regarded as showing a uniform spectroscopic change.

  Therefore, we introduced a new reference probe in the vicinity of the light-receiving probe, and used the Reference Subtraction technique (RS method) to calculate the amount of change in each hemoglobin (Hb) based on the difference in absorbance change measured at the two positions. Introduced.

<Measurement principle>
Conventionally, the calculation algorithm of each Hb change amount (Δxo, Δxd) in the fNIRS measurement, that is, MBL method, which has been performed using a three-wavelength stationary light source is expressed as follows.
Here, lout is the path length of light from the irradiation probe to the light receiving probe, ε _λ , o, ε _λ , d is the extinction coefficient of each Hb at wavelength λ, Δ A _λ , out is the light of wavelength λ It is the amount of change from the measurement start time of the absorbance when observed with the light receiving probe. + Indicates that a pseudo inverse matrix of the matrix in () is taken.

  On the other hand, a reference probe is newly introduced on the straight line connecting the irradiation probe and the light receiving probe as shown in FIG. Based on the knowledge of the simulation conducted by Okada et al., The light path captured by each probe in this case is considered to have the shape shown in FIG.

  Assuming that the temporal changes in the scattering and absorption of the light passing path directly under the light receiving probe and the reference probe are almost similar, the difference in absorbance observed at these two points is the brain indicated by the ellipse in the figure. It is thought to reflect mainly the hemoglobin dynamics of cortical tissue.

Assuming that the absorbance observed at each of the light receiving probe and the reference probe is Aλi, out, Aλi, ref, and the path length of light from the irradiation probe to the reference probe is lref, the following relationship is established.

Therefore, using the pseudo inverse matrix of the extinction coefficient matrix as in the MBL method, each Hb change amount can be calculated as follows.
In actual calculations, the extinction coefficients at the operating wavelengths of 776 nm, 809 nm, and 850 nm were in accordance with the literature (see Non-Patent Document 5).

  In comparison with the results of the conventional MBL method, in the MBL method, 1) there are motion artifacts due to body movements and body position changes, and sometimes it is several times as large as the brain activity signal. 2) Although there was a systemic response in the bilateral hemisphere that was far wider than the relevant brain activity area, a task-time-related variation (systemic response) was observed, but these variations were significantly reduced in the RS method.

  The best mode for carrying out the invention will be described with reference to the drawings.

<Measurement device>
For the optical probe and control measurement device for fNIRS measurement, an oxygen monitor NIRO-200 and a multi-probe adapter manufactured by Hamamatsu Photonics were used. Sampling frequency 0.5 [Hz], working wavelengths 776nm, 809nm, 850nm, probe tip opening diameter about 2mm. As the probe holder, a holder having a shape shown in FIG. 2 was used. The distance between each irradiation-light receiving probe (1, 3, 5, 7 in the figure) is 30 mm, and the reference probe (2, 4, 6, 8 in the figure) is 20 mm from the irradiation probe on this straight line. Introduced into position.

  These probes are placed symmetrically on both the left and right hemispheres, and probes 1, 2 and 7, 8 are positioned approximately 70mm on both sides from the Cz position of universal 10,20 method, which is the electrode placement method for electroencephalogram measurement. A probe holder was attached to the subject's head. As a result, simultaneous measurement was performed by the MBL method at positions 1, 3, 5, and 7 and the RS method at positions 1-2, 3-4, 5-6, and 7-8.

<Task>
As a verification of motion artifacts, a subject who sat in a sitting position imposed a task of tilting the upper body by about 45 ° by spontaneous movement (hereinafter referred to as “MA task”). Task design is the initial upper body upright
Following the 20s, the set of the body tilt 20s and the body erect 20s is repeated 5 times. In addition, as a verification of functional signal detection, the opposition of the thumb and index finger (hereinafter referred to as finger opposition, “FO task”) was imposed on subjects in the same position. The task design is an initial rest 20 s followed by a set of 5 left finger movements 20 s, rest 20 s, right finger movements 20 s, rest 20 s.

<Result>
[Comparison of results during motion artifact task]
FIG. 3 shows how hemoglobin changes in each probe obtained at the time of the MA problem with the probe arrangement B. Probes 1, 3, 5, and 7 correspond to each Hb variation measured by the MBL method. It can be seen that at each of these positions, each Hb changes significantly in correlation with the MA problem. Further, looking at these time-series changes in detail, it can be seen that different aspects are shown in terms of the presence or absence of drift and the magnitude relationship between the drift component and the problem-related component.

  On the other hand, it can be seen that the data measured by the reference probes 2, 4, 6, and 8 behaves very similar to the time series change of the data obtained at each measurement probe position. As a result, it can be seen that the fluctuation component correlated with the drift and the problem is remarkably reduced in each Hb variation calculated by the RS method. FIG. 4 shows the result of averaging five repetitions.

[Result comparison during finger opposition task]
Fig. 5 shows the data obtained by averaging the hemoglobin changes in each probe obtained during FO task with probe arrangement B over the interval from 10 seconds before the left finger movement to 20 seconds after the task ends and from 10 seconds before the right finger movement to 20 seconds after the task ends. Show. The MBL results obtained with the probes 1, 3, 5, and 7 show that Hb changes according to the FO task at either the same hemisphere or the opposite hemisphere of the motor finger.

  However, the intensity of Hb change in the same hemisphere tends to be slightly weaker than that in the opposite hemisphere. On the other hand, in the Hb change data obtained by the RS method, it can be seen that the change according to the FO task in the ipsilateral hemisphere is remarkably reduced.

  In addition, it can be seen that the change according to the FO task in the opposite hemisphere is remarkable in the probes 1, 2 and 7, 8 unlike the MBL method. In order to quantify these characteristics, as shown in FIG. 6, the difference between each Hb change amount in the 10 seconds immediately before the start of the FO task and 10 seconds after the start of the FO task is 10 seconds. The error is illustrated.

  The numerical values in the figure are t values when t-testing the difference between the opposite condition and the same condition. In the MBL method, t-values of oxy- and deoxy-Hb were low in all channels, and no statistically significant difference was observed between the contralateral and ipsilateral conditions.

  In contrast, in the RS method, it was found that positions showing high t values (corresponding to a risk rate of less than 5%) are limited to 1, 2 and 7, 8.

  As described above, it is well known from experience that fluctuations are superimposed on a measurement signal with body movement during fNIRS measurement. For example, the optical contact between the optical probe and the scalp surface changes with rapid body movement, resulting in irregular signal fluctuations. The results shown in Figs. In contrast to this, a certain offset is generated with the change of the posture, and the change returns to the original level with the return of the posture.

  For this reason, it is considered that this mainly reflects the hemoglobin change that occurs in the living body in the observation region with the change in posture. This value was observed to be the same as the intensity of the task correlation signal during the FO task, or more than several times larger.

  Therefore, when performing MBL measurement under an exercise task that includes such posture changes, a sufficiently careful experimental design is constructed and analyzed to separate and extract components derived from brain function from signals correlated with the task. There is a need to.

  In contrast, the measurement by the RS method significantly reduces these fluctuation components. For this reason, it is considered that there is a high possibility that the influence is canceled even in a task accompanied by a change in posture and a signal derived from the brain function is effectively extracted.

  It has been reported in the past that an increase in oxy-Hb and a decrease in deoxy-Hb that correlate with movement occur in the ipsilateral hemisphere as well as the opposite hemisphere during FO movement with the left or right body side finger.

  In addition, it has been reported that this kind of variation is observed simultaneously in both hemispheres over a wide area that is not limited to the related areas (primary motor areas and somatosensory areas).

  Our measured MBL results agree well with these reports. On the other hand, it is known that the active sites observed by fMRI during the same task are very limited. There is little discussion about the difference in results resulting from differences in measurement methods.

  Al-Rawi et al. Observed head blood flow dynamics associated with internal and external carotid artery clamp during brain surgery, and changes in oxy- and deoxy-Hb measured using the MBL method clamp the external carotid artery. It has been reported that this change is also significant, and this change correlates well with extracranial blood flow fluctuations measured by a laser Doppler blood flow meter.

  This strongly suggests that MBL measurements are susceptible to blood flow fluctuations in extracranial tissues. Blood flow control is generally not limited to local blood flow changes during brain activity, but is also under the control of mechanisms via autonomic nerves, etc., and naturally this mechanism also applies to the scalp and muscle tissue. It is unlikely that the tissue has outstanding site specificity.

  Therefore, if blood flow fluctuations occur in the scalp and muscles accompanying an exercise task, these fluctuations are likely not to have site specificity. One of the reasons why signals that have time correlation with the problems observed by the MBL method are not sufficiently localized compared to fMRI is the possibility that nonlocal fluctuation components due to the above mechanism may be mixed. .

  The present invention in which the reference subtraction technique is introduced in the fNIRS measurement can largely eliminate the influence of such extracranial tissue, and thus it can be understood that the signal localization is improved as compared with the MBL method.

Conceptual diagram explaining the present invention Probe layout Measurement value by MBL method (1, 3, 5, 7) for motion artifact problem, measurement value by reference probe (2, 4, 6, 8), measurement value by RS method (1-2, 3-4 , 5-6, 7-8). Vertical axis: hemoglobin change, horizontal axis: elapsed time, thick line: oxygenated hemoglobin, thin line: deoxygenated hemoglobin motion artifact Results of averaging the changes in each hemoglobin for 5 tasks. Comparison of measured values by MBL method (1, 3, 5, 7) and measured values by RS method (1-2, 3-4, 5-6, 7-8). Vertical axis: hemoglobin change, horizontal axis: elapsed time, thick line: oxygenated hemoglobin, thin line: deoxygenated hemoglobin Comparison of measured values by MBL method (1,3,5,7) and measured values by RS method (1-2, 3-4, 5-6, 7-8) during finger opposition task. Vertical axis: hemoglobin change, horizontal axis: elapsed time, thick line: oxygenated hemoglobin, thin line: deoxygenated hemoglobin, solid line: when performing the opposite hand, wavy line: when performing the same hand Statistical analysis of measurement results by MBL method and RS method during finger opposition task. Average and standard error of measured values for 10 seconds before starting task and 10 to 20 seconds after starting task. Black on the left: oxygenated hemoglobin, white on the right: deoxygenated hemoglobin, Contra: when performing the opposite finger, Ipsi: when performing the same finger. The numerical value is t value in t test between Contra and Ipsi in each condition.

Claims (3)

  An optical measurement device that irradiates a predetermined part of a living body with near infrared light, detects light emitted in the vicinity of the predetermined part, and acquires information about the living body, An optical measurement apparatus comprising a light source, a light receiving probe, and a reference probe in the vicinity of the light receiving probe, and acquiring information related to a living body based on a difference in absorbance change measured by the light receiving probe and the reference probe.   The optical measurement apparatus according to claim 1, wherein the predetermined part is a brain.   The optical measurement apparatus according to claim 1, wherein the information related to the living body is oxygenated hemoglobin and deoxygenated hemoglobin.