JPH0838480A - Biometric device - Google Patents

Abstract

(57) [Summary] [Purpose] Makes the TAC method more advantageous for weak light. [Configuration] 200 MHz modulated light from a laser diode 22 is applied to a subject 2, and diffused / transmitted reflected light from the subject 2 due to the irradiated light is detected by a photomultiplier tube 30. The photomultiplier tube 30 has a gain of 200.02 MH.
The output of the photomultiplier tube 30 is 200.00 MHz and 20.
A beat signal corresponding to a frequency difference of 2 MHz appears.
The output signal of the photomultiplier tube 30 passes through a preamplifier 36 and a discriminator 38, and n time gate circuits 40-1.
To 40-n are input in parallel to the time gate circuit 40-1.
40-n is 1 at 20 KHz by the gate drive circuit 44.
Counters 46-1 to 46-n count the pulse signals that are sequentially operated in each cycle and entered the time gate circuits 40-1 to 40-n while the time gate circuits 40-1 to 40-n are on.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical biometric apparatus for irradiating a subject of a living body with near-infrared light and detecting the diffuse transmitted / reflected light to nondestructively obtain information in the subject. Is. Such an optical biometric device is used, for example, as a biological oxygen monitor or optical CT.

[0002]

2. Description of the Related Art Near-infrared rays in the range of 600 to 1200 nm have good permeability to living organisms and maintain sufficient intensity for measurement even after passing a distance of several cm in living organisms. Conveniently, since the absorption spectra of hemoglobin and cytochrome oxidase, which are important substances that reflect biological functions, exist in this wavelength range, this property of near-infrared light is utilized to eliminate biological functions. Invasive measurements are performed.

A two-dimensional detector such as a CCD camera that irradiates a living body with near-infrared light, scatters the near-infrared light at various parts of the living body, and passes through the living body (called diffuse transmitted / reflected light). It is known that light is received by and the distribution of absorption inside the living body is imaged by calculation.

When an ultrashort light pulse is input to a part of the subject, the time response curve R of the light emitted from the other part of the subject
The analytical solution for (t) can be found in the article by Patterson et al. (APPLIED OPTICS,
It is given by the equation (7) in Vol.28, No.12, pp.2331-2336 (1989). In that document, the absorption coefficient μa and the equivalent scattering coefficient μs ′ (= (1-g) μ included in the time response waveform are described.
s; μs is the scattering coefficient, g is the anisotropic parameter of scattering)
One way to find is shown. (9) in that document
As shown in the formula, the value of the slope (time portion) of the time response waveform when the time is made infinite, that is, the slope of the tail of the time response waveform becomes −μa · c, The coefficient μa is obtained. The equivalent scattering coefficient μs ′ is given as a function of μa in the equation (10) of that document, and if μa is found, μs ′ can also be found by calculation.

The time-resolved measurement method requires an ultra-short pulse laser as a light source, and has a drawback that a large-scale apparatus such as a streak camera or a single photon counting method using a TAC is required as a high-time-resolved light receiving apparatus. As a known technique, FIG. 1A shows an example of a time-resolved measurement method by single photon counting for obtaining a time response curve of minute light emitted from a subject. Excitation pulsed light is emitted from the light source driven by the pulse driving unit 1 to the subject 2 of the living body, and diffuse transmitted reflected light from the subject 2 is detected by the photomultiplier tube 4 as a photodetector. After the output signal of the photomultiplier tube 4 is amplified by the preamplifier 6,
The dark current pulse having a low height is removed by the discriminator 8, and the TAC (Time to Amplit) is synchronized with the excitation pulse light.
The distribution of arrival times ti for a large number of pulses is detected by the ude converter 10 and integrated in the multi-channel analyzer (MCA) 12. (B) shows a time response waveform as a probability distribution of the arrival time of the photoelectron pulse which is the output signal of the photomultiplier tube 4, and the one in which the signals are integrated by the multi-channel analyzer 12 is as shown in (B). It becomes a time response waveform.

In the time-resolved measurement method using single photon counting, as shown in (C), one or more pulses should not come in the measurement time width T (for example, 10 nanoseconds). If two pulses come, the arrival time ti for the first pulse is measured and the subsequent pulses are not measured, which is an error. In practice, it is necessary to weaken the incident light intensity to the photomultiplier tube 4 to such an extent that the pulse does not double up and enter.

On the other hand, as an alternative method, a modulation method has been proposed as a simpler measuring method in terms of equipment (see SPIE Vol.1204, 481-491 (1990)). The modulation method uses high-speed modulated light as irradiation light, and turns on / off the detector with a gate signal having a frequency slightly different from that of the irradiation light, so that a so-called beat signal (irradiation light and detector It is measured as a signal having a frequency different from the gate frequency). When investigating the properties of a living body, the delay of light in the living body is important, and it is utilized that this appears as a phase difference of beat signals.

[0008]

The proposed modulation method is premised on analog signal processing. The analog method is effective when the light intensity is strong, but is disadvantageous in terms of S / N when the light intensity is weak. The photon count method is desirable as a measurement method when the light intensity is weak, but an example of applying the photon count method to the modulation method is not known.

The TAC system shown in FIG. 1 is a so-called single-photon counting method and is excellent in both time resolution and weak light measurement capability, but it is strong in addition to the drawback that the apparatus is complicated. There is a limitation that light cannot be measured (analog measurement cannot be performed). That is,
There is a drawback that the dynamic range for measuring from weak light to strong light is narrow. For this reason, optical CT using the TAC method
In order to configure the above, it is necessary to purposely combine a neutral density filter to weaken the light intensity in a strong light portion.

A first object of the present invention is to provide a biometric device by the photon counting method, which is more advantageous than the TAC method for strong light (light having a large number of counts). A second object of the present invention is to provide a biometric device having a wide dynamic range capable of measuring from weak light to strong light.

[0011]

The biological measuring apparatus of the present invention comprises a light irradiating means for irradiating a living body subject with irradiation light of a high-speed repeating optical pulse train or high-speed modulated light, and a living body subject with the irradiation light. A photodetector that receives the diffuse transmitted reflected light from the photodetector, a photodetector drive circuit that operates the photodetector with a gate signal whose frequency is slightly different from the irradiation light, and a photon pulse that is the detection signal of the photodetector are parallel. A number of time gate circuits for counting the number of times and a plurality of these time gate circuits are synchronized with the timing of the irradiation light, and one cycle of the frequency of the difference between the frequency of the irradiation light and the frequency of the gate signal. A gate drive circuit that sequentially operates for each phase and a counter connected to each time gate circuit are provided.

In order to widen the dynamic range, the biometric device of the present invention can be further configured, if necessary, with an analog processing section for taking in and processing the detection signal of the photodetector as an analog signal. In that case, the photon counting method using the time gate circuit and the counter is used when the received light intensity of the photodetector is small, and the analog photometric method by the analog processing unit is used when the received light intensity of the photodetector is large. Assemble the software.

[0013]

The modulation method is a method of operating a photodetector for detecting light from a subject irradiated with high-speed modulated light with a gate signal having a frequency slightly different from that of the irradiation light.
The photocount method is applied to the modulation method by counting the signals from the photodetector through a discriminator and a plurality of time gate circuits operating in mutually different phases and a counter connected to each of them.

Although only the time average output of the measurement light can be obtained only by counting the photons after the photodetector as it is, by using a large number of time gates and counters connected to the time gates, a time-varying signal can be obtained by the photon counting method. You can take it out. In that case, even if the original signal is a fast nanosecond signal, the rate of change of the signal detected by the photodetector by the modulation method is converted to a signal slower than, for example, microseconds. The hardware configuration of the circuit and the counter becomes simple and inexpensive, and it becomes possible to use these for the required number of divided channels.

[0015]

EXAMPLE FIG. 2 schematically shows an example. 20 KHz
The first stage oscillator 20 for generating the reference pulse signal of
Is provided, and the drive circuit 24 that drives the laser diode 22 that emits the irradiation light that irradiates the subject 2 is based on the reference pulse signal output from the oscillator 20.
It has a second-stage oscillator that generates a high-frequency signal of 00 MHz. The 200 MHz modulated light from the laser diode is applied to the subject 2 by the irradiation optical fiber 26. Diffuse transmitted / reflected light from the subject 2 due to the irradiation light is guided to the photomultiplier tube 30 via the light receiving optical fiber 28 and detected.

The gain of the photomultiplier tube 30 is determined by the high voltage applied to the dynode from the high voltage power source 34, and a high frequency pulse signal is generated in the high voltage power source 34 based on a 20 KHz reference pulse from the oscillator 20. The output signal from the second-stage oscillator 32 is applied to the high-voltage power supply 3
Reference numeral 4 applies a high voltage to the photomultiplier tube 30 at a frequency according to the high frequency pulse signal from the oscillator 32. The frequency of the high frequency pulse signal from the oscillator 32 is the laser diode 22.
Is set to, for example, 200.02 MHz so as to be slightly different from the driving frequency of 200 MHz, and the gain of the photomultiplier tube 30 is modulated at 200.02 MHz, so that the photomultiplier tube 30 is set to 200.02 MHz.
Turn on / off with. As a result, a beat signal corresponding to the frequency difference of 20.00 MHz and 200.02 MHz appears as the output of the photomultiplier tube 30. The configuration up to this point in FIG. 2 is based on the above-mentioned reference SPIE Vol.1204, 481-491.
It is basically the same as the modulation method described in (1990).

The output signal of the photomultiplier tube 30 is amplified by the preamplifier 36, the dark current pulse having a low height is removed by the discriminator 38, and then the n time gate circuits 40-1 to 40-n are provided. Input in parallel. Gate drive circuit 44 for driving the time gate circuits 40-1 to 40-n
20K from the gate signal generating circuit 42 created based on the 20KHz reference pulse from the oscillator 20.
A gate signal having a frequency of Hz is applied, and the gate drive circuit 44 causes the time gate circuit 40-
A gate drive signal for sequentially operating 1 to 40-n is generated. Counters 46-1 to 46-n are connected to the time gate circuits 40-1 to 40-n, respectively, and the time gate circuits 40-1 to 40-n are turned on while the time gate circuits 40-1 to 40-n are on. The pulse signals that have entered are counted. The count value of each of the counters 46-1 to 46-n is the data bus 48.
It is taken into the CPU 50 via the and is subjected to processing such as integration for each counter or fitting to a curve to calculate a phase.

The operation of FIG. 2 will be described with reference to FIG. Since the irradiation light (waveform a) irradiated from the laser diode 22 to the subject 2 is 200 MHz, the irradiation light has 1
There are 20 peaks in ^0-7 seconds. The diffuse transmitted reflected light emitted from the subject 2 is shown as a waveform b,
The light intensity is several orders of magnitude smaller than the irradiation light a, and a phase delay δ is generated. A waveform c is a gate signal of the photomultiplier tube 30 and is for applying a high voltage to a part of the dynode of the photomultiplier tube 30 to modulate the sensitivity. This gate signal is a pulse signal having a frequency of 200.02 MHz. The output signal of the photomultiplier tube 30 becomes a gentle signal of 20 KHz like the waveform d having the frequency of the difference between the waveforms b and c. The original signals a and b were high-speed signals of nanosecond level, but two signals b
And becomes a beat signal with a frequency of the difference between c and c, it is reduced to a slow signal of about microseconds.

When the input signal to the photomultiplier tube 30 is strong, it becomes a continuous signal like the waveform d (part 1), but when it is weak, individual photoelectron pulses like the waveform d (part 2). It is separated and observed, and the probability (frequency) of pulse generation is the signal. Only by counting the output signal of the waveform d (No. 2) as it is, only the average light intensity can be known. Therefore, each phase of one cycle is turned on by turning on each time gate circuit 40-1 to 40-n for a specific time obtained by dividing one cycle (50 microseconds in this example) into n equal parts, for example, 10 equal parts. The respective counters 46-1 to 46-n perform integration. The signals shown as the waveform e are the time gate circuits 40-1 to 40-.
It is a gate signal for opening n.

FIG. 4A shows counters 46-1 to 46-n.
It represents the result of integrating each count value of a number of cycles, for example, tens of thousands of cycles,
The original probability distribution appears. When the phase of the diffuse transmitted / reflected light changes from the irradiation light, the phase δ appears in this beat signal as shown in FIG. 4 (B). The signal received by the photomultiplier tube 30 is a fast nanosecond signal, but as shown in FIG. 4B, the signals obtained as the outputs of the counters 46-1 to 46-n are slow signals of the order of microseconds. Since the time gate circuits 40-1 to 40-n and the counters 46-1 to 46-n have a simple and inexpensive response speed of about microseconds, many of them are used. It will be possible.

As shown in FIG. 4B, when the signal obtained as the integration result of the counter is represented by F (t), the phase δ, the average value W ₀ and the amplitude A are parameters, and for example, (1) Formula F (t) = Asin (2πft + δ) + W ₀ (1) can be applied to obtain the phase δ.

Now, the method according to the present invention will be compared with the single photocounting method using TAC shown in FIG. 1 for comparison. (1) In the case of the TAC method, strong light is disadvantageous both in principle and in practice. As a disadvantage in principle, no more than two pulses can enter the photodetector during the measurement time T. Therefore, the more channels that divide the measurement time T, the more disadvantageous. For example, divide into 10 (T / 1
If used with the time resolution of 0), the channel utilization rate is 1/1
Although it is 0, if the resolution is further increased to divide it into 100, the utilization rate is reduced to 1/100. This means that the pulse comes to only 1/100 of the channel for one measurement time T. On the other hand, in the present invention, there is no such restriction, and even if two or more pulses come in the channel after division, they may be counted as they are. It is possible to measure up to the count rate limit until the pulses become unreadable due to overlapping.

In practice, the disadvantage of TAC is that the counting speed of the MCA (multi-channel analyzer) that counts in each divided channel is determined by the conversion speed of the built-in A / D converter. There is a limit to the speed. For example, the currently available speed is about 50 K pulses / sec for the Wilkinson type A / D converter and about 250 K pulses / sec for the successive conversion type A / D converter, so it is assumed that the maximum speed is 250 K pulses / sec. T = 1
Assuming that 0 nanosecond is 1/10 of one channel (decomposition is 1 nanosecond), 250 × 10 ³ × (1
/ 10) The count is maximum. That is, 25,000 counts / second / channel. If it is divided into 1/100, it will be reduced to 2500 pulses / second per channel.

(2) In the present invention in which the photocount method is used as the modulation method, the limit of the photocount method is TAC.
In addition to the advantage over the method, the method can be switched to the analog method, so that the dynamic range becomes wider. That is, in the modulation method, as shown by the waveform c in FIG. 3, there is a drawback that the effective use time is halved by connecting and disconnecting the photodetector, but as in the waveform d (part 2), the 50 ×
Multiple pulses are allowed during a period of 10 ^-6 seconds.
This point is in principle superior to TAC. There can be as many pulses as one pulse can count separately in time. For example, assuming that the circuit is capable of resolving pulses up to 0.1 microsecond, and the width of one gate is divided into 10 equal parts of 5 × 10 ⁻⁶ seconds, 50 pulses can be counted within 5 microseconds. That is, this is repeated 20 × 10 ³ times in 1 second, so 50 ×
20 × 10 ³ pulses / second = 10 ⁶ pulses / second. This is a numerical value which is about 50 times more advantageous than the case of 1/10 division of TAC.

If the light becomes stronger and the counting capability is exceeded, it is sufficient to switch to analog photometry and measure the waveform d (No. 1). Thus, compared to the TAC method, the method of the present invention can measure up to about two orders of magnitude stronger light with the photon counting method as it is. Furthermore, for stronger light, the measurement dynamic range can be widened by switching to the analog method. In principle, the TAC method can only perform photon counting, and therefore cannot shift to the analog method. It is also possible to prepare n or more sets of time gate circuits and counters, where n is one cycle, for example, a number corresponding to two cycles, that is, 2n sets. This makes it possible to deal with the case where the modulation frequency of the irradiation light is changed. In this case, even if the frequency is halved, the measurement can be performed without changing the hardware.

[0026]

In the present invention, the biometric apparatus such as optical CT is combined with the modulation method and the photon counting method, so that it is possible to count up to a stronger light than the TAC method, and moreover, it is an analog for a stronger light. Can move to law. Therefore, the dynamic range is widened, and a complicated neutral density filter mechanism for adjusting the intensity of light incident on the photodetector is not required. The frequency of the output signal of the photodetector is much lower than the frequency of the irradiation light, so a time gate circuit or counter is used, but they do not require high-speed response, so the structure is simple and inexpensive. I need it.

[Brief description of drawings]

1A and 1B are diagrams showing a conventional measuring apparatus using a single photon counting method using TAC, in which FIG. 1A is a block diagram showing a configuration, and FIG. 1B is a diagram showing a time response waveform;
(C) is a diagram showing a detection pulse in a measurement time width T.

FIG. 2 is a block diagram showing an embodiment.

FIG. 3 is a waveform diagram showing signals of respective parts in FIG.

FIG. 4A is a diagram showing a result of integrating counter count values in the same embodiment, and FIG. 4B is a diagram showing counter count values when there is a phase shift δ.

[Explanation of symbols]

 2 subject 20 first-stage oscillator for generating reference pulse 24, 32 second-stage oscillator 22 laser diode 30 photomultiplier tube 38 discriminator 40-1 to 40-n time gate circuit 46-1 to 46- n counter 44 gate drive circuit 48 bus line 50 CPU 51 analog phase detection circuit 52 A / D converter

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ichiro Oda 1 Kuwahara-cho, Nishinokyo, Nakagyo-ku, Kyoto Prefecture Kyoto Prefecture Shimazu Corporation Sanjo Factory

Claims (2)

[Claims] 1. A light irradiating means for irradiating a living body test object with irradiation light of a high-speed repeating light pulse train or high-speed modulated light, and light detection for receiving diffused / transmitted reflected light from the living body test object by the irradiation light. And a photodetector drive circuit that operates the photodetector with a gate signal having a frequency slightly different from the irradiation light, and a plurality of times for counting photon pulses that are detection signals of the photodetector in parallel. A gate drive that sequentially operates a gate circuit and a plurality of these time gate circuits in synchronization with the timing of the irradiation light and for each phase of one cycle of the frequency of the difference between the frequency of the irradiation light and the frequency of the gate signal. A biometric device comprising a circuit and a counter connected to each of the time gate circuits. 2. A differential frequency signal output from the photodetector is added to the circuit for inputting to a time gate circuit to analog-amplify the differential frequency signal output from the photodetector. An analog processing unit for analog-detecting the phase of the difference frequency signal is provided, and when the received light intensity of the photodetector is small, the photon counting method by the time gate circuit and the counter is used. The biometric device according to claim 1, wherein an analog photometric method is selectively used.