JP2006314462A - Biological photometric apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a biological photometric apparatus capable of improving a precision of detecting detecting light. <P>SOLUTION: The apparatus comprises a main lock-in processing part 12 for extracting signals synchronizing with reference signals by taking a modulation frequency as the reference signal, a first correcting lock-in processing part 13 for performing correcting lock-in processing by using the reference signals with a frequency lower than the modulation frequency, a second correcting lock-in processing part 14 for performing correcting lock-in processing by using reference signals with a frequency higher than the modulation frequency, and a noise correcting part 15 for correcting the intensity of the detecting light detected by the main lock-in processing by using results of the correcting lock-in processing by the first and second correcting lock-in processing parts 13, 14. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a biological light measurement apparatus that measures the optical characteristics of a subject using light.

  In general, a biological light measurement apparatus obtains an optical change accompanying a physiological change of a subject from a weak change in detected light intensity. Therefore, in order to perform highly accurate measurement excluding the influence of disturbance light, a lock-in light measurement method is used in which the irradiation light is modulated at a specific period and the detected optical signal is synchronized and measured by a lock-in circuit. ing. The lock-in type measurement circuit is a measurement circuit method capable of selectively measuring a signal having a specific frequency with high accuracy (see, for example, Patent Document 1).

JP 7-146151 A

  In the conventional biological light measurement apparatus as described above, the irradiation light is greatly attenuated by absorption and scattering when passing through the subject. Further, the intensity of irradiation light is limited from the viewpoint of safety. For this reason, the intensity of the detection light becomes weak, and the detection signal is easily affected by circuit noise. On the other hand, in order to detect weak detection light with high accuracy, a photodetector having an electron multiplication function, for example, a photomultiplier tube or an avalanche photodiode (APD) is used.

  However, since such an electron multiplication type photodetector accelerates and multiplies electrons at a high voltage, it generates shot noise due to spontaneous generation of electrons. Since this shot noise generates a pulsed signal in a short time, a broadband signal is generated intermittently. In addition, since shot noise has a wide band, it leaks into the modulation frequency range of the measurement signal and is difficult to remove even with a gorge band filter using a lock-in circuit. For this reason, when the light attenuation in the subject is large, sufficient detection accuracy may not be obtained even if a highly sensitive electron multiplier type photodetector is used.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to obtain a biological light measurement device capable of improving the detection accuracy of detection light.

  The biological light measurement apparatus according to the present invention modulates the intensity of detection light obtained from a subject by irradiating the subject with the light source unit that generates the illumination light that is intensity-modulated at a predetermined modulation frequency. An intensity detection unit that detects by a main lock-in process using a signal having the same frequency as the reference signal as a reference signal, and the intensity detection unit is a result of the correction lock-in process using a signal having a frequency different from the modulation frequency as the reference signal. Is used to correct the intensity of the detected light detected by the main lock-in process.

  The biological light measurement device of the present invention corrects the intensity of the detection light detected by the main lock-in process using the result of the correction lock-in process using a signal having a frequency different from the modulation frequency as a reference signal. The influence of shot noise can be reduced, and the detection accuracy of detection light can be improved.

The best mode for carrying out the present invention will be described below with reference to the drawings.
Embodiment 1 FIG.
1 is a block diagram showing a main part of a biological light measurement device according to Embodiment 1 of the present invention. In the figure, the subject 1 is irradiated with irradiation light generated by the light source unit 2. The intensity of the detection light obtained when the irradiation light is reflected or transmitted through the subject 1 is detected by the intensity detection unit 3. Specifically, the intensity detection unit 3 measures a temporal change in the intensity of the detection light. For example, if the subject is a human head, the information on the detected light intensity detected by the intensity detector 3 passes through the skin and the skull and includes information on the blood flow of the cerebrum.

  The light source unit 2 includes a laser light source 4 such as a semiconductor laser that generates laser light, and a laser drive circuit 5 that drives the laser light source 4. The laser light source 4 emits light having a wavelength in the visible to infrared wavelength region, for example, a wavelength in the vicinity of 780 nm. The wavelength is not limited to 780 nm, and the number of wavelengths is not limited to one wavelength.

The laser drive circuit 5 modulates the intensity of irradiation light based on a predetermined modulation frequency f ₀ given by the oscillator 6. As the modulation frequency f ₀ , for example, 10 KHz that can sufficiently measure a change in blood flow of a living body is selected.

Specifically, the laser drive circuit 5 applies a direct current bias current to the laser light source 4 and applies a modulation frequency f ₀ to modulate the irradiation light emitted from the laser light source 4. As this modulation method, digital modulation by a rectangular wave is used. Further, repetition of an arbitrary waveform such as a sine wave may be used as a modulation method.

The intensity detection unit 3 includes a photodetector 7, a lock-in amplifier processing unit 8, and a computer 9. The photodetector 7 converts the detected light intensity into an electric signal. As the photodetector 7, for example, a high-sensitivity electron multiplying detector such as APD is used. The lock-in amplifier processing unit 8 performs main lock-in processing using a signal having the same phase and frequency as the modulation frequency f ₀ obtained from the oscillator 6 as a reference signal.

  FIG. 2 is a block diagram showing a more specific configuration of the intensity detector 3 of FIG. The lock-in amplifier processing unit 8 includes an amplifier 10, an A / D converter 11, a main lock-in processing unit 12, a first correction lock-in processing unit 13, a second correction lock-in processing unit 14, and a noise correction unit. 15. The amplifier 10 converts the signal from the photodetector 7 into a voltage.

  The A / D converter 11 converts the analog signal from the amplifier 10 into a digital signal. The conversion cycle of the A / D converter 11 is set to 100 KHz, which is sufficiently larger than the modulation cycle of the light source unit 2. Thereby, the sample Nyquist frequency of the digital signal becomes 50 KHz, and the modulation signal 10 KHz can be sampled with sufficient accuracy.

  As the lock-in processing units 12 to 14, DSPs (Digital Signal Processors) are used. Digital signals output from the A / D converter 11 are processed in parallel by the lock-in processing units 12 to 14.

The main lock-in processing unit 12 performs the main lock-in process described above. That is, the main lock-in processing unit 12 extracts a signal synchronized with the reference signal using the modulation frequency f ₀ (10 KHz) as a reference signal. The cutoff frequency of the low-pass filter in the main lock-in processing unit 12 is 100 Hz, and the output signal from the main lock-in processing unit 12 is sent to the noise correction unit 15 as a digital signal sequence of 50 samples / second.

The first and second correction lock-in processing units 13 and 14 perform correction lock-in processing using a signal having a frequency different from the modulation frequency f ₀ as a reference signal. Specifically, the first correction lock-in processing unit 13 performs a correction lock-in process using a reference signal having a frequency f ₁ (for example, 8 KHz) lower than the modulation frequency f ₀ . The cutoff frequency of the low-pass filter of the first correction lock-in processing unit 13 is 100 Hz, and the output signal from the first correction lock-in processing unit 13 is a noise correction unit as a digital signal sequence of 50 samples / second. 15 is sent.

The second correction lock-in processing unit 14 performs a correction lock-in process using a reference signal having a frequency f ₂ (for example, 12 KHz) higher than the modulation frequency f ₀ . Here, the frequencies f ₁ and f ₂ are set so that the modulation frequency f ₀ is intermediate between the frequency f ₁ and the frequency f ₂ . The cutoff frequency of the low-pass filter of the second correction lock-in processing unit 14 is 100 Hz, and the output signal from the second correction lock-in processing unit 14 is a noise correction unit as a digital signal sequence of 50 samples / second. 15 is sent.

  The noise correction unit 15 corrects the intensity of the detected light detected by the main lock-in process using the result of the correction lock-in process by the first and second correction lock-in processing units 13 and 14. For example, a DSP is used as the noise correction unit 15.

  As the lock-in process, a phase-insensitive lock-in amplifier process is performed that outputs the average of the sum of squares of the phase detection signals based on the reference signals whose sine and cosine phases are shifted from each other by 90 degrees. Thereby, signal fluctuation due to phase difference fluctuation between the reference signal and the irradiation light can be prevented. In addition, the phase adjustment work is not necessary, and the influence of the phase shift of shot noise in the noise correction process can be eliminated, thereby enabling highly accurate noise correction.

Here, FIG. 3 is a graph showing the relationship between the frequency and intensity of the detection light normally detected by the intensity detector 3 of FIG. By modulating the intensity of the irradiation light at the modulation frequency f ₀ (10 KHz), the intensity of the detection light also has a sharp peak at the frequency f ₀ . Outputs in other frequency bands can be considered as noise due to stray light from the outside.

  Since the stray light is mainly natural light, the noise has a stable intensity in the time direction in a wide frequency band. That is, the noise power spreads uniformly in the frequency direction and the time axis direction. For this reason, the influence on the lock-in amplifier measurement signal which is a gorgeous band measurement is small.

  However, when weak light is measured using a high-sensitivity photodetector, pulsed shot noise having a large peak in a short time is generated in addition to noise due to stray light. For example, in the case where an APD is used as the photodetector 7, if the pressurization potential of the electron multiplier is increased, the shot noise occurrence rate increases. Such shot noise is an intermittent and narrow pulse shape that is intermittent at a long time interval compared to the response time of the measuring device, and thus propagates to the lock-in amplifier processing unit 8 after filtering and becomes a noise source. .

  FIG. 4 is a graph showing temporal changes in the output signals of the lock-in processing units 12 to 14 in FIG. 2 and the signals after correction by the noise correction unit 15. As shown in FIG. 4, when shot noise occurs, noise signals similar to the three signals after the lock-in process for each frequency are added. In this case, white noise caused by stray light or thermal noise of the circuit system also overlaps with shot noise, but white noise is smaller than shot noise.

Next, an example of specific correction processing in the noise correction unit 15 will be described. Signals from the lock-in processing units 12 to 14 are input to the noise correction unit 15 at a rate of 50 samples / second. In the present embodiment, since the three frequencies (f ₀ , f ₁ , f ₂ ) are close to each other, the power spectrum distribution of noise due to Schottky pulse noise in the frequency space is a substantially linear function of the frequency f. If LAn (f ₀ ) is an estimated value of the noise power caused by the shot pulse at the measurement frequency, the following equation is established.
LAn (f ₀ ) = (f ₀ −f ₁ ) (LA (f ₂ ) −LA (f ₁ )) / (f ₂ −f ₁ ) + LA (f ₁ )
As a result, the signal after shot noise correction can be obtained by the following equation.
LA (f ₀ ) ′ = LA (f ₀ ) −LAn (f ₀ )

  When the number of noise measurement windows is further increased, noise estimation and correction can be performed using a spectrum function estimated by shot noise. For example, if shot noise is a Gaussian function, the noise function becomes a Gaussian function that is the reciprocal of this 1 / e width σ. Therefore, by fitting the LA signal at the measurement frequency using the σ as a variable, accuracy can be obtained. High noise estimation and correction.

  If the process as described above is performed at 50 samples / second by the noise correction unit 15 configured by a DSP circuit, it is possible to correct the noise of the signal in real time. The corrected signal is input to the computer 9, and the signal change is displayed in real time on the display unit of the computer 9.

  As described above, in the biological light measurement device according to this embodiment, the detection light detected by the main lock-in process using the result of the correction lock-in process using a signal having a frequency different from the modulation frequency as a reference signal. Therefore, the influence of shot noise can be reduced, and the detection accuracy of detection light can be improved.

  The lock-in amplifier processing unit 8 includes a first correction lock-in processing unit 13 that uses a frequency lower than the modulation frequency as a reference signal, and a second correction use that uses a frequency higher than the modulation frequency as a reference signal. Since the lock-in processing unit 14 is provided, the intensity can be corrected more accurately.

In the above example, the main lock-in process, the correction lock-in process, and the noise correction process are executed by the DSP circuit, but may be executed by a computer if a sufficient processing speed is obtained.
In the above example, the configuration example of the digital lock-in circuit is shown. However, the same function can be obtained by using an analog lock-in circuit having the same function.
Furthermore, in the above example, the first and second correction lock-in processing units 13 and 14 are used, but the number of correction lock-in processing units may be one or three or more. That is, it is also possible to use third to nth correction lock-in processing units that use frequencies f _{2 to} f _n different from f ₁ and f ₂ as reference frequencies.

It is a block diagram which shows the principal part of the biological light measuring device by Embodiment 1 of this invention. It is a block diagram which shows the more concrete structure of the intensity | strength detection part of FIG. It is a graph which shows the relationship between the frequency and intensity | strength of the detection light normally detected by the intensity | strength detection part of FIG. It is a graph which shows the time change of the signal after the output signal of the lock-in process part of FIG. 2, and the correction | amendment in a noise correction | amendment part.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Subject, 2 Light source part, 3 Intensity detection part, 12 Main lock-in process part, 13 1st correction | amendment lock-in process part, 14 2nd correction | amendment lock-in process part

Claims (2)

A light source unit that generates irradiation light whose intensity is modulated at a predetermined modulation frequency;
An intensity detector that detects the intensity of the detection light obtained from the subject by irradiating the subject with the irradiation light by a main lock-in process using a signal having the same frequency as the modulation frequency as a reference signal; and
The intensity detector corrects the intensity of the detection light detected by the main lock-in process using a result of the correction lock-in process using a signal having a frequency different from the modulation frequency as a reference signal. A biological light measurement device.   The intensity detection unit includes a first correction lock-in processing unit that uses a frequency lower than the modulation frequency as a reference signal, and a second correction lock-in processing unit that uses a frequency higher than the modulation frequency as a reference signal. The living body light measuring device according to claim 1, wherein:

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