JP6045100B2 - Blood flow measurement device - Google Patents

Description

  The present invention relates to a blood flow measurement device, and more particularly to a technique for improving the measurement accuracy of a laser blood flow meter that measures the blood flow of biological tissue using laser light.

  As a device for measuring the blood flow state of a living tissue, a laser blood flow meter is widely used today in fields such as clinical medicine, preventive medicine, and sports medicine. This laser blood flow meter calculates the blood flow volume of a living tissue by irradiating a living tissue with a laser beam, receiving a part of the laser light scattered in the living tissue, performing photoelectric conversion, and performing arithmetic processing. It is.

  The laser light irradiated on the living body is scattered many times in the tissue, a part of which is scattered by the red blood cells flowing through the blood vessels, and is frequency-modulated. Since the laser light is scattered by biological tissue before reaching the red blood cells, the angle at which the red blood cells are irradiated is not constant, and the flow direction of the microcirculatory blood flow is not one direction. The power spectrum has a spread with respect to frequency as shown in FIG.

  FIG. 9 is a block diagram showing a calculation process of tissue blood flow in such a conventional laser blood flow meter. As shown in the figure and the following formula 1, in the conventional blood flow meter, the laser light emitted from the laser light emitting unit 13 is irradiated to the living tissue 10 through the optical fiber probe 14, and the scattered light is detected by the photodetector 15. Then, photoelectric conversion is performed, a blood flow signal is extracted from the spectrum signal S (ω) obtained by the photoelectric conversion by the bandpass filter 41, and the power spectrum P (ω) is weighted and integrated by the angular frequency ω. The blood flow volume of the tissue is calculated by normalizing this with the square of the received light intensity I.

Tissue blood flow = (∫ωP (ω) dω) / <I> ² (Formula 1)

  Moreover, there are the following patent documents related to such a laser blood flow meter.

JP-A-11-19074 JP 2003-194714 A JP 2007-125144 A

  By the way, the blood flow volume of the living tissue varies greatly depending on the site in the living body. The blood flow velocity ranges from about 0.1 mm / s to several mm / s, and the spread of the frequency of the power spectrum of the synthesized signal is different between the slow blood flow and the fast blood flow.

  On the other hand, the blood flow signal contained in the scattered light from the living tissue is extracted by the bandpass filter as described above. However, in the conventional blood flow meter, each part of the device is configured by hardware, and the detectable frequency range Is fixed to the passband of the bandpass filter (generally about 20 Hz to 20 kHz).

  Therefore, in the conventional blood flow meter, when the blood flow velocity is low, the error has to be large, and high-precision measurement cannot be performed. This is because when the blood flow velocity is low, only low frequency modulation of about several kHz exists, and this noise component is present only in the high frequency region (for example, 10 kHz or more) of the detection frequency band. This is because the S / N ratio deteriorates because of the uniform calculation.

  Conversely, when the blood flow velocity is high, the frequency range of the power spectrum of the blood flow signal may exceed the band set by the bandpass filter. In this case, the blood flow rate may be underestimated. There is.

  As described above, the conventional blood flow meter does not perform appropriate signal extraction in consideration of the difference in blood flow velocity, and leaves room for further improvement in this respect.

  Accordingly, an object of the present invention is to eliminate an error due to a difference in blood flow velocity and improve the measurement accuracy of the laser blood flow meter.

  In order to solve the above problems and achieve the object, a first blood flow measurement device according to the present invention includes a laser light irradiation unit that irradiates a living tissue with laser light, and scattered light of the laser light scattered by the living tissue. A scattered light receiving unit that receives light, a photoelectric conversion unit that photoelectrically converts the scattered light received by the scattered light receiving unit and outputs an analog electric signal, and an analog electric signal converted by the photoelectric conversion unit as a digital signal An A / D converter (analog-to-digital converter) for converting the digital signal, a fast Fourier transform for obtaining a power spectrum of a blood flow signal by performing fast Fourier transform on the digital signal converted by the A / D converter, and the high speed A power spectrum display unit that displays the power spectrum of the blood flow signal obtained by the Fourier transform unit, and is used to calculate the blood flow rate from the power spectrum of the blood flow signal. A frequency range setting unit capable of specifying a wave number range; and a blood flow rate calculation unit that calculates a blood flow rate using a power spectrum of the blood flow signal in a frequency range specified by the frequency range setting unit; Is provided.

  In the first blood flow measuring device of the present invention, before calculating the blood flow, the power spectrum of the blood flow signal is displayed by the power spectrum display unit to confirm the spread (frequency range) of the power spectrum. The frequency range used for calculating the blood flow rate can be specified by the frequency range setting unit.

  According to such an apparatus of the present invention, for example, an operator (user of the apparatus) selects an appropriate frequency range corresponding to the blood flow velocity of the biological tissue to be measured, whereby the selected optimum frequency. Since it is possible to calculate the blood flow volume with the power of the range, the measurement error (calculated blood flow volume) is calculated by adding the power in the high frequency region where only the noise component exists when the blood flow velocity is low as in the past. It is possible to prevent a situation in which the measurement accuracy decreases due to the exclusion of data that should be originally used for the calculation of the blood flow when the blood flow velocity is high.

  The blood flow measurement device further includes a noise calculation unit that calculates an offset value based on a noise component proportional to one or both of the amount of light received by the scattered light receiving unit and the frequency range, and the blood flow calculation The unit may calculate the blood flow by subtracting the offset value when calculating the blood flow (the same applies to the second blood flow measurement device described later).

  In an actual measurement device, noise that causes errors, specifically, noise (noise) from the electronic circuit that constitutes the device, and laser light reflected by the living tissue are generated by the scattered light receiving unit. Shot noise occurs. As described later in detail in the embodiment, these noises have a magnitude proportional to the amount of light received by the scattered light receiving unit and the frequency range. Accordingly, in one aspect of the present invention, an offset value proportional to one or both of the received light amount and the frequency range in the scattered light receiving unit is calculated, and correction for subtracting the offset value from the blood flow rate is performed when calculating the blood flow rate. Do. Thereby, the measurement accuracy can be further improved.

  In the noise correction, it is preferable to consider both the amount of received light and the frequency range in terms of performing highly accurate measurement. However, even if correction is performed based on one of these, the noise is not limited. Since the measurement accuracy can be improved as compared with the case where no consideration is taken into consideration, an apparatus for performing noise correction based on either the received light amount or the frequency range is also included in the scope of the present invention.

  Furthermore, in the present invention, the frequency range can also be set automatically (independent of the operator's operation).

  Specifically, the second blood flow measurement device according to the present invention includes a laser light irradiation unit that irradiates a living tissue with laser light, and a scattered light reception that receives scattered light of the laser light scattered by the living tissue. A photoelectric conversion unit that photoelectrically converts the scattered light received by the scattered light receiving unit and outputs an analog electric signal, and an A / D that converts the analog electric signal converted by the photoelectric conversion unit into a digital signal A fast Fourier transform unit that obtains a power spectrum of a blood flow signal by performing fast Fourier transform on the digital signal converted by the A / D converter, and power of the blood flow signal obtained by the fast Fourier transform unit Of the spectrum, the blood flow rate is calculated using a frequency range setting unit that detects up to a frequency corresponding to a preset threshold power and a power spectrum in the detection frequency range. And a blood flow calculator for.

  In the second blood flow measuring device, the frequency range setting unit does not wait for the operator's input operation, but sets the detection frequency range based on a preset threshold value. That is, a frequency showing a value larger than the preset threshold power is set as a detection frequency range, and the blood flow rate calculation unit calculates the blood flow rate based on the frequency and power in the detection frequency range.

  Whereas the conventional blood flow meter could not confirm the extent of the frequency spread of the blood flow signal power spectrum, according to the blood flow measuring device of the present invention, the spread of the power spectrum Blood flow can be calculated after confirming the above, or by selecting the optimal frequency range with less noise corresponding to the spread of the power spectrum, so blood flow can be measured with higher accuracy than before. It becomes.

  Other objects, features, and advantages of the present invention will become apparent from the following description of embodiments of the present invention described with reference to the drawings. In addition, in each figure, the same code | symbol shows the same or an equivalent part.

FIG. 1 is a block diagram showing a blood flow measuring device according to an embodiment of the present invention. FIG. 2 is a diagram illustrating a display screen by the display unit of the blood flow measuring device according to the embodiment. FIG. 3 is a diagram showing an example (forearm skin) of a power spectrum of a blood flow signal from a living tissue. FIG. 4 is a diagram showing another example (index finger skin) of a power spectrum of a blood flow signal from a living tissue. FIG. 5 is a diagram conceptually showing the relationship between the frequency and power of laser scattered light received from different (three types) biological tissues. FIG. 6 is a diagram showing temporal changes in blood flow obtained from the power spectrum of FIG. 4 (forearm skin). FIG. 7 is a diagram showing temporal changes in blood flow obtained from the power spectrum of FIG. 5 (forefinger). FIG. 8 is a diagram showing the magnitude of the integrated value of the power spectrum caused by noise in the blood flow measurement device. The output of the photoelectric conversion unit and the frequency when laser light is directly input to the photoelectric conversion unit. It is a figure which shows the relationship with the integral value (Σf * P (f) (DELTA) f mentioned later) of the multiplied power spectrum. FIG. 9 is a block diagram showing a conventional blood flow measuring device. FIG. 10 is a diagram conceptually showing the relationship between the frequency and power of laser scattered light received from a living tissue.

  As shown in FIG. 1, a blood flow measuring device 11 according to an embodiment of the present invention performs a blood flow calculation process with a laser unit 12 that irradiates a living tissue with laser light and receives scattered light, and obtains the result. An apparatus main body (computer) 21 for displaying.

  The laser unit 12 receives a laser light emitting unit 13 that emits laser light, an optical fiber probe 14 that irradiates the living tissue 10 with the laser light emitted from the laser light emitting unit 13 and receives the scattered light. A photoelectric conversion unit 15 that photoelectrically converts scattered light, and an A / D conversion unit 16 that converts an analog electric signal converted by the photoelectric conversion unit 15 into a digital signal are provided. The photoelectric conversion unit 15 includes a photodiode (PD) that performs photoelectric conversion and an amplifier that amplifies a weak electric signal output from the PD. Further, the A / D converter 16 may be included in the apparatus main body 21.

  The apparatus body 21 includes an FFT processing unit 22 that performs a fast Fourier transform on the digital signal converted by the A / D conversion unit 16, a blood flow calculation unit 23 that calculates a blood flow, and a frequency range that is used to calculate the blood flow. The frequency range setting unit 25 for setting the frequency range, the input unit 24 for inputting the threshold power serving as a reference for determining the frequency range to the frequency range setting unit 25, and the noise component (offset value) to be subtracted from the blood flow rate are calculated. And a display unit 27 for displaying the blood flow calculated by the blood flow calculation unit 23, the power spectrum of the blood flow signal obtained by the FFT processing unit 22, and the like. In addition, each part structure (FFT process part 22, blood flow volume calculating part 23, noise calculating part 26 grade | etc.,) Of the apparatus main body part 21 can be comprised by computer software.

  The laser light emitted from the laser light emitting unit 13 to the living tissue 10 through the optical fiber probe 14 is scattered by the living tissue 10, a part thereof is received through the optical fiber probe 14, and converted into an analog electric signal by the photoelectric conversion unit 15. After that, it is converted into a digital signal by the A / D conversion unit 16 and input to the FFT processing unit 22 of the apparatus main body unit 21.

  The FFT processing unit 22 performs a fast Fourier transform on the input digital signal to obtain a power spectrum of the blood flow signal. The blood flow signal power spectrum is output to the blood flow calculation unit 23 and used for calculation of the blood flow, and is displayed on the display screen of the display unit 27 (see reference numeral 31 in FIG. 2). FIG. 2 illustrates a display screen by the display unit 27. On the screen, in addition to the display 31 of the blood flow signal power spectrum, a display 32 that shows a temporal change of the blood flow value in a graph, A numerical display 33 showing the blood flow in real time is performed.

  The blood flow rate calculation unit 23 calculates the blood flow rate from the blood flow signal power spectrum, and this blood flow rate can be obtained by the following formula 2 which is a digital calculation formula of the formula 1.

Tissue blood flow = Σf · P (f) Δf / P ² (0) (Expression 2)

  In Equation 2, f is the frequency, P (f) is the power at the frequency f, and P (0) is the power at 0 Hz obtained by FFT. Further, a threshold power is set in advance in the frequency range setting unit 25 through the input unit 24, and the blood flow calculation unit 23 uses the threshold set in the frequency range setting unit 25 in calculating the blood flow. A frequency range of a blood flow signal power spectrum used for blood flow calculation is determined, calculation is performed using data (frequency value and power value) of the frequency range, and an offset value calculated by the noise calculation unit 26 is subtracted. To calculate the blood flow volume. The determination of these frequency ranges and the calculation of the offset value by the noise calculator 26 will be described next.

[Determination of frequency range]
3 to 4 are diagrams illustrating power spectra of blood flow signals from living tissue (FIG. 3 is forearm skin and FIG. 4 is index finger skin). As shown in these figures, depending on the portion of the living tissue, FIG. The spectrum distribution is very different. This is due to the difference in blood flow velocity in the tissue. The forearm skin (FIG. 3) has a signal component only up to about 10 kHz, whereas the index finger skin (FIG. 4) has a signal component up to 50 kHz or more. .

  Therefore, in the apparatus 11 of the present embodiment, a certain power level is set as a threshold value in the frequency range setting unit 25 via the input unit 24, and the blood flow rate calculation unit 23 is set in the frequency range setting unit 25. The blood flow rate is calculated using data in the frequency range corresponding to the threshold value.

  More specifically, FIG. 5 is a diagram conceptually showing the relationship between the frequency and power of laser scattered light received from three types of biological tissues having different blood flow velocities. As shown in FIG. Assuming that the threshold value is Pt, the blood flow rate calculation unit 23 treats power equal to or greater than the value Pt as a signal. For example, when the blood flow signal power spectrum is S1, the blood flow rate is calculated using the power in the frequency range W1 from 0 Hz to the frequency F1 corresponding to the Pt. Similarly, in the case of blood flow signal power spectrum S2, data from 0 Hz to frequency F2 (frequency range W2) is used, and in the case of blood flow signal power spectrum S3, data from 0 Hz to frequency F3 (frequency range W3) is used. Blood flow is calculated using the data.

  The specific value of the threshold value Pt is not limited to this value. However, in the example shown in FIGS. 3 to 4, it may be set to about −55 dB, for example.

  FIGS. 6 to 7 show temporal changes in the tissue blood flow obtained from the power spectra shown in FIGS. 3 to 4. FIG. 6 shows the result of calculating up to 10 kHz for the power spectrum shown in FIG. FIG. 7 shows the result of calculating up to 50 kHz for the power spectrum of FIG. 4 as the detection frequency range. As is clear from these results, a difference in power spectrum appears as a large difference in tissue blood flow value. Further, as can be seen from FIG. 6, since the noise component of 10 kHz or more can be removed by limiting the detection frequency range to 10 kHz, the blood flow synchronized with the heartbeat even if the tissue blood flow value is about 1-2. It becomes possible to measure changes stably.

  As described above, in the measurement apparatus according to the present embodiment, an appropriate signal in consideration of the difference in blood flow velocity is obtained by changing the frequency range W1 to W3 that is the basis of blood flow calculation in accordance with the degree of spread of the power spectrum. Extraction can be performed, which enables high-precision measurement independent of the blood flow velocity of the measurement target.

[Removal of noise]
In living tissue where blood flow does not exist, the value of tissue blood flow should be essentially zero, but in reality it is not zero due to noise generated from the electronic circuit and shot noise at the photodetector due to laser light reception Indicates the value. Further, if the detection frequency range (frequency range of data set by the frequency range setting unit 25 and used by the blood flow calculation unit 23) is widened, the integrated value of the power spectrum due to noise generated from the electronic circuit becomes large, As the intensity of the received laser beam increases, the shot noise increases.

  Therefore, in order to examine the integrated value of the power spectrum caused by noise, an experiment was conducted in which the laser light was attenuated and directly input to the photoelectric conversion unit 15. FIG. 8 shows the result of this experiment, that is, the light reception intensity I [V] in each detection frequency range ((1) 10 kHz, (2) 25 kHz, (3) 50 kHz, and (4) 100 kHz) and the square of the light reception intensity. It shows the relationship with the integral value Σf · P (f) Δf of the power spectrum multiplied by each frequency before conversion.

In FIG. 8, the received light intensity I is set so that the received light amount of about 3 nW becomes an output of 1 V at the output of the photoelectric conversion unit 15. The vertical axis represents an unnormalized integrated value of the power spectrum multiplied by each frequency, that is, Σf · P (f) Δf in Equation (2). In addition, the regression line formula (with the horizontal axis as the x-axis and the vertical axis as the y-axis) and the determination coefficient R ² (the square of the correlation coefficient R) outside the diagram area are shown for each detection frequency range (1) to (4). Value). In the same diagram, the value of Σf · P (f) Δf when I = 0 [V] indicates the noise of only the photoelectric conversion unit 15, and the value of Σf · P (f) Δf due to the increase in the amount of received light I. The increase corresponds to shot noise.

  From FIG. 8, in each detection frequency range, the integral value Σf · P (f) Δf of the power spectrum multiplied by the frequency due to the noise component is proportional to the amount of received light I, and the value (Σf · P (f) Δf) is the detection frequency range. It can be seen that it is proportional to. Therefore, by utilizing these relationships, an offset value due to a noise component can be obtained from several received light amount-power spectrum characteristics.

  Therefore, in the apparatus of the present embodiment, the noise offset value (the amount of blood flow value offset) is calculated from the detection frequency range set by the frequency setting unit 25 and the received light amount (output from the photoelectric conversion unit 15). The unit 26 calculates, and the blood flow rate calculation unit 23 removes (subtracts) this offset value to calculate the tissue blood flow rate. By performing such arithmetic processing, the measurement accuracy can be further improved.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to these, It is clear to those skilled in the art that a various change can be made within the range as described in a claim. .

  For example, in the above-described embodiment, the detection frequency range is automatically set (the threshold value preset in the frequency range setting unit 25 is used), but the device user manually (the device user's The detection frequency range may be set (by an input operation). In this case, for example, the setting function can be called by clicking the “SETUP” button on the screen shown in FIG. 2, and the threshold power is designated in the same way as the automatic setting, or the upper limit frequency value or It suffices to be able to specify the frequency range.

  Furthermore, regarding the setting of the detection frequency range, it is possible to provide an apparatus having both the automatic setting function using a threshold as in the above-described embodiment and the above-described manual setting function by an input operation of the apparatus user. In this case, for example, it is possible to switch between the automatic mode and the manual mode with a changeover switch, for example, and it is possible to provide a function for recalculating blood flow by manually inputting a detection frequency range after automatic calculation, for example. It is.

  Further, noise correction (calculation of offset value and removal from blood flow value) by the noise calculation unit 26 can also be performed manually in the same manner as the above detection frequency range. The offset removal by the component may be performed by subtracting the blood flow value output by measuring a plastic or the like having the same optical characteristics as the biological tissue from the blood flow value at the actual biological tissue measurement.

DESCRIPTION OF SYMBOLS 10 Living tissue 11 Blood flow measuring device 12 Laser unit 13 Laser light emission part 14 Optical fiber probe 15 Photoelectric conversion part 16 A / D conversion part 21 Apparatus main-body part 22 FFT processing part 23 Blood flow rate calculation part 24 Input part 25 Frequency range setting part 26 Noise calculation unit 27 Display unit 31 Display of blood flow signal power spectrum 32 Display showing temporal change of blood flow value in graph 33 Display of real time numerical value of blood flow 34 Setup button 41 Band pass filter

Claims (3)

A laser light irradiation unit for irradiating a living tissue with laser light;
A scattered light receiving unit that receives the scattered light of the laser light scattered by the biological tissue;
A photoelectric conversion unit that photoelectrically converts the scattered light received by the scattered light receiving unit and outputs an analog electric signal; and
An A / D converter that converts the analog electrical signal converted by the photoelectric converter into a digital signal;
A fast Fourier transform unit that obtains a power spectrum of a blood flow signal by performing a fast Fourier transform on the digital signal converted by the A / D conversion unit;
A power spectrum display unit for displaying a power spectrum of a blood flow signal obtained by the fast Fourier transform unit;
From the power spectrum of the blood flow signal, a frequency range setting unit that enables to specify a frequency range used for calculation of blood flow,
A blood flow rate calculation unit that calculates a blood flow rate using a power spectrum of the blood flow signal in a frequency range specified by the frequency range setting unit , and
The frequency range setting unit is capable of specifying a power level that determines an upper limit of the frequency range as a threshold,
The blood flow rate calculating device calculates a blood flow rate using a power spectrum from 0 Hz to a frequency corresponding to the threshold value . A laser light irradiation unit for irradiating a living tissue with laser light;
A scattered light receiving unit that receives the scattered light of the laser light scattered by the biological tissue;
A photoelectric conversion unit that photoelectrically converts the scattered light received by the scattered light receiving unit and outputs an analog electric signal; and
An A / D converter that converts the analog electrical signal converted by the photoelectric converter into a digital signal;
A fast Fourier transform unit that obtains a power spectrum of a blood flow signal by performing a fast Fourier transform on the digital signal converted by the A / D conversion unit;
Of the power spectrum of the blood flow signal obtained by the fast Fourier transform unit, a frequency range setting unit having a detection frequency range from 0 Hz to a frequency corresponding to a preset threshold power;
A blood flow rate measurement device comprising: a blood flow rate calculation unit that calculates a blood flow rate using a power spectrum in the detection frequency range. A noise calculation unit that calculates an offset value by a noise component proportional to either or both of the amount of light received by the scattered light receiving unit and the frequency range;
The blood flow measuring device according to claim 1 or 2, wherein the blood flow calculation unit calculates the blood flow by subtracting the offset value.

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