JPH021227A - Cycle measuring apparatus - Google Patents

Abstract

PURPOSE:To shorten a calculation time and to perform processing within a real time without being affected by noise by a method wherein the change range of the phase difference variable in the next calculation of an autocorrelation function is controlled according to the newest detected cycle so as to become the cycle range corresponding to a range exerting effect on a calculated cycle. CONSTITUTION:A transducer 2 is arranged on the abdomen W of a woman and detects the heartbeat signal of the embryo to generate an electric signal which is, in turn, sampled by a sampling circuit 4 to be stored in a data memory 6. A calculation range setting circuit 24 receives a signal showing a heart rate of + or -20BPM from a heart rate calculation circuit 18 to calculate the corresponding time and outputs a signal to a multiplier 8. The multiplier 8 reads two sampling data separated by a value of the phase difference variable gamma shown by the signal from the data memory 6 to multiply the same. An adder 10 reads the autocorrelation function calculated value relating to the phase difference variable gamma from a correlation memory 12 to add the calculated data thereto to again store the calculated value in the correlation memory 12. A cycle calculation circuit 16 calculates the cycle of the heart rate signal shown by a time corresponding to the phase difference variable gamma and the heart rate calculation circuit 18 calculates a heart rate.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a period measuring device that measures the period of an electrical signal representing a biological signal, particularly a fetal heartbeat signal, using an autocorrelation method.

[Prior art] The autocorrelation method samples a heartbeat signal at an appropriate sampling period, calculates an autocorrelation function of the heartbeat signal based on the sampled data, and measures the heartbeat signal from the calculated autocorrelation function. This is a method to do so. The autocorrelation function indicates how similar the wave number at the time of the heartbeat signal is to the waveform at a time shifted by the time from that time. In other words, it indicates the degree of similarity between repetitive waveforms of heartbeat signals.

To explain this with reference to FIG.
It can be expressed as J, which overlaps with the next succeeding part M2 with the highest accuracy if it is moved on the time axis by the following part M2.

By the way, when the biological signal is expressed as f (t), the autocorrelation function φ(te) can be obtained as follows.

The data obtained by sampling the signal to be measured is
(k) (k = 1, 2, -n), the above equation (1) is as follows: n is the number of times the heartbeat signal is sampled as an electrical signal in one sampling cycle, and k is the sampling ordinal number. .

(2) Sampling cycle refers to the process of calculating one autocorrelation function value for the values of the phase difference variable by sampling n times.

When formula (2) is expanded, it becomes as follows.

φ (τ) = − (f(1) f(at 1+) +f (
2) f (20 points) +...÷f (n) f (n
(3) In other words, it is obtained by the sum of the products of two data at times that differ by the phase difference.

In equation (1), T indicates the period of the signal.

In equations (1), (2), and (3), it is expressed as .

It shows the time between a certain time in a heartbeat signal and a certain time that is a certain amount of time off from that time. That is, it is a variable that gives a phase difference to the biological signal f (t), and changes within a range that can be considered as one cycle of the signal.

By the way, if we consider the case of measuring the cycle of a fetal heartbeat signal using an autocorrelation method, it begins by sampling the heartbeat signal at a predetermined sampling period.

The period of the fetal heartbeat signal is extremely wide, approximately 300m5 to 1500ms, as known from clinical experiments.
within the range of Therefore, conventionally, when measuring
It was varied in the range of 00m5 to 1500ms.

Actually, in the sampling method, the sampling period is T,
In this case, since /Ts is used as the value, /Ts is used as the value, so the value will be changed within the range of 300/TS to 1500/Ts. The autocorrelation function obtained in this range has a peak at the period T of the heartbeat signal and times 2T, 3T, etc., which are integral multiples thereof, so by detecting the peak corresponding to the period T, the heartbeat signal is The period of can be found.

[Problems to be Solved by the Invention] However, calculating the autocorrelation function over a wide range increases the time required for signal processing, which is not preferable in period measurement where real-time processing is strongly desired.

Furthermore, by measuring over a wide range, there is a possibility that the measurement will be affected by noise.

The present invention has been made in view of the above-mentioned circumstances, and an object of the invention is to provide a period measuring device that is not susceptible to noise and is capable of processing almost in real time.

Another object of the present invention is to propose a period measuring device that detects peaks at each sampling time, eliminates unnecessary calculations, and reduces the capacity of the correlation memory that stores the calculation results.

[Means for Solving the Problems] The present invention provides a method in which the predicted maximum number of changes in the fetal heartbeat signal is approximately ±1.
Noting that it was within 5 BPM, the calculation range of the autocorrelation function of the heartbeat signal was limited to the period range corresponding to the predicted maximum number of heartbeat changes in the latest heartbeat rate per unit time.

According to this invention, a sampling means samples an electrical signal representing a heartbeat signal at a predetermined sampling period, and an autocorrelation function for the heartbeat signal is calculated using data obtained by the sampling means for each sampling. , autocorrelation function calculation means for calculating by sequentially changing a phase difference variable that gives a phase difference to the electrical signal over a predetermined change range, and an autocorrelation function calculated over the predetermined change range of the phase difference variable; peak detection means for detecting a peak; period calculation means for calculating the period of the heartbeat signal from the autocorrelation function of the peak detected by the peak detection means; The predetermined change range is a period range corresponding to (latest heart rate per unit time ± predicted maximum number of changes in heart rate), where the heart rate corresponding to the period calculated by the period calculation means is the latest heart rate per unit time. A period measuring device is provided, comprising calculation range setting means for setting a calculation range.

When the heartbeat signal is that of a fetus, the predicted maximum heartbeat change rate is usually ±15 PMB, but since it may change more than that, it is preferable to set it to ±20 BPM with some margin.

However, even if set to approximately ±15 BPM, which is the normal maximum value, it is possible to obtain sufficient accuracy for practical use.
In this way, the calculation time can be further shortened, so if particularly high accuracy is not required, ±158 PM may be selected as the maximum variation range.

Furthermore, it is preferable to include means for changing the sampling period of the sampling means in accordance with a change in the period of the heartbeat signal.

Further, it is preferable that the rate of change between each change stage of the sampling period be a constant ratio.

[Embodiments] Examples of the present invention will be described below with reference to FIGS. 2 to 4.

FIG. 2 is for explaining the case where the period measuring device according to the present invention is used to measure the heartbeat signal period of a fetus. The horizontal axis shows the heartbeat signal period, and the range indicated by the arrow is the heartbeat signal period. This shows the autocorrelation function calculation range in the signal period, that is, the range in which the phase difference variable is changed.

From the viewpoint of real-time processing, it is desirable to set the autocorrelation function calculation range of not only fetal heartbeat signals but also biological signals in general as narrow as possible without substantially reducing the accuracy of measurement data. In other words, it is desirable to define only the signal range that substantially affects the measurement results as the calculation range, and perform calculation processing only within this range in real time without substantially reducing the accuracy of the measurement data. It will be done.

Furthermore, if the calculation range is set unnecessarily wide, there is a possibility that it will be affected by noise. From this point of view as well, it is desirable to control the autocorrelation function calculation range so that the calculation is performed within a range that substantially affects the measurement results.

Based on this idea, the present inventor determined the range of change in the phase difference variable because the maximum number of changes in the fetal heart rate per unit time is within the predictable range of changes as described above. That is, a period measuring device has been devised that is provided with a control means for controlling the autocorrelation function calculation range to a period range corresponding to (predicted maximum number of changes in heart rate based on the latest heart rate per unit time). In other words, by varying this phase difference variable within the above time range and calculating this autocorrelation function, it is possible to process in real time without substantially reducing measurement accuracy. be. To explain with reference to Fig. 2, the horizontal axis indicates the fetal heartbeat signal cycle, and the range indicated by the arrow indicates the autocorrelation function calculation range, which is the calculation range in the period range of 300m5 to 1500ms. As shown by the arrow range, is the time range corresponding to (the latest heart rate per minute ±20 BPM). In other words, the range in which the phase difference variable is changed is limited to the above-mentioned time range. As seen in this example, the predicted maximum number of changes in heart rate per minute should be selected to be a little wider than the maximum value of ±158PM obtained in clinical experiments, for example, about ±20BPM. is preferred. This can prevent the accuracy of measurement data from decreasing due to calculation omissions.

In the embodiment shown in FIG. 2, in other words, the counting range of the autocorrelation function as described above is the change range of the phase difference variable τ.
By controlling the calculation so that it is limited to only the range that has a substantial effect on the calculated period, there is no need to process large amounts of essentially meaningless data, which improves the practicality of the correlation method period measurement method. In addition to greatly contributing to real-time processing, which is highly desired from the viewpoint of

The period measurement method using the correlation method, as shown in Figure 2, is
This can be accomplished, for example, by a period measuring device configured as shown in FIG.

The transducer 2 is placed, for example, on the abdomen W of a woman, and detects the fetal heartbeat signal and generates a corresponding electrical signal. The electrical signal representing the heartbeat signal output from the transducer 2 is shaped into a waveform by a preprocessing circuit 3 connected thereto, and then sampled at a set sampling period by a sampling circuit 4 and converted into a digital signal from analog to digital. Converted (A-D conversion). The sampled data is stored in a data memory 6 connected to the sampling circuit 4. The data memory 6 is composed of a plurality of shift registers, and always stores the latest N data, for example, 256 data. A multiplier 8 is connected to the data memory 6, and an adder 10 is connected to the multiplier 8. Multiplier 8
The adder 10 substantially calculates the autocorrelation function shown in equation (3) based on the data stored in the data memory 6, and sends the result to the correlation memory 12 connected to the adder 10. Store. Therefore, the multiplier 8 and the adder 10 can be considered as an autocorrelation function calculation circuit for heartbeat signals.

A peak detector 14 is connected to the correlation memory 12, and the peak detector 14 detects a peak from the autocorrelation function data stored in the correlation memory 12.

A period calculation circuit 16 is connected to the peak detector 14, and the period calculation circuit 16 receives the peak detection signal from the peak detection circuit 14 and calculates the period of the heartbeat signal. The period T is the time corresponding to the value of the phase difference variable γ determined by the position of the obtained peak on the time axis, that is, T = γXt
(t is the sampling period). A heart rate calculation circuit 18 is connected to the period calculation circuit 16, and the heart rate calculation circuit 18 calculates the heart rate based on a signal indicating the period from the period calculation circuit 16. Heart rate calculation circuit 18 is connected to control circuit 20 . Control circuit 20
A display 22 comprising, for example, a light emitting diode (LED) is connected to the display. The display 22 is the control circuit 2
The heart rate of the heartbeat signal is displayed by light emission based on the signal output from the heart rate calculation circuit 18 via the heart rate calculation circuit 18. Furthermore, at this time,
The control circuit 20 displays the signal from the heart rate calculation circuit 18 on the display 2 when the signal from the heart rate calculation circuit 18 contains a noise component or when the probe is dislodged.
It is preferable to provide an auxiliary means to prevent the heart rate from being displayed incorrectly by controlling the heart rate so that it is not input to the heart rate. but,
Since this is not directly related to this invention, detailed explanation will be omitted. The control circuit 20 is further connected to a calculation range setting circuit 24 that sets the change range of the phase difference γ, that is, the calculation range of the autocorrelation function. The calculation range setting circuit 24 is further connected to the multiplier 8 and the adder 10. A reference level detector 26 is further connected to the control circuit 2o, and the reference level detector 26 is connected to the sampling circuit 4.

In the configuration as described above, the calculation range setting circuit 24 receives a signal indicating the heart rate from the heart rate calculation circuit 18 and calculates the time corresponding to (the heart rate ±20 BPM). Then, a signal regarding the phase difference variable γ within this time range is output to the multiplier 8. In this case, the control circuit 20 controls the signal from the heart rate calculation circuit 18 to be within the calculation range when the signal from the heart rate calculation circuit 18 contains a noise component or when the probe is misaligned. Control is performed to prevent input to the setting circuit 24. The multiplier 8 reads two pieces of sampling data separated by the value of the phase difference variable γ indicated by the signal input from the calculation range setting circuit 24 from the data memory 6 and multiplies them. The calculation range setting circuit 24 further outputs a timing signal to the adder 10, and the adder 10 responds to the timing signal by combining the data calculated by the multiplier 8 with the correlation memory 12.
The previous autocorrelation coefficient calculation value for the corresponding phase difference variable γ is read out and calculated. The addition result is stored again at a predetermined address in the correlation memory 12. Such calculations are performed every time data is sampled, and the autocorrelation function of the heartbeat signal is stored in the correlation memory 12.

As described above, the peak detector 14 detects a peak from the autocorrelation function stored in the correlation memory 12, and the period calculation circuit 16 uses a phase difference variable determined by the position of this peak on the time axis. The heart rate calculation circuit 18 calculates the period of the heartbeat signal that is generated at the time corresponding to the value of γ.
Calculate the heart rate from that cycle.

Control circuit 20 further outputs a signal to reference level detector 26 at appropriate time intervals. The reference level detector 26 receives the signal from the control circuit 20 and detects the optimum reference level (zero level) when coding the sampled data. To be more specific, the more accurately the positive and negative balance of the data is achieved when coding the sampled data, the more clearly the periodicity of the autocorrelation function curve is expressed, and the reference level detector 26 is provided for this purpose. During sampling, the maximum value, minimum value, or average value of the data is detected to determine the optimal value of the reference level.

In the embodiment shown in FIG. 3, the calculation range setting circuit 24 controls the change range of the phase difference variable γ, that is, the autocorrelation function range, to a time range corresponding to (the latest heart rate per minute ±20 BPM). By doing so, it is possible to obtain period measurement without unnecessarily increasing calculation time by sampling a large amount of substantially meaningless data, and without causing a substantial decrease in data accuracy.

By the way, from the perspective of wanting real-time processing, the distance is about 300 m.
It is not desirable to sample with a constant sampling period such as - over the entire fetal heartbeat signal period of 5 to 1500 ms. In the heartbeat signal region with a short period, it is desirable to set a short sampling period and perform dense data detection from the viewpoint of achieving high-precision measurement, but on the other hand, in the heartbeat signal region with a long period, it is desirable to Since the signal changes are not very sudden, setting a long sampling period will not substantially reduce the accuracy of the measured data; rather, the sampling period should be set to the same period as the sampling period of the short-period heartbeat signal. If it is set to , a large amount of data is substantially unnecessarily sampled, the number of calculations increases pointlessly, and this becomes a major hindrance to real-time measurement. Furthermore, it may be affected by noise.

From this point of view, in addition to limiting the autocorrelation function calculation range, as shown in Figure 4, the sampling period is changed in stages in response to changes in the period of the heartbeat signal, which makes it virtually meaningless. It is even more preferable to eliminate data arithmetic processing.

Another reason for changing the sampling period in stages in response to changes in the period of the heartbeat signal is as follows. That is, the period is inversely proportional to the heart rate, so for example, as the heart rate decreases, the period widens. For this reason, the time range corresponding to the upper 20 BP is expanded, and the sampling period must also be made longer. In this way, it is preferable to change the sampling period in accordance with the change in the heartbeat signal period in the upper 20 BP in response to the change in the heartbeat signal period.

As a specific means for this purpose, for example, a wide range of heartbeat signal regions is divided into several regions, and a sampling period of a corresponding size is determined according to the magnitude of the heartbeat signal in each region. That is, a short sampling period is set for an area where the heartbeat signal period is small, while a long sampling period is set for an area where the heartbeat rate is low, that is, an area where the heartbeat signal period is large.

An example of setting the sampling period will be explained with reference to FIG. 4. Two threshold values T H + ,
Define T H 2 and divide the fetal heartbeat signal period region into three regions:
It is divided into I, TI, and III, and a different sampling period is set for each region.

For example, the threshold values TH+ and TH2 are each 60
0m5, 10100O, in this case, the range of areas I, II, III is 300-600ms, respectively.
,600-1 000m5, 1 000-1500ms
becomes.

The sampling periods in these regions I, II, and III are respectively T, -I, T, -II, T. −
Assuming In, the relationship between each sampling period needs to be as follows.

T, − I <T. - II <T. -III sampling period T. -I.T. -II settings vary depending on the division form of regions I, II, and III, but the settings for regions I, II, and III are set to 300ms-600ms, 600ms-1000m as in the above example.
If you set s1 000m5 - 1 500m5, what is the sampling frequency? CIIT. - I, T. -II
For example, each time is 5ms, 7. 5ms. 1 1
．． It can be determined as 25m5.

In addition, when a change in the area occurs, if the sampling data obtained from measurements in the previous area is to be corrected and used for data with a period corresponding to the sampling period set in the new area, the correction calculation must be performed. For convenience, it is desirable that the rate of change in the sampling period between adjacent areas be constant. In particular, it is preferable that this fixed ratio be a fixed ratio expressed as a fractional ratio, such as 3/2.4/3.

Note that although the number of sampling period change areas can be set arbitrarily, increasing the number too much is not preferable because it only becomes complicated. Measurement target.

In consideration of accuracy, shortening of calculation speed, etc., it is appropriate to define, for example, about three regions as shown in the embodiment.

In the embodiment shown in FIG. 3, a region setting circuit 28 is provided in order to divide the entire period range of the heartbeat signal into, for example, three regions and change the regions appropriately in response to changes in the period of the heartbeat signal. There is. The area setting circuit 28 is connected to the control circuit 20. The sampling circuit 48 is connected to the period calculation circuit 16.

The area setting circuit 28 receives an area change instruction signal from the control circuit 20 and changes the area. The control circuit 20 receives a signal indicating the heart rate from the heart rate calculation circuit 18, calculates the period corresponding to the heart rate, and outputs a signal indicating the region to which the period belongs. Therefore, when the calculated period of the heartbeat signal exceeds the period range in the currently set area, the control circuit 20 sends a signal to the area setting circuit 20 instructing a new area of the period range to which the period belongs.
Output to 8. For example, the current cycle range is 300ms -60
Area ■ defined as 0ms is set, and area I
In the case where the period corresponding to the heart rate of the signal obtained from the heart rate calculation circuit 18 changes from 590 m5 to 610 m5, for example, the measurement area is changed to an area with a sampling period of 5 ms, for example. For example, a signal instructing a change to region II defining a period range of 600 ms to 1000 ms is output. The area setting circuit 24 receives this change instruction signal, and the sampling circuit 4
A sampling period change signal is output to change the sampling period in the sampling circuit 4 to a sampling period preset in region II, for example, 7.5 ms. In this way, when the period corresponding to the measured heart rate exceeds the predetermined period range in the set area, the area is changed and the sampling period is changed to the preset period in the new area. be done.

The area setting circuit 28 also sends a signal indicating the sampling period determined in the set area to the period calculating circuit 1.
Output to 6. The period calculation circuit 16 is connected to the peak detector 14
In response to the peak detection signal from , the cycle of the heartbeat signal T = t demand.

As described above, the sampling period is changed by changing the area, and the period of the heartbeat signal is calculated.

[Effects of the Invention] As described above, according to the present invention, in the period measurement of a heartbeat signal, the change range of the phase difference variable for the calculation of the next autocorrelation function is calculated according to the latest period detected. By controlling the cycle to a range that substantially affects the cycle, for example, the range of the cycle that corresponds to (the latest heart rate per unit time ± 20 B PM), it is possible to avoid sampling a large amount of data that is virtually meaningless. It is possible to omit unnecessary calculations and the storage capacity of the correlation memory, and there is no risk of being affected by noise, and a period measuring device is provided that can perform processing virtually in real time by shortening calculation time. be done.

Furthermore, in addition to limiting the calculation range of the autocorrelation function according to the present invention, by changing the sampling period in response to changes in the period of the heartbeat signal, calculation time can be further increased without substantially reducing data accuracy. By shortening the sampling period and changing the sampling period at a constant rate in stages, old data can be corrected and used as new data, which makes continuous measurement possible, making it possible to perform measurements in real time. A processed period measuring device is provided.

[Brief explanation of the drawing]

FIG. 1 is a heartbeat signal period diagram used to explain period measurement using the autocorrelation method, and FIG. 2 is a heartbeat signal period diagram used to explain the case where the period measuring device according to the present invention is applied to measurement of the heartbeat signal period of a fetus. FIG. 3 is a diagram schematically showing the configuration of an embodiment of the period measuring device of the present invention in the form of a block diagram, and FIG. FIG. 6 is a diagram for explaining a method of changing the sampling period in stages correspondingly. In the figure, 2...transducer, 3...preprocessing circuit, 4...sampling circuit, 6...data memory,
8... Multiplier, 12... Correlation memory, 14... Peak detector, 16... Period calculation circuit, 18... Heart rate calculation circuit, 20... Control circuit, 22... display,
24... Calculation range setting circuit, 26... Reference level detector, 28... Area setting circuit, T-I, T, -II. III-III... Sampling period. patent

Claims (5)

[Claims] (1) sampling means for sampling an electrical signal representing a heartbeat signal at a predetermined sampling period; autocorrelation function calculation means for calculating by sequentially changing a phase difference variable that gives a phase difference to a signal over a predetermined change range; and detecting a peak from the autocorrelation function calculated over the predetermined change range of the phase difference variable. peak detection means; period calculation means for calculating the period of the heartbeat signal from the autocorrelation function of the peak detected by the peak detection means; and the predetermined change range of the phase difference variable when the autocorrelation function calculation means calculates , if the heart rate corresponding to the period calculated by the period calculating means is the latest heart rate per unit time, then (the latest heart rate per unit time ± predicted maximum number of heart rate changes)
A period measuring device comprising calculation range setting means for setting a period range corresponding to. (2) The period measuring device according to claim 1, wherein the heartbeat signal is a fetal heartbeat signal, and the predicted maximum heartbeat change rate is 20 BPM. (3) The period measuring device according to claim 1, wherein the heartbeat signal is a fetal heartbeat signal, and the maximum predicted heartbeat change rate is 15 BPM. (4) The period measuring device according to claim 1, further comprising means for changing the sampling period of the sampling means in accordance with changes in the heartbeat signal. (5) The period measuring device according to claim 4, wherein the rate of change between each change stage of the sampling period is a constant ratio.