JP2007215816A - Pulse doppler measuring apparatus, its method and its program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pulse doppler measuring apparatus capable of preventing a phase difference from being erroneously measured due to the turn-back of a phase when measuring the speed of an object by using the phase difference of the reflected waves of ultrasonic pulses. <P>SOLUTION: The pulse doppler measuring apparatus 1 comprises: a Fourier transform means 31 for detecting the phase for each frequency from the reflected waves; a phase difference computing means 33 for computing the phase difference for the phase for each frequency; a phase difference correction means 341 for adding or subtracting the phase of the integral multiple of 2π by a plurality of predetermined different phase correction patterns to the phase difference; a linear approximation means 342 for approximating coordinate points for each frequency to a straight line for each phase correction pattern in a coordinate system wherein the frequency and the phase difference at the frequency are coordinate points and thereby computing the inclination of the straight line and the error; and a speed specifying means 36 for selecting the inclination of the straight line for which the error is minimum as the inclination of the phase difference corresponding to a moving speed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a pulse Doppler measurement apparatus, a method thereof, and a program thereof that measure the velocity of a moving object based on a reflected wave with respect to an ultrasonic wave transmitted to the object.

Conventionally, various diagnostic apparatuses using a pulse Doppler method for measuring a blood flow velocity in a living body have been developed as apparatuses for diagnosing a human body and are widely used in clinical settings. These diagnostic devices repeatedly transmit ultrasonic pulses to the blood flow in a living body at a constant cycle, and detect the phase change of the reflected wave caused by the movement of the red blood cells to be measured. Is measuring the speed.
Here, the concept of the conventional pulse Doppler method will be described with reference to FIG.
In the pulse Doppler method, as shown in FIG. 9 (a), an ultrasonic wave (transmitted pulse) is transmitted from a transmitter S to an object (for example, blood flow) B with movement at a constant period, and received. A wave R receives a reflected wave from the object B. At this time, if the object B moves by Δx between the first transmission and the second transmission, the received waveform shifts by the propagation time difference Δt as shown in FIG. 9B.
The relationship between the movement amount Δx and the propagation time difference Δt is expressed by the following equation (1), where c [m / s] is the speed of sound in the propagation medium.

  Here, assuming that the repetition period of the transmission pulse is T [s] and the velocity of the object B is v [m / s], the equation (1) becomes the following equation (2).

As shown in FIG. 9C, this propagation time difference Δt appears as a phase shift in the shifted received spectrum. In other words, the propagation time difference Δt is proportional to the phase difference (phase rotation) of the shifted received spectrum, as shown in FIG.
Here, the center frequency of the transmission pulse omega _c, when the phase difference between [Delta] [theta] _c in omega _c, the slope of the line shown in FIG. 9 (d) becomes Δθ _{c / ω c.} Since this corresponds to the propagation time difference Δt, the following equation (3) can be obtained from the equation (2).

Thus, the velocity v of the object B can be obtained from the center frequency ω _c of the transmission pulse and the phase difference Δθ _c .
The diagnostic apparatus using the pulse Doppler method is used for many diagnoses such as diagnosis of the heart, liver, etc., real-time diagnosis of hemodynamics, etc. by displaying the blood flow speed on the display device in color. .
However, in the diagnostic apparatus using the pulse Doppler method, as shown in the above equation (3), the blood flow measurement is performed by the phase change of the reflected wave. Therefore, as shown in FIG. Depending on the situation, phase folding (so-called aliasing) occurs, and there is a problem that a correct measurement result cannot be obtained.

Various proposals have been made as methods for solving this problem. For example, by sending a pulse having two different frequencies to an object in a predetermined cycle and taking the difference in phase difference between the transmitted pulse and the reflected wave, the phase difference exceeds ± π and aliasing is caused. Even if it occurs, a method for measuring the speed of an object (hereinafter referred to as a two-frequency method) has been proposed by utilizing the fact that the difference between two phase differences is within ± π and aliasing does not occur (Patent Patent) Reference 1 and Patent Reference 2).
In addition, as another method, by sending a pulse to an object at different periods, taking the phase difference between the transmitted pulse and the reflected wave, and utilizing the difference in phase difference generated by the period difference, A method for measuring the speed of an object (hereinafter referred to as a two-period method) has been proposed (see Patent Document 3).

Moreover, in these conventional methods, the accuracy of the measurement speed is increased by performing transmission and reception waves a plurality of times and averaging the phases. Furthermore, in the conventional method, the phase is detected based on the center frequency of the transmitted / received pulse. However, it is considered that it is advantageous in terms of S / N to narrow the pulse band, and the pulse length is relatively long. A long burst wave is used.
Moreover, in the conventional method, in the actual blood flow measurement, in order to remove the clutter component that picks up the reflected wave from the reflector without movement such as the blood vessel wall and bone, based on the phase difference of the reflected wave, An MTI (Moving Target Indication) filter that removes a reflected wave signal from a reflector that does not move is used.
Japanese Patent No. 2953083 (paragraph 0009) US Pat. No. 4,534,357 Japanese Patent Laid-Open No. 4-197249 (pages 3-5 and 3-5)

In the conventional method described above (two frequency method, two period method), from the viewpoint of S / N improvement, the phase of the received reflected wave is averaged or a burst wave having a relatively long pulse length is used. By doing so, it was trying to improve the accuracy of measurement.
However, the present inventor has clarified that the reason why the accuracy cannot be increased in the actual blood flow measurement in the living body is the phase fluctuation due to the interference of the reflected wave from each red blood cell.

In summary, when measuring blood flow in a living body, it is red blood cells that are the reflectors of ultrasonic waves. The erythrocytes are present in large numbers and randomly within one wavelength of the ultrasonic wave and behave as a large number of scatterers. When ultrasonic pulses are transmitted to such a large number of scatterers, the reflected waves from each red blood cell overlap and interfere with each other. As a result, even if the blood flow flows at a constant speed, the phase of the reflected wave fluctuates irregularly.
On the other hand, since the conventional method (two-frequency method, two-period method) does not consider the influence of phase fluctuation due to interference, the measurement accuracy cannot be improved. That is, in the conventional method, since the difference in phase difference is obtained, it is strongly influenced by the fluctuation of the phase, and even if the aliasing problem can be solved, the measurement accuracy is deteriorated conversely. .

  Further, in actual blood flow measurement, the conventional method removes clutter components by an MTI filter. However, since the conventional method detects the phase based on the center frequency of the transmitted / received pulse, the phase difference becomes zero for the reflector moved by a distance corresponding to an integral multiple of the transmission period. Actually, although the object is moving, it is removed as a clutter component. As described above, the conventional method has a problem that there is a speed that cannot be obtained (hereinafter referred to as a blind speed).

  The present invention has been made in order to solve the above-described problems. When the velocity of an object is measured by the phase difference of the reflected wave of an ultrasonic pulse, the phase is turned back (aliasing). An object of the present invention is to provide a pulse Doppler measurement device, a method thereof, and a program thereof that prevent erroneous measurement, increase the measurement accuracy of the speed of an object, and eliminate the blind speed.

  The present invention was devised to achieve the above object. First, the pulse Doppler measurement apparatus according to claim 1 transmits ultrasonic waves to an object at a constant period, and the reflected waves thereof. A pulse Doppler measurement device for measuring the moving speed of the object based on the following: a wave transmission means, a wave reception means, a spectrum analysis means, a phase difference calculation means, a phase difference correction means, and a linear approximation means And a speed specifying means.

In such a configuration, the pulse Doppler measurement device transmits an ultrasonic pulse to the target object whose speed is to be measured by the transmitting means, and receives the reflected wave reflected from the target by the receiving means. To do. At this time, if the object is moving, the phase of the reflected wave is shifted.
The pulse Doppler measurement device detects the phase for each frequency from the reflected wave by the spectrum analysis means. That is, the spectrum analysis means detects not only one frequency in the transmitted pulse but also phases at a plurality of predetermined frequencies.

Further, the pulse Doppler measurement device calculates a phase difference for each period with respect to the phase for each frequency detected by the spectrum analysis unit by the phase difference calculation unit. This phase difference corresponds to the amount of movement caused by the movement of the object, and therefore corresponds to the speed of the object.
Then, the pulse Doppler measurement device corrects the phase difference by adding or subtracting a phase that is an integral multiple of 2π with a plurality of different phase correction patterns determined in advance to the phase difference for each frequency by the phase difference correction means. . This phase correction pattern indicates which frequency phase difference is added or subtracted with an integer multiple of 2π. The phase correction pattern is preferably a pattern in which an integer multiple of 2π is added or subtracted in order from the maximum frequency. As a result, even if phase folding (aliasing) occurs, the phase difference is corrected in the direction in which the folding is corrected.

Further, the pulse Doppler measuring device approximates the coordinate point for each frequency to a straight line for each of a plurality of predetermined phase correction patterns in a coordinate system having the frequency and the phase difference at the frequency as coordinate points by the linear approximation means. Thus, the slope of the straight line and the error are calculated. This linear approximation can be calculated using the least square method. When this error is the smallest, the coordinate point between the frequency and the phase difference at the frequency is plotted on a substantially straight line, and the phase wrapping of the phase difference caused by aliasing is corrected. .
Then, the pulse Doppler measuring device selects, by the speed specifying means, the slope of the straight line that minimizes the error calculated by the straight line approximating means as the slope of the phase difference corresponding to the moving speed of the object.

  Further, the pulse Doppler measurement device according to claim 2 is the pulse Doppler measurement device according to claim 1, wherein the phase difference calculation means averages the phase difference for each frequency for each predetermined number of transmission and reception waves. It is characterized by doing.

  In such a configuration, the pulse Doppler measurement device suppresses fluctuations in phase due to noise and interference of reflected waves by averaging fluctuations in reflected waves by the phase difference calculation means.

  Furthermore, the pulse Doppler measurement apparatus according to claim 3 is a configuration comprising a filtering means for removing clutter components based on the phase detected by the spectrum analysis means in the pulse Doppler measurement apparatus according to claim 1 or 2. It was.

  In such a configuration, the pulse Doppler measurement apparatus removes the clutter component based on the phase detected by the spectrum analysis means by the filtering means. That is, the filtering means removes a spectral component having a phase rotation slower than a predetermined reference (low-frequency spectral fluctuation component) and extracts a spectral component having a fast phase rotation. As a result, signal components (clutter components) due to slow motion are removed.

  The pulse Doppler measurement method according to claim 4 is a pulse Doppler measurement method in which an ultrasonic wave is transmitted to a target object at a constant period, and the moving speed of the target object is measured based on the reflected wave. Thus, the method includes a transmission / reception step, a spectrum analysis step, a phase difference calculation step, a phase difference correction step, a linear approximation step, and a speed specifying step.

In this procedure, the pulse Doppler measurement method transmits an ultrasonic pulse to a target object whose speed is to be measured and receives a reflected wave reflected from the target in a transmission / reception step. In the pulse Doppler measurement method, the phase for each frequency is detected from the reflected wave in the spectrum analysis step.
Further, in the pulse Doppler measurement method, the phase difference is calculated for each period with respect to the phase for each frequency detected in the spectrum analysis step in the phase difference calculation step. This phase difference corresponds to the amount of movement caused by the movement of the object, and therefore corresponds to the speed of the object.

In the pulse Doppler measurement method, in the phase difference correction step, the phase difference is corrected by adding or subtracting a phase that is an integral multiple of 2π with a plurality of different predetermined phase correction patterns to the phase difference for each frequency. .
In the pulse Doppler measurement method, in the linear approximation step, the coordinate point for each frequency is approximated to a straight line for each of a plurality of predetermined phase correction patterns in a coordinate system having the frequency and the phase difference at the frequency as coordinate points. Thus, the slope of the straight line and the error are calculated.
In the pulse Doppler measurement method, the slope of the straight line that minimizes the error calculated in the straight line approximation step is selected as the slope of the phase difference corresponding to the moving speed of the object in the speed specifying step.

  Furthermore, the pulse Doppler measurement program according to claim 5, a computer for measuring a moving speed of the object based on a reflected wave with respect to an ultrasonic wave transmitted with a constant period to the object, It functions as a spectrum analysis means, a phase difference calculation means, a phase difference correction means, a straight line approximation means, and a speed specification means.

In such a configuration, the pulse Doppler measurement program detects the phase for each frequency from the reflected wave by the spectrum analysis means.
Further, the pulse Doppler measurement program calculates a phase difference for each period with respect to the phase for each frequency detected by the spectrum analysis unit by the phase difference calculation unit.
Then, the pulse Doppler measurement program corrects the phase difference by adding or subtracting a phase that is an integer multiple of 2π with a plurality of different phase correction patterns determined in advance to the phase difference for each frequency by the phase difference correction means. .

Further, the pulse Doppler measurement program approximates the coordinate point for each frequency to a straight line for each of a plurality of predetermined phase correction patterns in a coordinate system having the frequency and the phase difference at the frequency as coordinate points by a linear approximation means. Thus, the slope of the straight line and the error are calculated.
Then, the pulse Doppler measurement program selects, as the gradient of the phase difference corresponding to the moving speed of the target object, the gradient of the straight line that minimizes the error calculated by the linear approximation unit.

The present invention has the following excellent effects.
According to the first, fourth, or fifth aspect of the invention, even when aliasing occurs in the phase difference of the reflected wave, the generated aliasing is corrected by the phase difference at a plurality of frequencies. be able to. Thereby, this invention can prevent the erroneous measurement at the time of measuring the speed of a target object.

  According to the second aspect of the present invention, for example, a large number of scatterers such as blood flow are measured in order to suppress fluctuations in phase due to interference of reflected waves by averaging fluctuations in reflected waves. In the case of the target object, even if the reflected waves overlap and interfere with each other, the influence of the phase fluctuation can be reduced, and the target object can be accurately measured.

  According to the invention described in claim 3, since the filtering means removes the clutter component at a plurality of frequencies, even if the phase difference is less than a predetermined value at a certain frequency and the speed cannot be detected, the filtering means can be used at other frequencies. Since the phase difference is generated, the moving object is not recognized as an unmoving object. For this reason, the present invention can eliminate a so-called blind speed in which speed detection is impossible.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[Basic principle of pulse Doppler measurement method (broadband Doppler method)]
First, the basic principle of the pulse Doppler measurement method according to the present invention will be described with reference to FIGS. Here, the pulse Doppler measurement method according to the present invention is referred to as “broadband Doppler method”, and a general pulse Doppler measurement method is referred to as “conventional pulse Doppler method”.

FIG. 3 is a graph for explaining the difference between the conventional pulse Doppler method and the broadband Doppler method according to the present invention. FIG. 3A shows the amplitude spectrum of a pulse transmitted and received by the conventional pulse Doppler method and the propagation time difference. (B) shows the amplitude spectrum of the pulse transmitted and received by the wideband Doppler method and the phase difference of the amplitude spectrum in the propagation time difference. In (a-1) and (b-1) of FIG. 3, the horizontal axis represents frequency and the vertical axis represents amplitude. Further, in (a-2) and (b-2) of FIG. 3, the horizontal axis represents frequency and the vertical axis represents phase difference.
FIG. 4 is a graph for explaining the correction of the phase difference when aliasing occurs in the wideband Doppler method. In FIG. 4, the horizontal axis represents frequency and the vertical axis represents phase difference.

Normal pulse Doppler method, by obtaining the center frequency omega _c of the pulse P _N shown in FIG. 3 (a) (a-1) , the slope of the phase difference of the amplitude spectrum as shown in (a-2), Getting speed. In this case, for an object moving at a low speed where the phase difference is ± π or less, the speed can be obtained normally, but for an object moving at a high speed such that the phase difference exceeds ± π. In this case, aliasing of the phase difference (aliasing) occurs, and the speed due to the gradient of the phase difference cannot be obtained correctly. In the conventional pulse Doppler method, a method for solving this aliasing problem is the two-frequency method or the two-period method described in the background art.

On the other hand, the wideband Doppler method has a phase difference at a plurality of frequencies (ω _{1 to} ω _n ) as shown in (b-2) in the entire spectrum of the pulse P _W shown in (b-1) of FIG. The speed is obtained by obtaining the slope from the plurality of phase differences. Even in this case, phase reflection of the phase difference (aliasing) occurs for a reflector that moves at a high speed such that the phase difference exceeds ± π.

Therefore, the broadband Doppler method, as shown in FIG. 4, the phase difference of the above frequencies where there is the return of the phase (here omega ₇ or higher), by adding the phase of 2 [pi, modify the phase difference caused in the folded To do. When the phase wrapping occurs multiple times within the spectrum of the pulse _PW , a phase that is an integral multiple of 2π is added. In the broadband Doppler method, the velocity is obtained from the slope of the straight line L with the phase difference corrected. The method for obtaining this straight line will be described in detail later.
That is, since the conventional pulse Doppler method uses only the center frequency ω _c , a narrow band burst wave having a relatively long pulse length is used, whereas the wideband Doppler method conversely uses a plurality of frequencies. A broadband pulse wave is used.
In addition, although the example which adds 2 (pi) to a phase difference is shown here, since the inclination of a phase difference becomes negative when the motion of the target object B (refer FIG. 1) is reverse, 2 (pi) is added from a phase difference. The phase difference is corrected by subtraction.

In this way, the broadband Doppler method corrects the gradient of the phase difference that causes aliasing based on a plurality of frequencies, and therefore measures the speed of a moving body that moves at a speed that causes aliasing. Can do.
Hereinafter, the configuration and operation of a pulse Doppler measuring apparatus for realizing the broadband Doppler method will be described in detail.

[Configuration of pulse Doppler measurement device]
First, the configuration of a pulse Doppler measurement apparatus according to the present invention will be described with reference to FIG. FIG. 1 is a block diagram illustrating a configuration of a pulse Doppler measurement apparatus.
The pulse Doppler measuring device shown in FIG. 1 transmits ultrasonic waves to a target object at a constant period, and measures the moving speed of the target object based on the reflected wave. Here, the pulse Doppler measurement device 1 is assumed to function as a device for measuring the blood flow velocity in the living body.
As shown in FIG. 1, the pulse Doppler measuring device 1 includes a wave transmitting / receiving device 2 and a control device 3.

  The wave transmitting / receiving device 2 transmits ultrasonic waves to the object and receives reflected waves from the object. The wave transmitting / receiving apparatus 2 includes a wave transmitting means 21 and a wave receiving means 22.

The wave transmitting means 21 transmits ultrasonic waves to an object (here, blood flow [specifically, red blood cells]) B at a constant cycle. The wave transmitting means 21 transmits a broadband ultrasonic wave shorter than the pulse length of the wave transmission pulse used in the conventional pulse Doppler measurement apparatus.
For example, a transmission pulse having a pulse length of 1.0 μs is used for a conventionally used pulse length of 2.4 μs (microseconds). The pulse length may be a length that allows a plurality of frequencies (see FIG. 3B) to be set within the band.

  The wave receiving unit 22 receives a reflected wave reflected from the object (blood flow) B with respect to the ultrasonic wave transmitted from the wave transmitting unit 21. The reflected wave received by the wave receiving means 22 is converted into a digital signal and output to the control device 3.

  The control device 3 inputs the reflected wave received by the wave transmitting / receiving device 2 as a digital signal, and measures the moving speed of the object B based on a change in the phase of the reflected wave. Here, the control device 3 includes a Fourier transform unit 31, an MTI filter 32, a phase difference calculation unit 33, a phase difference slope calculation unit 34, a correction pattern storage unit 35, and a speed specification unit 36. .

The Fourier transform means (spectrum analysis means) 31 detects a phase (spectrum) for each frequency from the reflected wave received by the wave receiving means 22 of the wave transmitting / receiving apparatus 2. For example, a general FFT (Fast Fourier Transform) can be used for the Fourier transform means 31. The frequency sampling interval at this time is determined by the upper limit of the speed to be detected. That is, in the velocity measurement range, the sampling interval is determined in advance so that the phase rotation within the frequency sampling interval is within ± π.
Here, it is assumed that the (k−1) th reflected wave at time t is f _k−1 (t), and the spectrum value at the frequency ω after Fourier transform is F _k−1 (ω). Further, the kth reflected wave at time (t + T) is assumed to be f _k (t) = f _k−1 (t + Δt). Here, T is the time interval of the transmission pulse, and Δt is the amount of change in the propagation time caused by the movement of the object B. Then, the spectral value F _k (ω) at the frequency ω after Fourier transform of the _kth reflected wave f _k (t) is expressed by the following equation (4).

The spectrum value F _k (ω) calculated by the equation (4) is sequentially output to the MTI filter 32.

The MTI filter (filtering means) 32 removes clutter components from the spectrum obtained by the Fourier transform means 31. Note that the threshold value used as a reference for filtering here is determined in advance according to the speed range of the object to be measured.
The filtered signal filtered by the MTI filter 32 is output to the phase difference calculation means 33.

  Here, the principle of the MTI filter will be described with reference to FIG. 5A and 5B are explanatory diagrams for explaining the principle of the MTI filter. FIG. 5A shows the principle of the MTI filter in the conventional pulse Doppler method, and FIG. 5B shows the principle of the MTI filter in the broadband Doppler method.

Usually, the MTI filter is based on the difference between the first received wave and the second received wave, and the reflected wave from an object having no movement such as a blood vessel wall or a bone is selected if the difference is smaller than a predetermined value. (Clutter component) is removed. In this way, the MTI filter takes the difference between the received waves. Therefore, if the movement of the object moves by a distance corresponding to an integral multiple of the transmission period, the MTI filter regards it as a non-moving object and removes it as a clutter component. End up. That is, the transfer characteristic in the MTI filter has a comb shape as shown in FIG. 5A, and a speed region (blind speed) that cannot be measured is generated.
Therefore, in the conventional pulse Doppler method, as shown in FIG. 5A, the MTI filter performs filtering in a speed range where no blind speed is generated (for example, speed range W in the figure), and measures the speed. Yes.

On the other hand, in the wideband Doppler method, the basic MTI processing is the same as the conventional one, but as shown in FIG. 5B, the frequency band to be processed is wide and a plurality of frequencies (ω ₁ , ω ₂ , ω ₃ ,...), Filtering is performed, so that even if a blind speed occurs at each frequency, the blind speed does not occur in the frequency range to be measured, and the speed range that can be estimated can be expanded.
Returning to FIG. 1, the description of the configuration of the pulse Doppler measurement device will be continued.

The phase difference calculation means 33 calculates a phase difference between adjacent periods for each frequency in the signal filtered by the MTI filter 32. Here, the phase difference calculating means 33 uses the phase of the average cross spectrum as the phase difference. The average cross spectrum is an average obtained by multiplying two frequency components of two spectra.
That is, the phase difference calculation means 33 calculates the average cross spectrum C (ω) by the following equation (5) when performing transmission and reception waves a times. Note that * indicates a complex conjugate.

In this way, by averaging the phase difference for each predetermined number of transmission / reception times, ultrasonic pulses are transmitted to many scatterers, and even when the reflected waves overlap and interfere with each other, The influence of phase fluctuation can be suppressed.
The phase difference (average cross spectrum phase) calculated by the phase difference calculation means 33 is output to the phase difference slope calculation means 34.
Since the phase difference is usually expressed by the following equation (6), the phase difference calculating means 33 obtains the average phase difference Δθ by the following equation (7) when transmitting and receiving waves a times. It is good.

  The phase difference slope calculating means 34 is based on the phase difference for each frequency, and in the coordinate system using the frequency and the phase difference at the frequency as the coordinate point, the coordinate point is linearly obtained by the least square method for each of the plurality of phase correction patterns. To calculate the slope of the straight line (the slope of the phase difference) and its error.

Here, with reference to FIG. 6, the outline | summary is demonstrated about the process performed in the phase difference inclination calculating means 34. FIG. FIG. 6 is a conceptual diagram showing the concept of phase correction when phase folding occurs in the phase difference. In FIG. 6, the horizontal axis represents the frequency ω, the vertical axis represents the phase difference (phase of the average cross spectrum), and the frequencies to be used are ω ₁ to ω ₁₀ .

FIG. 6A shows a state in which a phase fold occurs in the phase difference at the frequency ω ₇ or higher. Here, the coordinates of the phase difference for each frequency are approximated to a straight line passing through the origin by the least square method. In this case, the slope of the straight line L _a determined is due to the influence of the phase after the return of the phase (frequency omega ₇ or higher), the error becomes large.

FIG. 6B shows a state in which 2π is added to the phase difference between the frequencies ω ₉ and ω _{10 from} the state of FIG. Here, the coordinates of the phase difference for each frequency, is approximated by the least squares method to the straight line passing through the origin, the error of the slope of the straight line L _b obtained, although smaller than the FIG. 6 (a), the still, It contains an error.

FIG. 6C shows a state in which 2π is added to the phase difference between the frequencies ω ₇ and ω ₈ from the state of FIG. 6B. Here, the coordinates of the phase difference for each frequency, is approximated by the least squares method to the straight line passing through the origin, the error of the slope of the straight line L _c obtained becomes minimum.

FIG. 6D shows a state in which 2π is added to the phase difference between the frequencies ω ₅ and ω _{6 from} the state of FIG. Here, the coordinates of the phase difference for each frequency, is approximated by the least squares method to the straight line passing through the origin, the error of the slope of the line L _d obtained, increases conversely.

In this way, by adding 2π to the phase difference in order from the maximum frequency of the frequencies ω _{1 to} ω ₁₀ , a straight line that minimizes the error in the least square method can be obtained. Here, in order to facilitate understanding of the figure, a phase of 2π is added for every two frequencies, but in reality, 2π is added for each frequency in order from the maximum frequency.
Here, an example is shown in which 2π is added to the phase difference in order from the maximum frequency, but 2π is subtracted when the movement of the object B (see FIG. 1) is reversed.

Returning to FIG. 1, the description of the configuration of the pulse Doppler measurement device will be continued.
In order to correct the phase inclination described with reference to FIG. 6, here, the phase difference inclination calculation means 34 includes a phase difference correction means 341 and a linear approximation means 342.

The phase difference correcting means 341 corrects the phase difference by adding or subtracting a phase that is an integer multiple of 2π in order from the maximum frequency among the phases of a predetermined number of phases (average cross spectrum phase). To do. Here, the phase difference correction means 341 includes as many phase correction patterns as the number of phase correction patterns to add or subtract phases indicated by a plurality of phase correction patterns determined in advance by the correction pattern storage means 35 (described later). Phase difference correcting means 341 _{1 to} 341 _n ).
Here, the phase difference correction means 341 _{1 to} 341 _n perform parallel processing of adding 2π to the phase difference for each frequency. Thereby, for example, the process of adding 2π to the phase difference shown in FIGS. 6B to 6D is processed in parallel by the individual phase difference correcting units 341 _{1 to} 341 _n .
The value of the phase difference (average cross spectrum phase) corrected by the phase difference correcting unit 341 is output to the linear approximation unit 342 corresponding to the phase difference correcting unit 341.

The straight line approximating means 342 approximates a coordinate point to a straight line passing through the origin by a least square method for each of a plurality of phase correction patterns in a coordinate system having a frequency and a phase difference at the frequency as a coordinate point. Here, the phase difference correcting means 341 is provided corresponding to the phase difference correcting means 341 by the number of phase correction patterns (linear approximation means 342 _{1 to} 342 _n ).
Then, the straight line approximation unit 342 outputs the approximate straight line slope (phase difference slope) and the error in the least squares method to the speed specifying means 36.

Here, a method will be described in which the straight line approximating unit 342 obtains the slope of the straight line and its error using mathematical expressions.
First, _let the phase θ _n of the average cross spectrum be the following equation (8), and the amplitude q _n be the following equation (9).

Here, Δω is the sampling interval of the cross spectrum, and n is an integer. Here, when the range gate length is _{T R,} it is expressed as Δω = 2π / _{T R.}
The slope of the straight line (phase difference slope) k is expressed by the following equation (10).

In equation (10), e is an error term.
At this time, the sum of squares of errors weighted by the amplitude (amplitude spectrum) q _n of the average cross spectrum, that is, the estimated amount k _e of the slope k that minimizes e ^T Qe is obtained by the following equation (11). .

Further, the estimated error e ^T Qe is obtained by the following equation (12).

The straight line slope (estimated amount k _e ) and its error (estimated error e ^T Qe) obtained in this way are output to the speed specifying means 36.

The correction pattern storage means 35 stores a plurality of phases added by the phase difference correction means 341 as phase correction patterns, and is a general storage device such as a hard disk.
Here, the correction pattern storage means 35 stores a phase correction pattern for the frequency to be phase corrected. In this phase correction pattern, the position indicating the frequency to which the phase is added and the addition amount are described.

Here, the contents of the phase correction pattern will be specifically described with reference to FIG. 7 (refer to FIG. 1 as appropriate). FIG. 7 is a pattern diagram showing an example of a phase correction pattern. Here, an example is shown in which ₁₅ frequencies ω ₁ to ω 15 are added to the phase to be added, and 25 phase correction patterns are ID _{0 to} ID ₂₄ . Here, ω ₁₅ indicates the maximum frequency.
In this phase correction pattern, “0” indicates that the phase is not added. Further, “1”, “2”, “3”, and “4” respectively add the phases of “1 × 2π”, “2 × 2π”, “3 × 2π”, and “4 × 2π”. Show.

In addition, in ID _{0 to} ID ₂₄ , in the phase difference inclination calculation unit 34, each of the plurality of phase difference correction units 341 reads a phase correction pattern with a unique ID and corrects the phase difference. As a result, the phase difference inclination calculating means 34 can correct the phase difference by parallel processing.

Here, 15 frequencies for adding phases that are integer multiples of 2π and 25 types of phase correction patterns are used, but the number is not limited to this. By increasing this phase correction pattern, a wide range of speeds can be estimated.
Further, here, only the phase correction pattern for adding a phase that is an integer multiple of 2π is shown, but this phase correction pattern indicates a pattern for subtracting 2π when the movement of the object is reverse. And If the direction of movement of the object is not determined in advance, it is desirable to set a pattern for adding 2π and a pattern for subtraction together.
Returning to FIG. 1, the description of the configuration of the pulse Doppler measurement device will be continued.

The speed specifying means 36 selects the slope of the straight line that minimizes the error calculated by the straight line approximation means 342 as the slope of the phase difference corresponding to the moving speed of the object B.
Note that the straight line that minimizes this error is obtained by appropriately correcting the phase aliasing (aliasing) of the phase difference. Therefore, even if the phase aliasing occurs due to the movement of the object B, The speed of the object B is specified.

  The control device 3 described above can be realized by a general computer including a CPU, a RAM, a ROM, and the like. That is, the control device 3 can be operated by a pulse Doppler measurement program that causes a computer to function as each of the above-described means.

Although the configuration of the pulse Doppler measurement device 1 has been described above, the present invention is not limited to this configuration.
Here, a plurality of phase difference correcting means 341 and a linear approximation means 342 are provided, and the inclination of the phase difference (straight line inclination) is obtained in parallel. However, the phase difference correcting means 341 and the linear approximation means 342 are each 1 It is also possible to sequentially calculate the slope of the phase difference (the slope of the straight line) serially.

Here, the pulse Doppler measurement apparatus 1 includes the MTI filter 32. However, in the case where measurement is performed in an environment where no object other than the target exists in the direction in which the ultrasonic wave is transmitted from the measurement point, the MTI filter 32 is provided. It may be omitted. For example, when the measurement target is an airplane flying over the measurement point, the MTI filter 32 is not necessarily provided.
The pulse Doppler measurement apparatus 1 can be used for measuring the speed of an object that moves, such as the above-described airplane speed measurement or weather radar that measures cloud movement, in addition to measuring blood flow in a living body. it can.

[Operation of pulse Doppler measurement system]
Next, the operation of the pulse Doppler measuring apparatus according to the present invention will be described with reference to FIG. FIG. 2 is a flowchart showing the operation of the pulse Doppler measurement apparatus.

(Transmission / reception step)
First, the pulse Doppler measurement apparatus 1 transmits ultrasonic waves to the object B at a constant period by the wave transmission means 21 of the wave transmission / reception apparatus 2 (step S1), and is reflected from the object B by the wave reception means 22. The reflected wave is received (step S2). The received reflected wave is converted into a digital signal and output to the control device 3.

(Spectral analysis step)
The pulse Doppler measurement device 1 detects the spectrum by performing fast Fourier transform (FFT) on the reflected wave by the Fourier transform unit 31 of the control device 3 (step S3).
(Filtering step)
Then, the pulse Doppler measurement apparatus 1 removes clutter components by the MTI filter 32 according to the magnitude of the phase rotation of the spectrum detected in step S3 (step S4).

(Phase difference calculation step)
And the pulse Doppler measuring device 1 calculates the phase difference between adjacent periods for every frequency by the phase difference calculating means 33 (step S5). Here, as the phase difference, the phase of the average cross spectrum is calculated as shown in the equation (5).

Thereafter, the pulse Doppler measurement apparatus 1 uses the phase difference inclination calculation means 34 to make the coordinate points straight by the least square method for each of the plurality of phase correction patterns in the coordinate system having the frequency and the phase difference at the frequency as the coordinate points. By approximation, the slope of the straight line (the slope of the phase difference) and its error are calculated. Here, this operation is performed in parallel for each phase correction pattern.
That is, pulse Doppler measuring apparatus 1, the phase difference correction means 341 _1, it reads the phase correction pattern corresponding to the phase difference correcting unit 341 ₁ from the modified pattern storage unit 35 (Step _S6 1).

(Phase difference correction step)
Then, the phase difference correcting unit 341 ₁ adds a phase that is an integer multiple of 2π to the phase difference for each frequency (phase of the average cross spectrum) corresponding to the phase correction pattern (step S7 ₁ ).

(Linear approximation step)
After that, the pulse Doppler measurement apparatus 1 uses the linear approximation means 342 _{1 to} approximate each coordinate point to a straight line passing through the origin by the least square method in a coordinate system having the frequency and the phase difference at the frequency as the coordinate point. Then, the inclination of the straight line (refer to the equation (11)) and the error (refer to the equation (12)) are calculated (step S8 ₁ ).
The operations of Step S6 ₂ to Step S8 ₂ ,..., Step S6 _n to Step S8 _n are not described because only the phase correction pattern is different in the operations of Step S6 ₁ to Step S8 ₁ .
Here, the phase difference slope calculating unit 34 is operated in parallel for each phase correction pattern. However, the single phase difference correcting unit 341 and the straight line approximating unit 342 perform steps S6 ₁ to S61. The operations of S8 ₁ , Step S6 ₂ to Step S8 ₂ ,..., Step S6 _n to Step S8 _n may be sequentially performed.

(Speed identification step)
Then, the pulse Doppler measuring apparatus 1 uses the speed specifying means 36 to change the slope of the straight line that minimizes the error calculated in steps S8 ₁ , S8 ₂ ,..., S8 _n to the phase difference corresponding to the moving speed of the object B. (Step S9).
With the above operation, the pulse Doppler measurement device 1 performs measurement using a wideband frequency. Therefore, even when phase wrapping occurs in the phase difference due to the movement of the object B, the speed of the object B is correctly set. Can be identified. Further, even if a blind speed is generated at each frequency, the pulse Doppler measuring device 1 does not generate a blind speed in the frequency range to be measured, so that the estimable speed range can be expanded as compared with the conventional case.

[simulation result]
Finally, the evaluation results of the speed estimation between the conventional method (conventional pulse Doppler method, two-frequency method) and the broadband Doppler method by the pulse Doppler measurement apparatus 1 according to the present invention will be described.
Here, a simulation of velocity estimation was performed using a one-dimensional multiple scatterer model. The simulation conditions are as shown in the following table.

  In this simulation, only the pulse length of the transmission pulse to be used is changed between the conventional method and the broadband Doppler method, 2.4 μs in the conventional method, and 1.0 μs in the broadband Doppler method. In the broadband Doppler method, 81 types of phase correction patterns were used.

  FIG. 8 is a graph of the simulation results in each method, where (a) shows the result of the conventional pulse Doppler method, (b) shows the result of the conventional two-frequency method, and (c) shows the result of the broadband Doppler method according to the present invention. Show. Here, the two-frequency method uses the method described in Patent Document 2. In each graph of FIG. 8, the horizontal axis indicates the preset speed of the scatterer that is the target of speed measurement, and the vertical axis indicates the speed estimated by simulation. Ideally, the preset speed matches the estimated speed, and it is desirable that the plot position (“+” in the figure) is on a straight line having a slope of “1”.

As shown in FIG. 8A, in the conventional pulse Doppler method, since the Nyquist limit is 0.42 m / s, aliasing occurs at a speed exceeding this limit. In addition, there is a variation in the speed estimated by the interference of the reflected wave from the scatterer.
Further, as shown in FIG. 8B, in the conventional two-frequency method, although aliasing does not occur, it is affected by phase fluctuations due to interference, and thus there is a large variation in the estimated speed. This simulation result can be said to be the same for the conventional two-period method.
However, as shown in FIG. 8 (c), the wideband Doppler method according to the present invention does not cause aliasing, and the estimated speed variation is smaller than that of the conventional pulse Doppler method of FIG. The speed is estimated accurately in the range of 4 m / s.
As described above, the broadband Doppler method according to the present invention does not cause aliasing, is less influenced by interference of reflected waves from the scatterer, and can perform speed estimation with high accuracy.

It is a block diagram which shows the structure of the pulse Doppler measuring device which concerns on this invention. It is a flowchart which shows operation | movement of the pulse Doppler measuring device which concerns on this invention. It is a graph for demonstrating the difference between the conventional pulse Doppler method and the broadband Doppler method concerning this invention. It is a graph for demonstrating correction | amendment of the phase difference when the aliasing generate | occur | produces in a broadband Doppler method. It is explanatory drawing for demonstrating the principle of a MTI filter. It is a conceptual diagram which shows the concept of the correction | amendment of a phase when the return of a phase generate | occur | produces. It is a pattern diagram which shows an example of a phase correction pattern. It is a graph which shows a simulation result. It is explanatory drawing for demonstrating the concept of the conventional pulse Doppler method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Pulse doppler measuring device 2 Wave transmitter / receiver 21 Wave transmitting means 22 Wave receiving means 3 Control apparatus 31 Fourier transform means (spectrum analysis means)
32 MTI filter (filtering means)
33 Phase difference calculation means 34 Phase difference inclination calculation means 341 Phase difference correction means 342 Linear approximation means 35 Correction pattern storage means 36 Speed identification means

Claims (5)

A pulse Doppler measurement device that transmits ultrasonic waves to a target object at a constant period and measures the moving speed of the target object based on the reflected wave,
Wave transmitting means for transmitting the ultrasonic wave to the object;
Receiving means for receiving a reflected wave with respect to the ultrasonic wave from the object;
Spectral analysis means for detecting a phase for each frequency from the reflected wave received by the wave receiving means;
About the phase for each frequency detected by this spectrum analyzing means, a phase difference calculating means for calculating a phase difference for each period;
Phase difference correcting means for correcting the phase difference by adding or subtracting a phase that is an integer multiple of 2π with a plurality of different predetermined phase correction patterns to the phase difference for each frequency;
In the coordinate system having the frequency and the phase difference at the frequency as coordinate points, by approximating the coordinate point for each frequency to a straight line for each of the plurality of phase correction patterns, the slope of the straight line and the error A linear approximation means for calculating
Speed specifying means for selecting the slope of the straight line that minimizes the error calculated by the linear approximation means as the slope of the phase difference corresponding to the moving speed;
A pulse Doppler measurement device comprising:   The pulse Doppler measurement apparatus according to claim 1, wherein the phase difference calculation means averages the phase difference for each frequency for each predetermined number of transmission / reception times.   The pulse Doppler measurement apparatus according to claim 1, further comprising a filtering unit that removes a clutter component based on the phase detected by the spectrum analysis unit. A pulse Doppler measurement method for transmitting ultrasonic waves to a target object at a constant period and measuring a moving speed of the target object based on the reflected wave,
Transmitting and receiving the ultrasonic wave to the object and receiving a reflected wave from the object;
A spectral analysis step of detecting a phase for each frequency from the reflected wave;
About the phase for each frequency detected by this spectrum analyzing means, a phase difference calculating step for calculating a phase difference for each period;
A phase difference correction step of correcting the phase difference by adding or subtracting a phase that is an integer multiple of 2π with a plurality of different predetermined phase correction patterns with respect to the phase difference for each frequency;
In the coordinate system having the frequency and the phase difference at the frequency as coordinate points, by approximating the coordinate point for each frequency to a straight line for each of the plurality of phase correction patterns, the slope of the straight line and the error A linear approximation step for computing
A speed specifying step of selecting a slope of the straight line that minimizes the error calculated in the linear approximation step as a slope of a phase difference corresponding to the moving speed;
A pulse Doppler measurement method comprising: In order to measure the moving speed of the object based on the reflected wave with respect to the ultrasonic wave transmitted with a constant period to the object, a computer is provided.
Spectral analysis means for detecting a phase for each frequency from the reflected wave,
About the phase for each frequency detected by this spectrum analyzing means, a phase difference calculating means for calculating a phase difference for each period,
Phase difference correction means for correcting the phase difference by adding or subtracting a phase that is an integer multiple of 2π with a plurality of different predetermined phase correction patterns with respect to the phase difference for each frequency;
In the coordinate system having the frequency and the phase difference at the frequency as coordinate points, by approximating the coordinate point for each frequency to a straight line for each of the plurality of phase correction patterns, the slope of the straight line and the error Linear approximation means for calculating
Speed specifying means for selecting the slope of the straight line that minimizes the error calculated by the linear approximation means as the slope of the phase difference corresponding to the moving speed;
Pulse Doppler measurement program characterized by functioning as

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