JP2012147894A - Ultrasonic diagnostic apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve selectivity of a target position in a technique utilizing continuous waves of ultrasonic waves.SOLUTION: Within the frequency spectrum of demodulation signals, a Doppler signal fis included in reception signals obtained from a target position, and the Doppler signal fappears in a relatively low frequency band. Doppler signals f-findicate the Doppler signals included in reception signals of positions other than the target position. The Doppler signals f-fof positions other than the target position appear near frequencies f, 2f, 3f, ..., that is, the Doppler signals f-fof positions other than the target position appear in a relatively high frequency band compared to the Doppler signal fof the target position. Then, by utilizing an LPF of a characteristic 70, the Doppler signals f-for the like, which are unrequired waves of positions other than the target position, are reduced or eliminated. Thus, position selectivity related to the target position is improved.

Description

  The present invention relates to an ultrasonic diagnostic apparatus, and more particularly to an ultrasonic diagnostic apparatus using a continuous wave.

  A continuous wave Doppler is known as a technique using a continuous wave of an ultrasonic diagnostic apparatus. In continuous wave Doppler, for example, a transmission wave that is a sine wave of several MHz is continuously emitted into the living body, and a reflected wave from the living body is continuously received. The reflected wave includes Doppler shift information by a moving body (for example, blood flow) in the living body. Therefore, by extracting the Doppler shift information and performing frequency analysis, a Doppler waveform reflecting the velocity information of the moving body can be formed.

  Continuous wave Doppler using a continuous wave is generally superior in speed measurement at a higher speed than pulse Doppler using a pulse wave. Under such circumstances, the inventor of the present application has conducted research on continuous wave Doppler. As one of the results, Patent Document 1 proposes a technique related to continuous wave Doppler (FMCW Doppler) subjected to frequency modulation processing.

  On the other hand, with continuous wave Doppler, position measurement is difficult due to the use of continuous waves. For example, a conventional general continuous wave Doppler device (a device that does not use FMCW Doppler) cannot perform position measurement. On the other hand, the inventor of this application has proposed a very epoch-making technique in Patent Document 2 that can selectively extract Doppler information from a desired position in a living tissue by FMCW Doppler. Furthermore, the inventor of the present application proposes a technique related to a continuous wave subjected to digital modulation processing in Patent Document 3.

JP 2005-253949 A JP 2008-289851 A JP 2009-291294 A

  The technique of continuous wave Doppler described in Patent Documents 1 to 3 is an epoch-making technique having the possibility of ultrasonic diagnosis that has never existed before. The inventor of the present application has conducted further research and development on this revolutionary technology improvement. In particular, research and development have been repeated with a focus on techniques for selectively extracting in vivo information from target positions using continuous waves.

  The present invention has been made in the course of research and development, and an object thereof is to improve the selectivity of a target position in a technique using a continuous wave of ultrasonic waves.

  An ultrasonic diagnostic apparatus suitable for the above-described object includes a transmission signal processing unit that outputs a continuous wave transmission signal having periodicity obtained based on a periodic numerical pattern, and an ultrasonic wave corresponding to the transmission signal. Between the target position in the living body and the ultrasonic wave transmitting / receiving unit that receives the ultrasonic wave from the living body and obtains a reception signal by using the reference signal obtained based on the numerical pattern A demodulation processing unit that obtains a demodulated signal by performing demodulation processing on the received signal while adjusting the correlation, and a relatively low frequency component obtained from the target position and a relatively high frequency component obtained from other than the target position An unnecessary wave processing unit that removes at least a part of the relatively high frequency component by performing a filtering process on the demodulated signal included, and a target position based on the filtered demodulated signal And having the in vivo information extraction unit that extracts in-vivo information.

  In a preferred embodiment, the unnecessary wave processing unit includes a low-pass filter that passes a relatively low frequency component obtained from the target position, and the in-vivo information extraction unit outputs a Doppler signal from a demodulated signal as the in-vivo information. It is characterized by extracting.

  In a preferred embodiment, the pass band of the low-pass filter is controlled according to the maximum measurable frequency of the Doppler signal.

  In a preferred embodiment, the numerical pattern is a sine pattern obtained from a sine function and a cosine pattern obtained from a cosine function, and the transmission signal processing unit has a period according to a phase pattern obtained by synthesizing the sine pattern and the cosine pattern. It outputs a continuous wave transmission signal whose phase has been changed.

  In a preferred embodiment, the sine pattern is composed of N sine function values corresponding to N (N is an even number) phase values, and the cosine pattern is N corresponding to the N phase values. The continuous wave transmission signal includes a phase pattern having a pattern length N corresponding to the N phase values.

  In a preferred embodiment, when the filtered demodulated signal is added in the period direction, the pattern length N is divided into q blocks (q is a natural number) every p (p is a natural number), and each block is divided. And a demodulated signal is added to obtain a partially added demodulated signal.

  In a preferred embodiment, in a signal sequence in which a plurality of partial addition demodulated signals obtained one after another for each block are arranged in the order in which they are obtained, the range of q blocks is shifted step by step while shifting the range of one block at a time. By extracting q partial addition demodulated signals over q blocks constituting the length N and adding them, the addition processing of the demodulated signals over the pattern length N is realized.

  According to the present invention, selectivity of a target position is improved in a technique using a continuous wave of ultrasonic waves.

1 is a diagram illustrating an overall configuration of an ultrasonic diagnostic apparatus that is preferable in the practice of the present invention. It is a figure which shows the time change waveform regarding the transmission signal of a phase shift continuous wave. It is a figure which shows the time corresponding relationship of a reference signal and a received signal. It is a figure which shows the specific example of the phase change of a demodulated signal. It is a figure which shows the frequency spectrum of a demodulated signal. It is a figure which shows a mode that a demodulation signal is added. It is a figure which shows the specific example of the calculation result regarding an electric power spectrum. It is a figure which shows the specific example of the electric power spectrum obtained by removing an unnecessary wave. It is a figure for demonstrating the addition process which expands the maximum Doppler frequency.

  FIG. 1 is a diagram showing an overall configuration of an ultrasonic diagnostic apparatus suitable for implementing the present invention. The transmitting vibrator 10 continuously transmits ultrasonic waves into the living body, and the receiving vibrator 12 continuously receives reflected ultrasonic waves from the living body. In this way, transmission and reception are performed by different vibrators, and transmission / reception is performed by a so-called continuous wave Doppler method. The transmitting transducer 10 includes a plurality of vibration elements, and the plurality of vibration elements are controlled to form an ultrasonic transmission beam. The receiving vibrator 12 also includes a plurality of vibration elements, and signals obtained by the plurality of vibration elements are processed to form a reception beam.

  The transmission beamformer (transmission BF) 14 outputs transmission signals to a plurality of vibration elements included in the transmission transducer 10. The transmission beamformer 14 is supplied with a continuous wave transmission signal from the synthesis processing unit 24, and the transmission beamformer 14 applies a delay process corresponding to each vibration element to the transmission signal to correspond to each vibration element. The transmitted signal is formed. Note that power amplification processing may be performed on the transmission signal corresponding to each vibration element formed in the transmission beam former 14 as necessary. In this way, an ultrasonic transmission beam is formed, and the transmission beam is scanned in a two-dimensional plane or in a three-dimensional space.

  The continuous wave transmission signal supplied to the transmission beam former 14 is formed by a sine pattern processing unit 22B, a cosine pattern processing unit 22A, and a synthesis processing unit 24.

  The sine pattern processing unit 22B performs processing based on the sine pattern on the RF wave (carrier wave signal) obtained from the RF wave oscillator 20. On the other hand, the cosine pattern processing unit 22A performs processing based on the cosine pattern on the RF wave (carrier wave signal) obtained from the RF wave oscillator 20 via the π / 2 shift circuit 21.

  Then, the two signals output from the sine pattern processing unit 22B and the cosine pattern processing unit 22A are synthesized in the synthesis processing unit 24, and a continuous wave (phase shift continuous wave) having a predetermined phase pattern is formed. The continuous wave transmission signal formed by the sine pattern processing unit 22B, the cosine pattern processing unit 22A, and the synthesis processing unit 24 will be described in detail later.

  The reception beam former (reception BF) 16 forms a reception beam by phasing and adding a plurality of reception signals obtained from a plurality of vibration elements included in the reception transducer 12. That is, the reception beamformer 16 performs a delay process corresponding to the vibration signal obtained from each vibration element and adds a plurality of reception signals obtained from the plurality of vibration elements. Form a beam. Note that a plurality of received signals may be supplied to the reception beam former 16 after processing such as low noise amplification is performed on the received signals obtained from the respective vibration elements. Thus, a reception beam corresponding to a transmission beam scanned in a two-dimensional plane or a three-dimensional space is formed, and a reception RF signal is collected along the reception beam.

  The reception mixer 30 is a circuit that performs quadrature detection on the received RF signal to generate a complex baseband signal, and is composed of two mixers 32 and 34. Each mixer is a circuit that mixes the received RF signal with a predetermined reference signal.

  The reference signal supplied to each mixer of the reception mixer 30 is generated based on the transmission signal output from the synthesis processing unit 24. That is, the transmission signal output from the synthesis processing unit 24 is subjected to delay processing in the delay circuit 25, and the transmission signal subjected to delay processing is directly supplied to the mixer 32 as a reference signal, while the transmission signal subjected to delay processing is supplied to the mixer 34. The signal is supplied as a reference signal via the π / 2 shift circuit 26.

  The π / 2 shift circuit 26 is a circuit that shifts the phase of the delayed reference signal by π / 2. As a result, an in-phase signal component (I signal component) is output from one of the two mixers 32 and 34, and a quadrature signal component (Q signal component) is output from the other. The high-frequency components of the in-phase signal component and the quadrature signal component are cut by LPFs (low-pass filters) 36 and 38 provided at the subsequent stage of the reception mixer 30, and a demodulated signal of only a necessary band after detection is obtained. Extracted.

  The unnecessary wave processing unit 40 reduces or eliminates unnecessary waves included in the demodulated signal, and includes an LPF (low pass filter) corresponding to each of the I signal component and the Q signal component of the demodulated signal. The demodulated signal obtained from the receiving mixer 30 via the LPFs 36 and 38 includes a relatively low frequency component obtained from the target position and a relatively high frequency component obtained from other than the target position, as will be described in detail later. Yes. Therefore, the unnecessary wave processing unit 40 uses a relatively high frequency component obtained from other than the target position as an unnecessary wave, and removes at least a part of the unnecessary wave.

  The addition units 46 and 48 add the demodulated signals obtained from the unnecessary wave processing unit 40 over a predetermined period. Thereby, the addition process regarding the phase pattern of the phase shift continuous wave is executed, and the demodulated signal from the target position that matches the phase pattern of the reference signal is selectively extracted. This position selectivity will be described in detail later.

  The FFT processing unit (fast Fourier transform processing unit) 50 performs an FFT operation on each of the demodulated signals (in-phase signal component and quadrature signal component) obtained from the addition units 46 and 48. As a result, the demodulated signal is converted into a frequency spectrum in the FFT processing unit 50. The frequency spectrum output from the FFT processing unit 50 is output as frequency spectrum data having a frequency resolution δf depending on circuit setting conditions and the like.

  The Doppler information analysis unit 52 extracts a Doppler signal from the demodulated signal converted into a frequency spectrum. As will be described in detail later, in the ultrasonic diagnostic apparatus of FIG. 1, a target position is set by delay processing in the delay circuit 25, and a Doppler information analysis unit 52 selectively extracts a Doppler signal from the target position. For example, the Doppler information analysis unit 52 forms a display waveform of a Doppler signal that changes with time. In addition, Doppler signals are extracted for each depth (each position) in the living body, and for example, the velocity of the tissue in the living body is calculated for each depth on the ultrasonic beam (sound ray) and output in real time. May be. Alternatively, the velocity of each position of the in-vivo tissue may be calculated two-dimensionally or three-dimensionally by scanning an ultrasonic beam.

  The display unit 54 displays the waveform of the Doppler signal formed in the Doppler information analysis unit 52. Each unit in the ultrasonic diagnostic apparatus shown in FIG. 1 is controlled by the system control unit 60. That is, the system control unit 60 performs transmission control, reception control, display control, and the like.

  As described above, in the ultrasonic diagnostic apparatus in FIG. 1, a reception signal is obtained by transmitting and receiving an ultrasonic wave corresponding to a phase shift continuous wave, and a reference signal and reception are received according to the depth of the target position in the living body. The Doppler information from the target position is selectively extracted by adjusting the delay relationship between the signals and strengthening the correlation between the received signal from the target position and the reference signal to perform demodulation processing. Therefore, the phase shift processing in the ultrasonic diagnostic apparatus of FIG. 1 and the position selectivity from which Doppler information from the target position is selectively extracted will be described in detail below. In addition, about the part (structure) shown in FIG. 1, the code | symbol of FIG. 1 is utilized also in the following description.

<About phase shift processing>
In the ultrasonic diagnostic apparatus of FIG. 1, phase shift processing is performed using two rows of numerical patterns that are complementary to each other. That is, the sine pattern processing unit 22B uses a sine pattern, and the cosine pattern processing unit 22A uses a cosine pattern.

The sine pattern and cosine pattern, which are two-row numerical patterns, are defined by In the following formula, a _n is obtained from is a cosine function cosine pattern. On the other hand, b _n is a sine pattern and is obtained from a sine function. N is a natural number indicating the pattern length, and n is the number of each numerical value (each code) constituting the pattern. Incidentally, N is an arbitrary natural number and an even number, and is not limited to a power of 2.

The sine pattern processing unit 22B changes the amplitude of the RF wave (sine wave) obtained from the RF wave oscillator 20 according to the sine pattern. On the other hand, the cosine pattern processing unit 22A changes the amplitude of the RF wave (cosine wave) obtained via the π / 2 shift circuit 21 according to the cosine pattern. Then, the continuous wave output from the sine pattern processing unit 22B and the continuous wave output from the cosine pattern processing unit 22A are combined in the combining processing unit 24 to form a continuous wave transmission signal represented by the following equation. Note that the A ₁ the amplitude of the transmission signal in the following equation. A specific example of the continuous wave (phase shift continuous wave) is shown in FIG.

  FIG. 2 is a diagram illustrating a time-varying waveform related to a phase-shift continuous wave transmission signal. The waveform of the transmission signal shown in FIG. 2 is obtained from Equation 2, where the pattern length in Equation 1 is N = 8 (n = 0 to 7). For example, a continuous wave obtained by repeating a phase pattern as shown in FIG. 2 is used. When actually used in the ultrasonic diagnostic apparatus, the pattern length N is set to about several hundreds, for example. Of course, the pattern length N may be set according to the measurement object, the type of diagnosis, and the like.

  The reception signal corresponding to the transmission signal arrives at the reception system after a delay time τ shown in the following equation from the time when the transmission signal is transmitted. In the following expression, T is a time length of 1 bit (each numerical value) of the numerical pattern, that is, a bit length, and k is an arbitrary natural number. Also, ξ is a time of 1/2 bit length or less.

When the delay time (transmission / reception time difference) of the received signal with reference to the transmitted signal is τ and the amplitude of the received signal is A ₂ , the received signal with the phase change amount ω _d due to the Doppler shift is expressed as follows: Expressed. In the following equation, the phase rotation amount φ at the time of reflection is also taken into consideration.

In the ultrasonic diagnostic apparatus of FIG. 1, a reference signal obtained by delaying the transmission signal in the delay circuit 25 is multiplied by the reception signal in the reception mixer 30. When the delay amount of the reference signal in the delay circuit 25 is 1T (l: English letter L), the reference signal v _ref1 sent from the delay circuit 25 to the mixer 32 and the mixer from the delay circuit 25 via the π / 2 shift circuit 26. The reference signal v _ref2 sent to 34 is expressed as follows.

FIG. 3 is a diagram illustrating the temporal correspondence between the reference signal and the received signal. In comparison with a transmission signal (not shown), the reference signal is delayed by the delay amount lT in the delay circuit 25, and therefore the number of code shifts associated with this delay is l (el). Therefore, in FIG. 3, the codes of the 1-bit period T shown at the center in the reference signal are a _n−l and b _n−l .

  On the other hand, in comparison with a transmission signal (not shown), the reception signal is delayed by a transmission / reception time difference τ (Equation 3). Since the received signals are obtained from various continuous depths, FIG. 3 shows received signals from I to III in consideration of ξ which is a minute time of ½ bit length or less.

  The received signal II is in the case of ξ = 0, and the timing of code switching is the same between the received signal II and the reference signal. On the other hand, the received signal I and the received signal III are in the case where ξ is not 0, and the timing of code switching is shifted between the received signal and the reference signal.

  Therefore, for example, the central time point of each code having a time length T is set as a time of multiplication sampling (multiplication timing) with reference to the code of the reference signal. As a result, when the ξ is relatively small as in the received signals I and III shown in FIG. 3, the same multiplication result as in the case of the received signal II is obtained, and the influence of ξ can be ignored.

The multiplication in the mixer 32 of FIG. 1 is expressed by the following equation, and a baseband component (demodulated signal) is obtained as a multiplication result. Incidentally, the term 2 [omega _c t In the calculation process of the following equation is removed by LPF36 disposed downstream of the mixer 32.

  Further, if the portion irrelevant to the time t in the phase term of the final result of Formula 6 is defined as Formula 7, the final result of Formula 6 is simply expressed as Formula 8.

  Furthermore, Formula 9 is obtained by applying Formula 1 to Formula 8, and Formula 11 is obtained when the definition of Formula 10 is used in Formula 9. In this way, the in-phase signal component of the demodulated signal output through the LPF 36 after multiplication in the mixer 32 is finally expressed as in Expression 11.

On the other hand, the multiplication in the mixer 34 of FIG. 1 is expressed by the following equation, and a baseband component (demodulated signal) is obtained as a multiplication result. In the calculation process of the following equation, the 2ω _c t term is removed by the LPF 38 provided at the subsequent stage of the mixer 34, and the definition of Equation 7 is used for the portion unrelated to the time t.

  Furthermore, when Formula 1 and Formula 10 are applied to Formula 12, Formula 13 is obtained. That is, the quadrature signal component of the demodulated signal output through the LPF 38 after multiplication in the mixer 34 is finally expressed as in Expression 13.

  When the in-phase signal component (Equation 11) and the quadrature signal component (Equation 13) are combined in a complex expression, the demodulated signal Z (t) can be expressed in a complex form as shown in the following expression.

As shown in Equation 14, the demodulated signal Z (t) has a phase that changes by 2πf _{kl at} every time interval T in addition to a temporal change in phase according to the Doppler frequency f _d . In Equation 14, Ψ _kl is a constant term that does not change with time.

  Further, the demodulated signal Z (t) is obtained for each time interval T, for example, at the timing of multiplication sampling shown in FIG. Therefore, when the time of multiplication sampling is expressed as t = nT using the code number n (see Equation 1) and T which is also the time length of each code, Equation 14 to Equation 15 are obtained. Then, the phase of the demodulated signal obtained from equation (15) according to the change of the value of the number n is specifically shown in FIG.

FIG. 4 is a diagram illustrating a specific example of the phase change of the demodulated signal. FIG. 4 shows the phase value obtained when the value of number n is changed in equation (15). As shown in FIG. 4, the phase of the demodulated signal of Formula 15 is rotated by 2π (f _kl −f _d ) NT radians during time NT, that is, during one period of the numerical pattern (Formula 1). It becomes a sine wave. The frequency of the sine wave is as follows:

The characteristics of the demodulated signal obtained from the above analysis results are summarized as follows. (1) The demodulated signal includes _kl (l: English letter L), that is, a frequency component f _kl -f _d corresponding to the phase difference between the received signal and the reference signal.
(2) When k = 1, that is, the received signal from the target position where the phase difference with respect to the reference signal is 0 is f _kl = 0 from Equation 10, the frequency is f _d in the demodulated signal. It becomes. It is the Doppler frequency f _d that this frequency f _d is the observation target.
(3) When k ≠ l, that is, a received signal from other than the target position has a frequency of f _kl −f _d in the demodulated signal. Further, as f _kl increases as the _distance from the target position increases, the frequency f _kl −f _d shifts from the Doppler frequency f _d to the higher frequency side.
(4) frequency _{f kl} is proportional to kl as shown in Equation 10 Equation, since k and l are natural numbers, an integer multiple of the frequency _{f N.} Note that the maximum value of the frequency f _kl is expressed by the following equation because the sampling interval of the demodulated signal is the time T.

FIG. 5 is a diagram showing the frequency spectrum of the demodulated signal. FIG. 5 shows a specific example of the frequency spectrum related to the demodulated signal of Formula 15. Doppler signal f _d1 is the Doppler signals Fukumaru the reception signal obtained from the target position, a Doppler signal f _d in the case of the f _{kl = 0} in the case of k = l ie equation (15). Since f _kl = 0, the Doppler signal f _d1 appears in a relatively low frequency band.

On the other hand, Doppler signals f _{d2 to} f _d5 indicate Doppler signals included in received signals other than the target position. Outside target position k ≠ l, and the equation (15) of _{f kl} is an integer multiple of _{f N.} Therefore, the Doppler signals f _{d2 to} f _d5 other than the target position appear in the vicinity of the frequencies f _N , 2f _N , 3f _N. That is, compared to the Doppler signal f _d1 at the target position, Doppler signals f _{d2 to} f _d5 other than the target position appear in a relatively high frequency band.

FIG. 5 shows a characteristic 70 of an LPF (low-pass filter) of the unnecessary wave processing unit 40 (FIG. 1). The unnecessary wave processing unit 40 reduces or eliminates Doppler signals f _{d2 to} f _d5 that are unnecessary waves other than the target position by using, for example, the LPF having the characteristic 70 shown in FIG. Thereby, the position selectivity regarding the target position is improved. Therefore, the principle of position selection and the improvement of position selectivity will be described below.

<About the principle of position selection>
In the ultrasonic diagnostic apparatus of FIG. 1, in order to selectively extract the Doppler information from the target position, the numerical value pattern (sine pattern and cosine pattern) represented by the equation 1 is over one period, that is, the pattern length N Then, the demodulated signal is subjected to addition processing.

The demodulated signal Z (t) of Expression 14 is sampled at every time interval T. Therefore, using the code number n (an integer from 0 to N-1), the number of code repetitions m (an integer from 0 to M-1), and T, which is also the time length of each code, sampling of the demodulated signal When the time is expressed as t _n = nT + mNT, Expression 14 is obtained from Expression 14. Moreover, Formula 19 is derived based on Formula 10.

  In Equation 19, m, k, and l are all integers, so the result of Equation 19 is always an integral multiple of 2π. When the result is applied to Equation 18, the following equation is obtained.

  In order to selectively extract the Doppler information from the target position, the demodulated signal expressed by Equation 20 is expressed by the following equation over one period of the numerical pattern (sine pattern and cosine pattern), that is, over the pattern length N: As shown in FIG.

Applying the formula of Formula 22 for the geometric series having the first term of 1 and the common ratio of r to the result of Formula 21 yields Formula 23 when f _kl = f _d , and f _kl number 24 expression is obtained when the ≠ f _d.

Signal obtained by equation 23 or equation (24) equation is a complex sine wave having a frequency f _d. Therefore, for example, the frequency f _d, that is, the frequency of the Doppler signal can be extracted by performing frequency analysis processing of Formula 23 or Formula 24. In particular, a Doppler signal from the target position is selectively extracted.

FIG. 6 is a diagram showing how the demodulated signals are added. (A) in FIG. 6 represent the results of the sampled every T the demodulated signal Z _S having 20 Formula Time in multiple pulses. Further, (B1) and (B2) in FIG. 6 represent w _m in Equation 23 and Equation 24 obtained by adding the demodulated signal Z _S over one period (N samples) by a plurality of pulses. Yes.

The power spectrum of w _m shown in Equation 24 is expressed as the following equation. Note that B in Expression 24 is a constant and is omitted in the following expression.

First, the characteristics of the power spectrum obtained from the fixed target will be confirmed. In the case of a fixed target, f _d = 0, and when Formula 10 is further applied, the power spectrum obtained from the fixed target is as follows.

Since k−1 ≠ 0 except for the target position, the result of Expression 26 is always 0 (zero). Since _kl = 0 at the target position, f _kl = 0, and for a fixed target, f _d = 0, and the amplitude of w _m is N from Equation 23. That is, in the case of a fixed target, the selection characteristic is sharp at the target position.

Next, the characteristics of the power spectrum obtained from the moving target will be confirmed. The length of the code sequence (pattern length of the numerical pattern) is N = 200, the code length is T = 1 μs (microseconds), the code sequence repetition length is TN = 200 μs, and the code sequence repetition frequency is f _N = 5 kHz. FIG. 7 shows a specific example of the calculation result obtained from the equation (25).

FIG. 7 is a diagram illustrating a specific example of a calculation result regarding the Doppler power spectrum. That is, FIG. 7 shows the calculation result regarding the Doppler power spectrum P ₁ obtained from the equation (25) when N = 200, T = 1 μs, TN = 200 μs, and f _N = 5 kHz.

The horizontal axis in FIG. 7 represents the distance from the target position in k−1. That is, since k = 1 at the target position, the distance becomes 0 (zero), and k−1 increases as the position deviates from the target position, so the distance also increases. The actual distance in the living body can be obtained from Equation 27, where c is the ultrasonic propagation velocity. The vertical axis of FIG. 7 shows the value of the power spectrum P ₁ (relative value).

Figure 7, for a plurality of Doppler frequency f _d, and the calculation results are shown for each Doppler frequency f _d. As overall characteristics regarding a plurality of Doppler frequencies f _d , the target position, that is, the position of k−1 = 0 on the horizontal axis, and a distance k− apart from the target position by a distance corresponding to one period (N = 200) of the code sequence. in the position of the l = 200, the value of the Doppler power spectrum _{P 1} is the maximum value. It should be noted that the position of kl = 200 is a position where the code sequence (N = 200) makes a round and coincides again.

Then, looking at the individual of the Doppler frequency f _d, the Doppler frequency f _d is about maximum trend at the target position is strong small. In particular, when the Doppler frequency f _d = 0, that is, in the case of a fixed target, sharp selection characteristics are obtained only at the target position (including a position separated by one cycle) as described above.

In contrast, according to the Doppler frequency f _d becomes larger, the value of the Doppler power spectrum P ₁ at the other target position (including one period apart position) is increased, the position selectivity in the target position is degraded.

  Therefore, in the ultrasonic diagnostic apparatus of FIG. 1, the unnecessary wave processing unit 40 improves the position selectivity of the target position by suppressing the deterioration of the position selectivity at the target position. Next, improvement in position selectivity will be described.

<About improvement of position selectivity>
The Doppler power spectrum P ₁ shown in Formula 25 is obtained by adding the demodulated signal of Formula 20 obtained from the mixer 30 of FIG. 1 through the LPFs 36 and 38 over one period of the numerical pattern, that is, over NT time. It is obtained by processing. Formula 20 before the addition includes a frequency component of f _kl −f _d , and f _kl is an integral multiple of the frequency f _N as shown in Formula 10.

On the other hand, the required signal is a Doppler signal from a target position that satisfies the condition of k = 1. For example, when the demodulated signal is added N samples over one period of the numerical pattern by the simple addition process shown in FIG. 6, the addition result is obtained every NT time. That is, in this case, the required Doppler signal is sampled every NT time. Its sampling frequency is _{f N.} Therefore, the maximum frequency of the Doppler signal that can be measured in this case is f _N / 2 from the sampling theorem.

Thus, while the maximum frequency of the Doppler signal for the required target position is f _N / 2, as shown in FIG. 5, the Doppler signals other than the target position have frequencies f _N and 2f _N. , 3f _N ... That is, the Doppler signals f _{d2 to} f _d5 other than the target position appear in a relatively high frequency band as compared to the Doppler signal f _d1 at the target position where the maximum frequency is f _N / 2.

Therefore, the unnecessary wave processing unit 40 in FIG. 1 sets a relatively high frequency component other than the target position included in the demodulated signal as an unnecessary wave, and removes at least a part of the unnecessary wave from the demodulated signal. For this purpose, the unnecessary wave processing unit 40 includes an LPF corresponding to each of the I signal component and the Q signal component of the demodulated signal. Each of these LPFs has a characteristic 70 shown in FIG. 5, for example. That is, the Doppler signal f _d1 at the target position in the relatively low frequency band is allowed to pass, and the Doppler signals f _{d2 to} f _d5 other than the target position in the relatively high frequency band are blocked. For example, the cutoff frequency (cut-off frequency) of each LPF is set to f _N / 2.

As a specific example of the LPF included in the unnecessary wave processing unit 40 of FIG. 1, for example, there is a Butterworth LPF of order i having the following transfer characteristic. In the following equation, f is a frequency of a signal to be processed, and f _C is a cutoff frequency (cut-off frequency).

Since the demodulated signal processed in the unnecessary wave processing unit 40 includes a frequency component of f _kl −f _d, the frequency f in Equation 28 is set to f _kl −f _d as shown in the following equation.

The power spectrum of the signal obtained by applying the LPF having the transfer characteristic shown in Equation 29 to the demodulated signal and then adding the demodulated signal over one period (N samples) is expressed by Equation 25 and Equation 29. Can be calculated as: In the following equation, the cut-off frequency is set to f _C = f _N / 2 for the first-order Butterworth LPF having the order i = 1.

Then, under the same conditions as the specific example in FIG. 7, that is, the code sequence length is N = 200, the code length is T = 1 μs (microseconds), the code sequence repetition length is TN = 200 μs, and the code sequence repetition is A specific example of a calculation result obtained from Equation 30 using a first-order (i = 1) Butterworth LPF with a frequency of f _N = 5 kHz and a cutoff frequency of f _C = f _N /2=2.5 kHz. As shown in FIG.

FIG. 8 is a diagram illustrating a specific example of a Doppler power spectrum obtained by removing unnecessary waves. The calculation result shown in FIG. 8 is obtained from the equation (30). In the unnecessary wave processing unit 40 (FIG. 1), the cut-off frequency is set to f _C = f _N /2=2.5 kHz (i = 1) 7) except that the Butterworth LPF is used. 7, the horizontal axis in FIG. 8 represents the distance from the target position in k−1, and the vertical axis in FIG. 8 represents the value (relative value) of the power spectrum P _1. Yes.

FIG. 8 also shows the calculation results for each of the Doppler frequencies f _d for a plurality of Doppler frequencies f _d . As overall characteristics regarding a plurality of Doppler frequencies f _d , the target position, that is, the position of k−1 = 0 on the horizontal axis, and a distance k− apart from the target position by a distance corresponding to one period (N = 200) of the code sequence. in the position of the l = 200, the value of the Doppler power spectrum _{P 1} is the maximum value. It should be noted that the position of kl = 200 is a position where the code sequence (N = 200) makes a round and coincides again.

Compared with the calculation result of FIG. 7, in FIG. 8, the value of the Doppler power spectrum P ₁ other than the target position (including a position separated by one cycle) is extremely small. That is, the position selectivity regarding the target position is improved.

  Note that the position selectivity can be further improved by increasing the order i of the Butterworth LPF shown in Equation 29, that is, by using a second-order, third-order, etc. Butterworth LPF. Of course, other known filters such as Chebyshev type and Gaussian type (also called Thomson type) LPFs may be used instead of the Butterworth type LPF.

  As described above, by using the LPF in the unnecessary wave processing unit 40 of FIG. 1, a relatively high frequency component other than the target position included in the demodulated signal is made an unnecessary wave, and at least a part of the unnecessary wave is removed from the demodulated signal. Thus, position selectivity regarding the target position can be improved.

<About expansion of maximum Doppler frequency>
In the ultrasonic diagnostic apparatus of FIG. 1, in order to selectively extract the Doppler information from the target position, as shown in FIG. 6, one of the numerical patterns (sine pattern and cosine pattern) represented by Equation 1 is used. The demodulated signal is added over the period, that is, over the pattern length N. For example, when the demodulated signal is added N samples over one period of the numerical pattern by a simple addition process shown in FIG. 6, for example, an addition result is obtained every NT time. That is, in this case, the required Doppler signal is sampled every NT time. Its sampling frequency is _{f N.} Therefore, the maximum frequency of the Doppler signal that can be measured in this case is f _N / 2 from the sampling theorem. On the other hand, the maximum frequency (maximum Doppler frequency) of the measurable Doppler signal can be expanded by the addition processing described below.

  In the ultrasonic diagnostic apparatus of FIG. 1, the synthesis processing unit 24 outputs a continuous wave transmission signal so as to repeat a phase pattern having a pattern length N (see FIG. 2). Then, in the reception processing from the reception mixer 30 to the FFT processing unit 50, the pattern length N is divided into q blocks (q is a natural number) every p pieces (p is a natural number), and a partial demodulated signal for each block. Thus, a partial demodulated signal over q blocks corresponding to the pattern length N is extracted. The partial demodulated signal over the q blocks extracted in this way is subjected to frequency analysis processing.

  FIG. 9 is a diagram for explaining an addition process for enlarging the maximum Doppler frequency. This addition process is executed for each of the in-phase signal component processed by the adder 46 from the mixer 32 and the quadrature signal component processed by the adder 48 from the mixer 34.

  FIG. 9 shows a demodulated signal obtained when a continuous wave transmission signal that repeats a phase pattern having a pattern length N is used. That is, the demodulated signal sequence shown in FIG. 9 is a signal sequence in which a plurality of partial demodulated signals obtained sequentially for each block are arranged in the order in which they are obtained.

  SUM1, SUM2,... SUMq included in the demodulated signal sequence indicate demodulated signals (partially added demodulated signals) that are partially added for each p bit length. For example, SUM1 is a partial demodulated signal addition result corresponding to the first block, and SUM2 is a partial demodulated signal addition result corresponding to the second block. In this way, the demodulated signal is added for each block. This addition processing is executed by the addition units 46 and 48, for example. Since the pattern length N is composed of q blocks, q addition results (SUM) are obtained within the period of the pattern length N.

  Then, a partial demodulated signal over q blocks corresponding to the pattern length N is extracted step by step while shifting the range of q blocks one block at a time in the demodulated signal sequence. That is, the signal strings Y1, Y2, Y3,... Shown in FIG. 9 are extracted one after another and stored in a memory or the like.

  The signal string Y1 is composed of q addition results from SUM1 to SUMq with SUM1 at the head. The signal sequence Y2 extracted next to the signal sequence Y1 is composed of q addition results obtained by adding SUM1 after the addition results from SUM2 to SUMq with SUM2 as the head. Further, the signal sequence Y3 extracted next to the signal sequence Y2 is composed of q addition results obtained by adding SUM1 and SUM2 after the addition results from SUM3 to SUMq with SUM3 as the head. In this way, the signal strings Y1, Y2, Y3,... Are extracted one after another while the leading block is shifted in stages.

  If the time length of 1 bit is T, the time length of p bits is pT, and the time interval of SUM1, SUM2,... Obtained for each p bit is pT. Therefore, the time interval between the signal trains Y1, Y2, Y3,... Extracted one after another is also pT. The extracted signal sequences Y1, Y2, Y3,... Are stored in a memory or the like, and subjected to frequency analysis processing in the FFT processing unit 50 (FIG. 1).

  The signal sequences Y1, Y2, Y3,... Are subjected to an FFT operation for each signal sequence in the FFT processing unit 50 (FIG. 1). As a result, the demodulated signal is converted into a frequency spectrum for each signal sequence, and a frequency spectrum SP1 corresponding to the signal sequence Y1, a frequency spectrum SP2 corresponding to the signal sequence Y2,... Are formed one after another at a time interval pT, for example. The And if the result of frequency spectrum SP1-SPq is obtained, these frequency spectrum will be added in the FFT process part 50. FIG.

  Adding a plurality of frequency spectra SP1 to SPq corresponding to the plurality of signal sequences Y1 to Yq corresponds to adding signals at the same time (time corresponding to each other) included in the plurality of signal sequences. For example, this corresponds to the addition of head blocks of a plurality of signal sequences Y1 to Yq. In other words, this is equivalent to adding all the partial demodulated signals from SUM1 to SUMq obtained over one period of the phase pattern. In addition to the first block, a plurality of addition results (SUM) from signal sequences Y1 to Yq are added, and all partial demodulated signals from SUM1 to SUMq are added. That is, all the demodulated signals are added over one period of the numerical pattern.

  When the frequency spectra SP1 to SPq are obtained, the next frequency spectrum SP1 can be obtained after the time interval pT. Therefore, for example, after the addition results for the frequency spectra SP1 to SPq are obtained, the addition results for the frequency spectra SP2 to SPq and SP1 can be obtained after the time interval pT. That is, it is possible to obtain frequency spectrum addition results one after another at time intervals pT.

Compared with the simple addition process shown in FIG. 6, in FIG. 6, the addition result is obtained every NT time. That is, in this case, the required Doppler signal is sampled every NT time. The sampling frequency is f _N = 1 / NT. Therefore, the maximum frequency of the Doppler signal that can be measured in this case is 1/2 NT from the sampling theorem.

  On the other hand, in the addition process described with reference to FIG. 9, an addition result is obtained every pT time. Since N ≧ p, pT is NT or less. In this case, the maximum frequency of the Doppler signal that can be measured is 1/2 pT from the sampling theorem. That is, the maximum Doppler frequency is expanded from 1/2 NT in the case of FIG. 6 to 1/2 pT by the addition process of FIG.

  Thus, when the addition result of the frequency spectra SP1 to SPq is obtained in the FFT processing unit 50 of FIG. 1, the Doppler information analysis unit 52 extracts the Doppler signal from the frequency spectrum of the addition result, and based on the Doppler shift amount and the like. The flow rate of the blood flow existing at the target position is calculated. Since quadrature detection is performed in the receiving mixer 30, it is possible to determine the polarity such as the flow velocity.

  Further, as the maximum Doppler frequency is increased, the cutoff frequency (cutoff frequency) of each LPF included in the unnecessary wave processing unit 40 of FIG. 1 may be adjusted. For example, the system control unit 60 sets the cutoff frequency of each LPF of the unnecessary wave processing unit 40 to 1/2 pT according to the expanded maximum Doppler frequency 1/2 pT, and controls the passband of each LPF. Accordingly, each LPF of the unnecessary wave processing unit 40 is appropriately adjusted to improve the position selectivity regarding the target position, and the maximum Doppler frequency can be expanded by the addition process shown in FIG.

  The preferred embodiments of the present invention have been described above, but the above-described preferred embodiments of the present invention are merely examples in all respects, and do not limit the scope of the present invention. The present invention includes various modifications without departing from the essence thereof.

  22A cosine pattern processing unit, 22B sine pattern processing unit, 24 synthesis processing unit, 25 delay circuit, 30 reception mixer, 40 unnecessary wave processing unit, 46, 48 addition unit, 50 FFT processing unit, 52 Doppler information analysis unit.

Claims (7)

A transmission signal processing unit for outputting a continuous wave transmission signal having periodicity obtained based on a periodic numerical pattern;
An ultrasonic transmission / reception unit that obtains a reception signal by transmitting an ultrasonic wave corresponding to the transmission signal to the living body and receiving the ultrasonic wave from the living body;
Using a reference signal obtained based on the numerical pattern, a demodulation processing unit that obtains a demodulated signal by performing demodulation processing on the received signal while adjusting the correlation with the target position in the living body,
Filtering the demodulated signal including a relatively low frequency component obtained from the target position and a relatively high frequency component obtained from other than the target position, thereby at least a part of the relatively high frequency component is obtained. An unnecessary wave processing section to be removed;
An in-vivo information extracting unit that extracts in-vivo information of the target position based on the demodulated signal subjected to the filter processing;
Having
An ultrasonic diagnostic apparatus. The ultrasonic diagnostic apparatus according to claim 1,
The unnecessary wave processing unit includes a low-pass filter that passes a relatively low frequency component obtained from the target position,
The in-vivo information extraction unit extracts a Doppler signal from a demodulated signal as the in-vivo information.
An ultrasonic diagnostic apparatus. The ultrasonic diagnostic apparatus according to claim 2,
The pass band of the low-pass filter is controlled according to the maximum measurable frequency of the Doppler signal.
An ultrasonic diagnostic apparatus. The ultrasonic diagnostic apparatus according to any one of claims 1 to 3,
The numerical pattern is a sine pattern obtained from a sine function and a cosine pattern obtained from a cosine function,
The transmission signal processing unit outputs a continuous wave transmission signal whose phase is periodically changed according to a phase pattern obtained by synthesizing a sine pattern and a cosine pattern.
An ultrasonic diagnostic apparatus. The ultrasonic diagnostic apparatus according to claim 4,
The sine pattern is composed of N sine function values corresponding to N (N is an even number) phase values,
The cosine pattern is composed of N cosine function values corresponding to the N phase values.
The continuous wave transmission signal includes a phase pattern having a pattern length N corresponding to the N phase values.
An ultrasonic diagnostic apparatus. The ultrasonic diagnostic apparatus according to claim 5,
When the filtered demodulated signal is added in the period direction, the pattern length N is divided into q blocks (q is a natural number) every p (p is a natural number), and the demodulated signal is added to each block. Processing to obtain a partial additive demodulated signal,
An ultrasonic diagnostic apparatus. The ultrasonic diagnostic apparatus according to claim 6,
A pattern length N is formed step by step while shifting the range of q blocks one block at a time in a signal sequence in which a plurality of partial addition demodulated signals obtained sequentially for each block are arranged in the order in which they are obtained. By extracting q partial addition demodulated signals over q blocks and adding them, the addition of the demodulated signals over the pattern length N is realized.
An ultrasonic diagnostic apparatus.

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