JPH11113911A - Doppler ultrasonograph - Google Patents

Abstract

PROBLEM TO BE SOLVED: To limit the turning-over of a noise component by a sampling, and improve the S/N ratio of a diagnosing image. SOLUTION: This apparatus comprises an ultrasonic probe 1 which transmits/ receives ultrasonic waves, an orthogonal detecting circuit 4 which orthogonally detects the receiving signal of the ultrasonic probe 1, a low-pass filter 24 which eliminates a high frequency component from the output of the orthogonal detecting circuit 4, an A/D converter 7 which samples and digitizes the output, and a frequency analyzer 8 which performs a frequency analysis of the output or the like, and is a doppler ultrasonograph having a zero shift function which makes the frequency region of a blood stream spectrum variable, and when it is taken that the sampling frequency of the A/D converter 7 is fs, and the zero shift amount is Zr, the cut-off frequency of the low-pass filter 24 is controlled by a CPU 13 in such a manner that the cut-off frequency may become fs/2+|Zr|.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diagnostic apparatus to which the ultrasonic Doppler method is applied, and more particularly to a continuous wave Doppler diagnostic apparatus for displaying a spectrum of blood flow information obtained by transmitting and receiving continuous wave (CW) ultrasonic waves.

[0002]

2. Description of the Related Art This type of diagnostic apparatus is used for noninvasively visualizing blood flow information of various parts in a living body, and is used for diagnosis in various clinical fields such as circulatory organs. The ultrasonic Doppler diagnostic method uses the ultrasonic Doppler effect that, when an ultrasonic wave is reflected by a moving object such as a blood cell, the frequency of the reflected wave shifts in proportion to the moving speed. In particular, a continuous wave (CW) Doppler diagnostic device
A continuous wave ultrasonic wave is transmitted into a living body, the reflected wave is received, and phase detection is performed to obtain a frequency shift due to the Doppler effect, and this is imaged as blood flow information. The flow information includes the speed of the blood flow, the direction of the flow, the degree of turbulence, and the like. Furthermore, in addition to video information, a Doppler signal heard as an audible sound is also useful diagnostic information. Therefore, this type of apparatus generally has a function of outputting a Doppler signal as an audio signal from a speaker or the like.

Here, a conventional general continuous wave ultrasonic Doppler device will be described. FIG. 4 is a configuration diagram showing a conventional example of a continuous wave ultrasonic Doppler device. Reference numeral 1 denotes an ultrasonic probe applied to a living body, which is composed of a transmitting and receiving transducer. Reference numeral 12 denotes an oscillator for supplying a master clock. The output of the oscillator 12 is frequency-divided by a frequency divider 11 to generate a clock having an ultrasonic oscillation frequency. By exciting the trust transducer, the ultrasound is transmitted into the body as a continuous wave. The reflected wave due to the blood flow or the like is converted into an electric signal by the receiving transducer of the ultrasonic probe 1 and is amplified by the receiving amplifier 3. The quadrature detection circuit 4 includes a frequency divider 11
(Sin) synchronized with the transmission frequency supplied from
Signal) and a clock (C
os signal), and by mixing (multiplying) each with the output signal of the receiving amplifier, a Doppler signal is extracted and separated into a synchronous component (I signal) and a quadrature component (Q signal) with respect to the oscillation frequency.

The high-pass filter 5 is a two-system filter (for I and Q signals) having high-pass characteristics, and removes low-frequency components from the output of the quadrature detection circuit 4 due to wall motion of an organ or the like. Further, the low-pass filter 6 is a two-system filter (for I signal and for Q signal) having low-pass characteristics, and removes unnecessary high-frequency components from the output of the high-pass filter 5. The A / D converter 7 includes two types of converters, samples the output signal of the low-pass filter 6, and converts it into a digital signal. Here, a clock obtained by dividing the master clock of the oscillator 12 by the divider 14 is used as the sampling clock fs. Reference numeral 8 denotes a frequency analyzer that performs a Fourier transform at a fixed period of several ms to several tens ms to calculate a power spectrum of the Doppler signal. The power spectrum is stored in the display memory 9 as display data.
Further, the display device 10 reads data in the display memory 9 at each refresh cycle of the screen, and displays the data in a so-called sonogram format (a horizontal axis represents time, a vertical axis represents frequency, and a luminance represents power) on a monitor screen. ), The power spectrum is displayed as a flow velocity pattern.

[0005] Reference numeral 13 denotes a control CPU. When the sampling frequency and the filter characteristics are changed via an operation panel (not shown), the cutoff frequencies of the high-pass filter 5 and the low-pass filter 6 and the A / D The frequency division ratio of the frequency divider 14 that supplies the sampling clock of the D converter 7 is changed. Also, a low-pass filter 6
Is input to the Doppler sound processing circuit 15. The Doppler sound processing circuit 15 is composed of two systems of 90 ° phase shifters and adders. The Doppler sound processing circuit 15 processes the in-phase component (I signal) and the quadrature component (Q signal) of the Doppler signal to provide the ultrasonic probe 1 with the Doppler sound. The incoming flow velocity component (positive frequency component) and the moving velocity component (negative frequency component) are separated. Further, the output is amplified by an audio amplifier 16 including two systems of amplifiers and output from speakers 17 and 18 as Doppler sound.

According to the sampling theorem, since a signal component equal to or more than の of the sampling frequency becomes aliasing noise, the cutoff frequency of the low-pass filter 6 which is usually provided before the A / D converter is determined by the sampling frequency. Of 1
This is cut by setting it to / 2 or less. However, some devices of this type have a function of expanding the observable frequency range by using aliasing by sampling. This is generally called a zero shift function because it is used together with the function of moving the zero axis of frequency. Hereinafter, this will be described. In an apparatus having a zero shift function, the cutoff frequency of the low-pass filter 6 is usually set to the same value as the sampling frequency fs. At this time, the characteristics of the low-pass filter 6 are as shown in FIG.
As shown by the symbol a in (a), the pass band is from -fs to fs.
As a result, the observable frequency range is expanded to -fs to fs when the return is included. Reference symbol b in FIG. 5B indicates the spectrum of the blood flow component after passing through the low-pass filter 6. Further, in FIG. 5 (c), symbols d and d ′ indicate the spectrum of the blood flow component after A / D conversion. Here, d ′ is a folded component.

FIG. 6A shows the spectrum of FIG. 5C and the spectrum before and after the spectrum shown in FIG.
0 is displayed as a flow velocity pattern on a monitor screen. In the figure, the symbol a indicates a display image, and b indicates a zero value of the frequency. The upper and lower ends of the image correspond to the frequencies fs / 2 and -fs / 2, respectively. Symbol c is an image of the spectrum of FIG. Thus, the flow velocity pattern of the spectrum including the return is fs
The portion exceeding / 2 is folded in the lower half of the screen as shown in FIG. Therefore, in such a case, the position of the frequency zero is shifted downward by Zr as shown in FIG. The shift range is a maximum of ± fs / 2, so that the flow velocity from the frequency −fs to fs can be observed without turning back.

[0008]

As described above, in the apparatus having the zero shift function, the low pass filter 6 is used.
Is set to the same value as the sampling frequency fs of the A / D converter 7. As a result, the observable flow velocity range expands, but on the other hand, a part of the broadband noise included in the received signal returns by sampling,
The problem of lowering the S / N ratio occurs. FIG.
(B) shows the spectrum of the noise component included in the output signal of the low-pass filter 6 by the code c. The noise components are distributed in the frequency range -fs to fs. Symbols e and e 'in FIG. 5C are the spectrums of the noise components after the A / D conversion, and e' is the frequency range fs / 2 to fs and -fs to -fs shown in FIG. / 2 is a folded noise component. As described above, in the device having the zero shift function, the noise components in the frequency ranges fs / 2 to fs and −fs to −fs / 2 are not cut by the low-pass filter, and the sampling is performed by −fs to −fs /.
Since it is returned to the range of 2 and added to the blood flow component, there is a problem that the S / N ratio of the diagnostic image is deteriorated as a result.
Therefore, an object of the present invention is to reduce the influence of the aliasing component of noise and suppress the deterioration of the S / N ratio.

[0009]

According to the first aspect of the present invention, there is provided an ultrasonic probe for transmitting and receiving a continuous ultrasonic wave, and a receiving signal of the ultrasonic probe. , A first low-pass filter that removes high-frequency components from the output of the quadrature detection circuit, and an A / D converter that samples and digitizes the output of the first low-pass filter. And a frequency analyzer for frequency-analyzing a signal digitized by the A / D converter to obtain a blood flow spectrum, and having a zero-shift function for varying the output frequency range of the blood flow spectrum. In the Doppler diagnostic apparatus, the sampling frequency of the A / D converter is set to f
s, the amount of zero shift is Zr (−fs / 2 ≦ Zr ≦ fs /
In the case of 2), a control means for controlling the cutoff frequency of the first low-pass filter to be equal to fs / 2 + | Zr | is provided. According to the first aspect of the present invention, a second low-pass filter is provided in parallel with the first low-pass filter on an output side of the quadrature detection circuit,
Doppler sound output means for outputting the Doppler signal as an audio signal is added, and the A / D converter converts the Doppler signal input to the Doppler sound output means by the second low-pass filter regardless of the zero shift amount. The pass band can be limited by the cut-off frequency equal to the sampling frequency of the present invention (the invention of claim 2).

[0010] By doing so, the frequency range of the aliasing due to the sampling of the noise component is-(fs
/ 2 + | Zr |) -fs / 2 and fs / 2-fs /
2+ | Zr |. That is, when the zero shift amount is small, the aliasing component of the noise can be almost ignored, and as a result, the S / N ratio of the signal is improved and a good diagnostic image can be obtained. Further, the second low-pass filter provided separately in front of the Doppler sound output means has a cutoff equal to the sampling frequency fs of the A / D converter regardless of the amount of zero shift, so that the frequency zero axis shifts. Thus, the problem that the voice is changed can be avoided.

[0011]

FIG. 1 is a block diagram showing an embodiment of the present invention. The master clock supplied from the oscillator 12 is frequency-divided by the frequency divider 11 to generate a clock having an ultrasonic oscillation frequency. This is, after being amplified by the transmission amplifier 2, by exciting the transmission transducer of the ultrasonic probe 1, the ultrasonic wave is transmitted into the living body as a continuous wave. The reflected wave due to the blood flow is converted into an electric signal by the receiving transducer of the ultrasonic probe 1 and amplified by the receiving amplifier 3. The quadrature detection circuit 4 generates a clock (Cos signal) having a phase shifted by 90 ° from the clock (Sin signal) synchronized with the transmission frequency supplied from the frequency divider 11 and outputs each clock to the output of the receiving amplifier. By mixing (multiplying) with the signal, the Doppler signal is extracted and separated into an in-phase component (I signal) and a quadrature component (Q signal) with respect to the oscillation frequency.

The high-pass filter 5 removes low-frequency components from the output signal of the quadrature detection circuit 4 due to wall motion of an organ or the like. The first low-pass filter 24 is a high-pass filter 5
Unnecessary high frequency components are removed from the output. The A / D converter 7 has two types of converters, samples the output signal of the first low-pass filter 24, and converts it into a digital signal. Here, a clock obtained by dividing the master clock of the oscillator 12 by the divider 14 is used as the sampling clock fs. Reference numeral 8 denotes a frequency analyzer, which is several ms to
Fourier transform is performed at a fixed period of several tens of ms, and the power spectrum of the Doppler signal is calculated. The power spectrum is stored in the display memory 9 as display data, and the display device 10 reads the data in the display memory 9 at each refresh cycle of the screen, and displays the so-called sonagram format (time on the horizontal axis and frequency on the vertical axis) on the monitor screen. , And a power spectrum is displayed as a flow velocity pattern in accordance with a display format in which luminance corresponds to power.

The second low-pass filter 25 is composed of two low-pass filters, and removes unnecessary high-frequency components from the output of the high-pass filter 5. The output is input to the Doppler sound processing circuit 15, and the in-phase component (I
The signal and the quadrature component (Q signal) are separated into a flow velocity component (positive frequency component) approaching the ultrasonic probe 1 and a flow velocity component (negative frequency component) moving away. Further, the output is amplified by the audio amplifier 16 and output from the speakers 17 and 18 as Doppler sound. Reference numeral 13 denotes a control CPU, which changes the cutoff frequency of the first and second low-pass filters 6 and changes the sampling clock to the A / D converter 7 when the sampling frequency is changed via an operation panel (not shown). Is changed. When the amount of zero shift is changed, the cutoff frequency of the first low-pass filter 24 is changed.

FIG. 2 shows a specific example of the first low-pass filter 24. Reference numerals 20 and 21 denote switched capacitor filters, each constituting a filter having a low-pass characteristic having a cutoff frequency of 1/50 of the frequency of the input clock f1, and outputting two systems from the high-pass filter 5 shown in FIG. Signals (I signal and Q signal) are input, and the high frequency components are cut. The output is provided to the A / D converter 7 in FIG. Reference numeral 23 denotes a programmable frequency divider which receives a frequency division ratio from the CPU 13 of FIG.
The base clock of frequency f0 input from 2 is divided by N, and the clock of frequency f1 is supplied to the switched capacitor filters 20 and 21, respectively.

The operation of FIG. 1 will be described with reference to FIGS. The CPU 13 calculates the frequency division ratio N given to the programmable frequency divider 23 based on the sampling frequency fs and the zero shift amount Zr set via an operation panel (not shown) as follows. FIG. 3A shows the zero shift amount Zr, the display frequency range a of the spectrum, and the characteristic b of the low-pass filter 24 at that time. That is, the cutoff frequency fc of the low-pass filter 24
Is given by the following equation (1). fc = fs / 2 + | Zr | (1) In order to set the cutoff frequency given by the above equation (1) for the switched capacitor filters 20 and 21, the cutoff frequency may be set to 50 times (100 times). ), The input clock f
1 is: f1 = 50 * (fs / 2 + | Zr |) (2) Therefore, the dividing ratio N at that time is as follows: N = f0 / (50 * (fs / 2 + | Zr |)) (3)

FIG. 3B shows a case where the present invention is implemented.
This is a spectrum of an output signal of the low-pass filter 24, and symbols c and d indicate a blood flow component and a noise component, respectively. Here, the noise component is determined by the low-pass filter 24 from-(fs / 2 + | Zr |) to (fs / 2 +
| Zr |). FIG. 3C shows A /
The spectrum after D conversion is shown, e and e 'show blood flow components, f, f', and f "show noise components. Of these, f 'and f" show the aliasing of the noise components. 3 (b), frequency range of noise component d
(Fs / 2 + | Zr |) to -fs / 2, and fs / 2
To (fs / 2 + | Zr |) correspond to each other, and |
Zr |. That is, the range of the return of the noise component is equal to the absolute value of the zero shift amount when the sampling frequency is fixed, and can be ignored when the zero shift amount is small.

The operation related to the Doppler sound is as follows. When the sampling frequency fs is changed via an operation panel (not shown), the CPU 13 of FIG. 1 sets the cutoff frequencies of the two filters of the second low-pass filter 25 to the respective sampling frequencies fs. As a result, the second low-pass filter 25
5 is set as shown in FIG. 5A, and the Doppler signal in the frequency range from -fs to fs is pass-band-limited and given to the Doppler sound processing circuit 15. Therefore, the frequency band of the Doppler sound is not affected by the zero shift amount.

[0018]

According to the present invention, according to the present invention,
In a continuous wave ultrasonic Doppler diagnostic apparatus having a zero shift function, when the amount of zero shift is small, the deterioration of the S / N ratio of the signal due to the return of the noise component can be suppressed lower than before, and as a result, A Doppler image with high diagnostic performance can be provided. Further, by moving the frequency zero axis, it is possible to reduce the risk of erroneous diagnosis at the time of diagnosis due to Doppler sound, and the like.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram showing a specific example of a first low-pass filter of FIG.

FIG. 3 is an explanatory diagram of a waveform example of each unit in FIG. 1;

FIG. 4 is a configuration diagram showing a conventional example.

FIG. 5 is an explanatory diagram of low-pass filter characteristics and waveforms of a blood flow component and a noise component after passing through the low-pass filter and after A / D conversion.

FIG. 6 is an explanatory diagram of a zero shift function.

[Explanation of symbols]

1 ... ultrasonic probe, 2 ... transmitting amplifier, 3 ... receiving amplifier,
4: Quadrature detection circuit, 5: High-pass filter, 6, 24,
25: low-pass filter, 7: A / D converter, 8: frequency analyzer, 9: display memory, 10: display device,
11, 14 ... frequency divider, 12, 22 ... oscillator, 13 ... CP
U, 15: Doppler sound processing circuit, 16: Audio amplifier, 17, 18: Speaker, 20, 21: Switched capacitor filter, 23: Programmable frequency divider.

Claims (2)

[Claims] 1. An ultrasonic probe for transmitting and receiving an ultrasonic wave as a continuous wave, a quadrature detection circuit for quadrature detecting a reception signal of the ultrasonic probe and extracting a Doppler signal, and a quadrature detection circuit for the quadrature detection circuit. A first low-pass filter for removing high-frequency components from the output, an A / D converter for sampling and digitizing the output of the first low-pass filter;
An ultrasonic Doppler diagnostic apparatus comprising: a frequency analyzer that frequency-analyzes a signal digitized by the / D converter to obtain a blood flow spectrum; and a zero-shift function that varies an output frequency region of the blood flow spectrum. When the sampling frequency of the A / D converter is fs and the amount of zero shift is Zr (−fs / 2 ≦ Zr ≦ fs / 2), the cutoff frequency of the first low-pass filter is f
An ultrasonic Doppler diagnostic apparatus, comprising: a control unit for performing control so as to be equal to s / 2 + | Zr |. 2. An output terminal of the quadrature detection circuit,
A second low-pass filter is added in parallel with the low-pass filter, and a Doppler sound output unit that outputs the Doppler signal as an audio signal is added. The Doppler signal input to the Doppler sound output unit is output by the second low-pass filter. The ultrasonic Doppler diagnostic apparatus according to claim 1, wherein a pass band is limited by a cutoff frequency equal to a sampling frequency of the A / D converter regardless of the zero shift amount.