JPH0431696B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an ultrasonic Doppler apparatus, and particularly to an ultrasonic Doppler apparatus that displays on a screen the speed and other conditions of a moving reflector such as cardiac blood flow.

[Prior Art] Ultrasonic Doppler equipment is well known, which emits ultrasonic pulse waves at a constant repetition frequency into a living body, etc., and displays an image of the motion state of a moving reflector. The velocity distribution state of the reflector is determined by receiving the reflected echo and detecting the Doppler effect received by the reflected echo as a frequency shift of the ultrasonic carrier frequency.

Although such ultrasonic pulse wave devices sometimes use continuous wave ultrasonic waves to obtain velocity information, pulse wave devices are superior in terms of range resolution, good beam focusing, and ease of use. Ultrasound is often used.

[Problems to be Solved by the Invention] However, since the ultrasonic pulse wave repeats ultrasonic waves at a constant frequency, a folding phenomenon occurs, and there is a problem that the speed of a moving reflector cannot be detected satisfactorily.

In other words, the repetition frequency of the ultrasonic pulse wave is selected according to the distance to the moving reflector, but the Doppler shift frequency for determining the velocity is within the frequency range of ±1/2 of this repetition frequency (Nyquist frequency ) (sampling theorem).

Figure 5 shows the Doppler shift frequency (Doppler signal)
As shown in the figure, the signal level increases and the velocity component that goes outside the upper limit of the Nyquist frequency is folded back and appears at the lower limit of the Nyquist frequency. Therefore, the Doppler signal becomes discontinuous, making it difficult to accurately determine velocity.

OBJECTS OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and its purpose is to provide an ultrasonic Doppler apparatus that can eliminate the aliasing phenomenon and can satisfactorily determine speed.

[Means for Solving the Problem] In order to achieve the above object, the present invention emits an ultrasonic pulse wave at a constant repetition frequency into a subject, demodulates the reflected echo signal from a moving reflector, and modulates the frequency. An ultrasonic Doppler device that detects a Doppler shift frequency using an analyzer includes a sampler that captures a received signal at a predetermined sampling frequency corresponding to the Nyquist frequency, and a low-pass filter that has a cutoff frequency set to twice the sampling frequency. , a frequency shift detector that performs frequency analysis on the output of the sampler and then detects the maximum frequency as a Doppler shift frequency; and a frequency shift detector that compares the output of this frequency shift detector with a reference value that approximates the Nyquist frequency and calculates this reference value. a comparison circuit that outputs a detection signal when the output of the frequency shift detector exceeds the reference value based on the output of the comparison circuit; The present invention is characterized by comprising an image display position shift circuit that shifts the display position of frequency information on the screen to a predetermined position during the overflow time.

[Operation] According to the above configuration, since the sampling frequency is 1/2 of the cutoff frequency of the low-pass filter, the sampled signal contains Doppler shift frequency components (high velocity components) remain. That is, in the past, in order to remove as much of the aliased signal as shown in FIG. 5, high frequency (Doppler shift frequency) components were removed in advance using a filter. In the present invention, on the contrary, the cutoff frequency is expanded to include frequency components outside the Nyquist frequency (outside the range).

In this state, it is detected on the time axis whether or not there is a Doppler shift frequency signal other than the Nyquist frequency, and the Doppler shift frequency signal during this time is shifted by a predetermined frequency. Therefore, on the display, a frequency shift signal (information) outside the Nyquist frequency is displayed in a state connected to a frequency shift signal within the Nyquist frequency.

[Embodiments] Hereinafter, preferred embodiments of the present invention will be described based on the drawings.

FIG. 1 shows a circuit schematically showing an ultrasonic Doppler device. A transceiver 12 is connected to a transducer 10 which is an electro-acoustic transducer, and based on the control of this transceiver 12, Sound waves can be emitted into living organisms. A demodulator 14 is connected to this transceiver 12, and the echo signal received by the transceiver 12 is demodulated in a form that allows easy extraction of the Doppler signal.

In addition, the ultrasonic Doppler apparatus is provided with a display unit 30 made of a CRT or the like, and in the present invention, the Doppler signal is frequency-analyzed and the frequency information is displayed as a spectrum, or changes in frequency deviation are displayed on the time axis. Displaying M mode, etc.

A characteristic feature of the present invention is that the Doppler shift frequency information of the Nyquist frequency, which was conventionally discarded, is extracted and connected on the screen.

That is, the frequency analysis section 100 is first provided with a low-pass filter 16, a sampler 18, and a frequency analyzer 20.
The received signal is captured at a predetermined sampling frequency corresponding to the Nyquist frequency, and in the embodiment, the sampling frequency is set to a frequency fo (several kHz) that is almost the same as the Nyquist frequency. On the other hand, the cutoff frequency of the low-pass filter 16 is set to twice the sampling frequency.

In FIG. 2, the signal spectrum obtained by the low-pass filter 16 and sampler 18 is shown as a frequency-analyzed function P(f), and the output of the low-pass filter 16 is as shown in FIG. If the signal is
Since fo is 1/2 of the cutoff frequency fc,
As shown in Fig. b, the spectrum in Fig. a is an overlapping spectrum for each sampling frequency fo. Note that the analysis range of the frequency analyzer 20 is 0 to fo.
is within the range of

The output of the sampler 18 is supplied to an image display position shift circuit 28 via a frequency analyzer 20 having a configuration similar to the conventional one, and is also supplied to a circuit for extracting Doppler shift frequency signals other than the Nyquist frequency. be done. This extraction circuit includes a frequency shift detector 22 and a comparator 2 as a comparison circuit.
4 and a flip-flop 26.

Since the frequency-analyzed values of the frequency shift detector 22 vary, in the embodiment, the maximum frequency of the Doppler signal is detected as the Doppler shift frequency.
Further, as shown in FIG. 3, the comparator 24 compares the output of the frequency shift detector 22 with a reference value fo' (voltage value) that is almost the same as the Nyquist frequency.
If the frequency deviation is larger than the reference value fo′, then
It outputs the signal shown in Figure b. The flip-flop 26 receives the output of the comparator 24,
As shown in FIG. 3c, a predetermined signal is output for an overflow time during which the Doppler shift frequency signal exceeds the reference value.

Next, the image display position shift circuit 28 shifts the display position of the frequency information in the image to a predetermined position during the overflow time shown in FIG. Display Doppler shift frequency signal 3
0 by the Nyquist frequency and connect it to the Doppler shift frequency signal found within the Nyquist frequency.

The embodiment has the above configuration, and its operation will be explained below.

The echo signal obtained by the transducer 10 in FIG. signal components are removed.

Since the sampler 18 samples this filter output at a frequency that is substantially the same as the Nyquist frequency fo, the output of the sampler 18 becomes a signal containing high-velocity Doppler frequency components.

The output of this sampler 18 is transmitted to a frequency analyzer 20.
The frequency is analyzed in the range of 0 to fo and then supplied to the image display position shift circuit 28 as well as to the frequency shift detector 22, where the Doppler shift frequency signal ( The maximum value) is determined by the voltage value. This Doppler shift frequency signal is then compared with a reference value corresponding to the Nyquist frequency by the comparator 24, and as described above, an overflow signal is output from the flip-flop 26 indicating that the Doppler shift frequency signal is outside the Nyquist frequency. A time signal is output.

This overflow time signal is supplied to the image display position shift circuit 28, which shifts the display position of the frequency shift information existing within that time by the Nyquist frequency and displays it on the screen. become.

Shifting to this display position will be explained based on FIG. 4. That is, in the conventional device, the echo signal is sampled at a sampling frequency that is almost the same as the cutoff frequency fc of the low-pass filter 16, and in this case, as shown in Figure a, the range of the Nyquist frequency fo is displayed. range and frequency
Doppler shift frequency information between fo and fc is not displayed. In contrast, in the present invention, as shown in FIG. b, the frequency component 200 obtained by reducing the sampling frequency is shifted to the position 201 in the diagram and displayed. This frequency component 200
is a high velocity component, which makes it possible to increase the detection speed of the motion reflector to twice that of the conventional method.

[Effects of the Invention] As explained above, according to the present invention, the sampling frequency is set to be almost the same as the Nyquist frequency, the cutoff frequency of the low-pass filter is set to twice the sampling frequency, and
By detecting the Doppler shift frequency that exceeds the Nyquist frequency and shifting the display position of this part of the image to the original position, the Doppler shift frequency signal outside the Nyquist frequency, which was previously discarded, is replaced with a Doppler shift frequency signal within the Nyquist frequency. It can be connected to a Doppler shift frequency signal, making it possible to display speeds twice as fast as conventional speeds.

Therefore, it is possible to accurately display an image of a moving reflector up to high speeds while eliminating the aliasing phenomenon that could not be eliminated conventionally.

[Brief explanation of drawings]

FIG. 1 is a circuit block diagram schematically showing the ultrasonic Doppler apparatus according to the present invention, FIG. 2 is a waveform diagram illustrating the output signals of the low-pass filter and sampler, and FIG.
4 is a waveform diagram for explaining the operation of the image display position shift circuit, and FIG. 5 is a waveform diagram for explaining the folding phenomenon in the conventional device. 12... Transmitter/receiver, 14... Demodulator, 16...
Low pass filter, 18... Sampler, 20...
Frequency analyzer, 22... Frequency shift detector, 24
...Comparator, 26...Flip-flop, 28...
...Image display position shift circuit, 30...Display section.

Claims (1)

[Claims] 1 Ultrasonic Doppler equipment emits ultrasonic pulse waves at a constant repetition frequency into the subject, demodulates the reflected echo signal from a moving reflector, and detects the Doppler shift frequency using a frequency analyzer. Compatible with the Nyquist frequency. a sampler that captures a received signal at a predetermined sampling frequency, a low-pass filter whose cutoff frequency is set to twice the sampling frequency, and a maximum frequency detected as a Doppler shift frequency after frequency analysis is performed from the output of the sampler. a frequency shift detector that compares the output of this frequency shift detector with a reference value approximated to the Nyquist frequency, and a comparison circuit that outputs a detection signal when the reference value is exceeded; a flip-flop that outputs a detection signal during an overflow period in which the output of the frequency shift detector exceeds the reference value; and an image display that shifts the display position of frequency information on the screen to a predetermined position during the overflow period based on the flip-flop output. An ultrasound Doppler device comprising a position shift circuit.