JPH0213442A - Ultrasonic wave doppler device - Google Patents

Abstract

PURPOSE:To enable the display having the speed of two times former speed by setting the cut-off frequency of a low-pass filter to two times sampling frequency, and also shifting the display position of the picture image of a detected part to an originally due position by detecting the doppler deviation frequency exceeding a Nyquist frequency. CONSTITUTION:A received signal is incorporated with a sampler 18 on a given sampling frequency corresponding to a Nyquist frequency. A cut-off frequency is set to two times sampling frequency with a low-pass filter 16. The maximum frequency is detected as a doppler deviation frequency with a frequency deviation detector 22 after frequency analysis 20 is made from the output of the sampler 18. The output of the detector 22 is compared to the reference value approximate to the Nyquist frequency with a comparison circuit 24, and when exceeds the reference value, a given signal is outputted by the overflow time. The display position of the frequency information on a picture surface 30 is shifted to a given position with a shift circuit 28 in the overflow time based on the output of the circuit 24.

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. by detecting the Doppler effect on the reflected echo as a frequency shift of the ultrasound carrier frequency.
The velocity distribution state of the reflector is being determined.

Although such ultrasound Doppler devices sometimes use continuous-wave ultrasound to obtain velocity information, pulsed-wave ultrasound is superior in terms of range resolution, good beam focusing, and ease of use. Sound waves are often used.

[Problem 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 well.

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 ±1/2 of this repetition frequency.
is limited to the frequency range (Nyquist frequency) (sampling theorem).

Figure 5 shows the change in the Doppler shift frequency (Doppler signal), and as shown in the figure, the signal level increases and the velocity component that goes outside the Nyquist frequency is folded back. 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 an object thereof 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 and demodulates the reflected echo signal from a moving reflector. In an ultrasonic Doppler device that detects the Doppler shift frequency using a frequency analyzer, there is a sampler that captures a received signal at a predetermined sampling frequency corresponding to the Nyquist frequency, and a low-pass device whose cutoff frequency is set to twice the sampling frequency. A filter, 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 the frequency shift detector with a reference value that approximates the Nyquist frequency. A comparison circuit that outputs a predetermined signal for the overflow time when the reference value is exceeded, and an image display position that changes the display position of the frequency information on the screen to a predetermined position at the overflow time based on the output of the comparison circuit. It is characterized by comprising a shift circuit.

According to the above configuration, since the sampling frequency is 1/2 of the cutoff frequency of the low-pass filter,
After sampling, the signal retains the Doppler shift frequency component (high velocity component), which was conventionally cut. That is, in the past, high frequency (Doppler shift frequency) components were removed in advance using a filter in order to remove as much of the aliased signal as shown in FIG. 5, but the present invention So,
Conversely, the cutoff frequency is widened 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, the signal is displayed on the display in a state where it is connected to a frequency shift signal (information) outside the Nyquist frequency or a frequency shift signal within the Nyquist frequency.

Embodiments] Preferred embodiments of the present invention will be described below with reference to the drawings.

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

Further, the ultrasonic Doppler apparatus is provided with a display unit 30 such as a CRT, and in the present invention, the Doppler signal is frequency-analyzed and the frequency shift information is displayed as a spectrum, or changes in frequency shift on the time axis. The M mode display is displayed.

A distinctive feature of the present invention is that Doppler shift frequency information outside the Nyquist frequency, which was previously discarded, is extracted and displayed on the screen.
The idea is to connect them with two.

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 corresponding predetermined sampling 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 low-pass filter 16 sets its cutoff frequency to twice the sampling frequency.

FIG. 2 shows the low-pass filter 16 and the sampler 1.
Function P that frequency-analyzes the signal spectrum obtained in 8.
(f), and assuming that the output of the low-pass filter 16 is a signal as shown in Figure (a), the sampling frequency fO of the sampler 18 is 1/2 of the cutoff frequency fc. Therefore, as shown in figure (b), the spectrum in figure (a) is at sampling frequency f
.

The result is a spectrum that overlaps each other. Note that the analysis range of the frequency analyzer 20 is from 0 to fo.

The output of the sampler 18 is sent to an image display position shift circuit 28 via a frequency analyzer 20 having a configuration similar to the conventional one.
and to a circuit for extracting Doppler shift frequency signals outside the Nyquist frequency. This extraction circuit is composed of a frequency shift detector 22, a comparator 24 as a comparison circuit, 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, and the frequency shift is When the value is larger than the value fO', the signal shown in FIG. 2(b) is output. The flip-flop 26 receives the output of the comparator 24, and outputs a predetermined signal for the overflow time during which the Doppler shift frequency signal exceeds the reference value, as shown in FIG. 3(C). .

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. 3(c) based on the output of the comparison I-circuit. The Doppler shift frequency signal outside the Nyquist frequency is shifted by the Nyquist frequency on the display 30'2, and connected 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. Extra 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 measured in the frequency analyzer 20 from 0 to
The frequency is analyzed in the range of fo, and then supplied to the image display position shift circuit 28 and also to the frequency shift detector 22, where the Doppler shift frequency signal (its maximum value) is It can be 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, the signal from the flip-flop 26 indicates that the Doppler shift frequency signal is outside the Nyquist frequency. An overflow 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. It turns out.

This shift of the 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.
), the display range is the Nyquist frequency fo, and Doppler shift frequency information between frequencies fo and fc is not displayed. In contrast, in the present invention, as shown in Figure (b), a frequency component 200 obtained by reducing the sampling frequency is shifted to a 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 approximately the same as the Nyquist frequency, and the cutoff frequency of the low-pass filter is set to 2 times the sampling frequency.
In addition to detecting the Doppler shift frequency that exceeds the Nyquist frequency, the display position of this part of the image is shifted to the original position, so Doppler shift frequencies outside the Nyquist frequency that were previously discarded The signal can be coupled to a Doppler shift frequency signal within the Nyquist frequency, 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 Description of the Drawings] Fig. 1 is a circuit block diagram showing the outline of the ultrasonic Doppler device according to the present invention, Fig. 2 is a waveform diagram illustrating the output signals of the low-pass filter and sampler, and Fig. 3 is FIG. 4 is a waveform diagram for explaining the operation of the image display position shift circuit; 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) In an ultrasonic Doppler device that 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, the Nyquist frequency a sampler that captures a received signal at a predetermined sampling frequency corresponding to the sampling frequency, a low-pass filter whose cutoff frequency is set to twice the sampling frequency, and a Doppler shift of the maximum frequency after performing frequency analysis on the output of the sampler. A frequency shift detector that detects the frequency as a frequency, and a comparison circuit that compares the output of the frequency shift detector with a reference value approximated to the Nyquist frequency and outputs a predetermined signal for the overflow time if the reference value is exceeded. and 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 based on the output of the comparison circuit.