JP2000139909A - Ultrasonograph - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively reduce flicker of an image caused following the removal of fixed noise, in an ultrasonograph. SOLUTION: This ultrasonograph is constituted so that a cross section of an object is scanned with ultrasonic wave to provide a received signal, the received signal is filtered by a fixed noise removing filter 4 to remove fixed noise with relatively small time variation, the received signal passing through the fixed noise removing filter 4 is detected by a detector 5, the detected signal is filtered and relatively large components of the time variation as a cause of flicker is damped by a smoothing filter 6, and the detected signal damped by the smoothing filter 6 is logarithmically compressed by a logarithmic compressor 7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for scanning a cross section of an object with an ultrasonic wave, detecting a reception signal obtained thereby,
The present invention relates to an ultrasonic diagnostic apparatus that generates an ultrasonic image by performing logarithmic compression on the detection signal.

[0002]

2. Description of the Related Art FIG. 9 shows a configuration of a conventional ultrasonic diagnostic apparatus. A high-frequency voltage pulse is applied to the ultrasonic probe 31 from the transmission system 32. Thereby, an ultrasonic wave is transmitted from the ultrasonic probe 31 to the subject. This ultrasonic wave is reflected at the boundary of the acoustic impedance in the subject, returns to the ultrasonic probe 31, and is converted into an electric signal.
This electric signal is amplified in the receiving system 33, converted into a digital signal, and subjected to phasing addition.

The received signal fi obtained by the phasing addition is
Contains noise with little variation in time, so-called fixed noise. The fixed noise is removed by a high-pass fixed noise removal filter 34.

The received signal fi from which the fixed noise has been removed by the fixed noise removal filter 34 stores phase information and amplitude information, and only the amplitude information is extracted by the detector 35. . The dynamic range of the detection signal Ai is very wide, and the dynamic range of the display unit 37 is insufficient to express the detection signal Ai as it is. Therefore, it is general that the logarithmic compressor 36 converts the detection signal Ai into an appropriate dynamic range. It has been done.
The video signal Vi after logarithmic compression is output to the display unit 37.
And is displayed as a so-called B-mode image representing the tissue tomographic structure.

By the way, the fixed noise removing filter 3
4, it is necessary to finely control the cutoff frequency according to the heartbeat time phase so that, for example, the stationary myocardium does not disappear together with the fixed noise.
For example, in a heartbeat phase in which the motion of the heart wall is stationary, such as in late diastole, the cutoff frequency is lowered to suppress the attenuation of myocardial echo, while a heartbeat in which the motion of the heart wall is intense, such as in the early stage of contraction. In the time phase, increase the cutoff frequency,
Attenuate fixed noise sufficiently.

However, the movement of the heart is very complicated,
In other words, the whole myocardium does not move regularly at the same speed, but includes a part that moves violently and a part that hardly moves. Therefore, even if the cutoff frequency is controlled according to the heartbeat time phase, it is almost impossible to reliably remove the fixed noise and reliably leave the myocardial echo in all the heartbeat time phases, and the fixed noise is partially eliminated. It was inevitable that they would remain or myocardial echoes would partially disappear.

Therefore, in the display of the myocardial component as an example, the echo on the image display is attenuated by the fixed noise removal filter 34 in the portion that hardly moves, so that the brightness on the image display (corresponding to the power of the echo signal) However, since the echo is not attenuated in the moving part, such a decrease in luminance is not observed.

As a result, the myocardium is not displayed with the same level of brightness, but the brightness is partially lowered. Furthermore, since the portion where the luminance is reduced changes every moment according to the complicated movement of the heart, there is a problem that the myocardial image flickers and becomes very difficult to see.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to effectively reduce flickering of an image caused by removal of fixed noise in an ultrasonic diagnostic apparatus.

[0010]

(1) The present invention provides scanning means for scanning a cross section of a subject with ultrasonic waves, receiving means for obtaining a reception signal based on an output signal of the scanning means, and reception signal. Filtering means for attenuating a component having a relatively small temporal variation, an image generating means for generating an ultrasonic image based on a reception signal passing through the first filter means, In an ultrasonic diagnostic apparatus including a display unit that displays a sound wave image, the image generation unit includes:
Amplitude extracting means for extracting amplitude information contained in a received signal passed through the first filter means, and a second filter for filtering an output signal of the amplitude extracting means to attenuate a component having a relatively large temporal variation. Means, and compression means for compressing the dynamic range of the output signal of the second filter means.

(2) The present invention relates to the apparatus of (1),
The output signal of the amplitude extracting means is substantially arithmetically averaged in the second filter means.

(3) An ultrasonic diagnostic apparatus according to the present invention scans a section of a subject with ultrasonic waves to obtain a received signal, and filters the received signal to obtain a component having a relatively small temporal variation. High-pass filter means for attenuating, a detection means for detecting an output signal of the high-pass filter means, and filtering an output signal of the detection means to remove a component having a relatively large temporal variation. Attenuating low-pass filter means, a compression means for compressing a dynamic range of an output signal of the low-pass filter means, and a display for displaying an image relating to the cross section based on an output signal of the compression means. Means.

(4) According to the present invention, a cross section of an object is scanned by an ultrasonic wave, a received signal obtained by the scanning is filtered to attenuate a component having a relatively small temporal variation, and the filtered received signal is detected. Then, in an ultrasonic diagnostic apparatus that compresses the dynamic range of the detection signal and displays an image related to the cross section based on the compressed detection signal, the detection signal before the compression is filtered to relatively change the temporal variation. It is characterized by attenuating large components.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An ultrasonic diagnostic apparatus according to the present invention will be described below with reference to the drawings. FIG. 1 shows a configuration of an ultrasonic diagnostic apparatus according to the present embodiment. At the tip of the ultrasonic probe 1, a plurality of vibrators, such as piezoelectric ceramics, that convert electric vibration into mechanical vibration and vice versa, are arranged. Generally, 1
The vibrators constitute one channel. The ultrasonic probe 1 is connected to the transmission system 2 during transmission, and is connected to the reception system 3 during reception. Transmission system 2
Although not shown, it is composed of a clock generator, a frequency divider, a transmission delay circuit, and a pulser. The clock pulse generated by the clock generator is dropped by the frequency divider to a rate pulse of, for example, about 6 KHz, and this rate pulse is A high-frequency voltage pulse is applied to a pulser through a transmission delay circuit to drive the vibrator, that is, to mechanically vibrate. The ultrasonic wave generated in this manner is reflected at the boundary of the acoustic impedance in the subject, returns to the ultrasonic probe 1, and mechanically vibrates the vibrator. As a result, an electric signal is generated in the vibrator. This electric signal is received by the receiving system 3
It is taken in. Although not shown, the receiving system 3 includes a preamplifier, an analog-to-digital converter, a receiving delay circuit, and an adder.
The signal is converted to a digital signal by an analog-to-digital converter. In the analog-to-digital converter, the amplified electric signal is sampled on one ultrasonic scanning line at a sampling frequency corresponding to, for example, 0.5 mm intervals. Occurs. This digital signal is subjected to so-called phasing addition by the reception delay circuit and the adder. As a result, a reception signal fi including a difference in acoustic impedance between tissues, that is, amplitude information representing a tissue structure and phase information representing a velocity of a moving body such as a blood flow is obtained.

The received signal fi obtained by the phasing addition
Contains noise with relatively little variation over time, so-called fixed noise. This fixed noise is removed by a fixed noise removal filter 4 designed for a high-pass type for each sample point. ing. In this fixed noise removal filter 4, the cutoff frequency is finely controlled by the system controller 9 according to the heartbeat time phase so that, for example, the stationary myocardium does not disappear together with the fixed noise. It has become. For example,
In the cardiac phase where the heart wall motion is stationary, such as in late diastole, the cutoff frequency is lowered to reduce the attenuation of myocardial echo, while the cardiac wall phase in which the heart wall motion is intense, such as during early systole. Then, the cutoff frequency is raised to sufficiently attenuate the fixed noise.

The received signal fi from which the fixed noise has been removed by the fixed noise removing filter 4 stores the phase information and the amplitude information.
It is taken out by.

By the way, the movement of the heart is very complicated, that is, the myocardium does not always move at the same speed regularly, and a part that moves violently and a part that hardly moves are mixed. . Therefore, even if the cut-off frequency of the fixed noise removal filter 4 is controlled in accordance with the heartbeat phase, it is almost impossible to reliably remove the fixed noise and reliably leave the myocardial echo in all heartbeat phases. It is inevitable that fixed noise partially remains or myocardial echo partially disappears. Therefore, when the display of the myocardial component is taken as an example, the echoes are attenuated by the fixed noise removal filter 4 in the portion that hardly moves, so that the luminance on the image display greatly fluctuates with time and flicker occurs. I will.

In order to reduce this flicker, the detector 5
And a logarithmic compressor 7, a smoothing filter 6 designed to be of a low-high-pass type is provided, and the detection signal Ai after detection and before logarithmic compression is filtered to have a relatively large temporal variation. The component is attenuated for each sample point, in other words, averaging processing is performed between adjacent frames, that is, averaging processing is performed on a plurality of temporally continuous detection signals Ai at the same sample point.

The dynamic range of the detection signal ￣Ai averaged by the smoothing filter 6 is very wide, and the dynamic range of the display unit 8 is insufficient for expressing the signal as it is. The range is narrowed down, and the logarithmically compressed video signal Vi is supplied to the display unit 8. In the present embodiment, the dynamic range is compressed by logarithmic compression. However, the dynamic range may be compressed by using a non-logarithmic non-linear transformation. The display unit 8 displays a so-called B-mode image representing a tissue tomographic structure according to the video signal Vi.

A console 10 is connected to the system controller 9, and a filter characteristic selection switch 11 in the console 10 is operated to allow the operator to set a reference value of the cutoff frequency of the fixed noise removing filter 4 to the subject. And the number of additions of the smoothing filter 6 can be selected. Others
The console 10 has a ROI setting device 1 for an operator to set a region of interest ROI on the displayed B-mode image.
Various switches, such as a 2nd, a mouse, a keyboard, etc. are equipped.

FIG. 2 shows an N-order FIR filter as a configuration example of the smoothing filter 6. As the smoothing filter 6, either an FIR (non-recursive) digital filter as shown in FIG. 2 or an IIR (recursive) digital filter may be used. The smoothing filter 6 has a general configuration of a digital filter. A plurality of frame memories 23 are cascaded to an input terminal IN with a filter controller 21 as a control center. This frame memory 23
Functions as a delay circuit that gives a reciprocal time difference of the frame rate Z between input and output by input / output control by the filter controller 21. This makes artificial terminal I
From the cascade connection of N and the plurality of frame memories 23, (N + 1) detection signals (Ai, Ai-1,..., Ai-N) up to N-pieces before the same sample point are synchronously output all at once. Is done. The (N + 1) detected signals (Ai, Ai-
, Ai-N) are supplied to (N + 1) multipliers 26, respectively. The filter controller 21 stores identification information for identifying the filter characteristic selected via the filter characteristic selection switch 11 in the system controller 10.
The filter controller 21 generates an address FCA according to the identification information coefficient of the filter characteristic and supplies the address FCA to the multiplication coefficient ROM 22. A plurality of multiplication coefficients (k0, k1,..., KN) stored at the location corresponding to the coefficient address FCA are stored in the multiplication coefficient ROM 22.
And is supplied to each multiplier 26. The multiplier 26 multiplies the detection signals (Ai, Ai-1,..., Ai-N) by the multiplication coefficients (k0, k1,..., KN), and sends them to the adder 30.

The adder 30 adds the outputs of the respective multipliers 26 and outputs them as a temporally filtered detection signal ￣Ai.

[0023]

(Equation 1)

Table 1 shows the correspondence between coefficient addresses FCA and multiplication coefficients (k0, k1,..., KN) in this embodiment. The multiplication coefficient kj in Table 1 represents a case where the order N of the filter is 4, and the coefficient kj is set to 1 / M so as to obtain a simple moving average characteristic. M represents the number of frames to be added, and the cutoff frequency in this case is determined only by the number M of added frames.
In Table 1, the coefficient address FCA is equal to the number M of added frames, and the number M of added frames can be switched between 1 to 5 by selecting the coefficient address FCA from 1 to 5. Note that the number of added frames M
Is 1 when the filter is turned off.

[0025]

[Table 1]

Here, as described above, a low-pass smoothing filter 6 is provided between the detector 5 and the logarithmic compressor 7.
Is provided, a particularly remarkable effect can be achieved. To explain this particularly remarkable effect, for comparison, consider an example in which a low-pass smoothing filter 6 is provided after the detector 5 and the logarithmic compressor 7 shown in FIG. Here, as shown in FIG. When considered as a function of i, as is well known, the reception signal fi before detection and the detection signal A
i and the video signal Vi are given by the following equations, respectively.

[0027]

(Equation 2)

In the comparative example of FIG. 3, the smoothing filter 6 performs averaging processing on the video signal Vi after detection and logarithmic compression. The averaged video signal ￣Vi is given by the following equation. Note that the number of added frames M (moving average number) is 3.

[0029]

(Equation 3)

As can be seen from this equation, in the comparative example of FIG. 3, the averaging process in the smoothing filter 6 substantially corresponds to taking the geometric mean of the detection signal Ai.

On the other hand, in this embodiment, after the detection,
Averaging processing is performed by the smoothing filter 6 on the detection signal Ai before logarithmic compression. The averaged detection signal ￣Ai and the video signal Vi obtained by logarithmically compressing the detection signal ￣Ai are given by the following equations.

[0032]

(Equation 4)

As can be seen from this equation, as in the present embodiment, when the averaging process is performed by the smoothing filter 6 on the detected signal Ai after detection and before logarithmic compression, the processing is substantially performed. Corresponds to taking the arithmetic average of the detection signal Ai.

Thus, the flicker can be reduced more effectively by taking the arithmetic mean as in the present embodiment than by taking the geometric mean as in the comparative example. The reason will be described below. Hereinafter, the number of added frames FCA = M = 3 is shown as an example.

First, FIG. 5A shows an example of the time variation of the detection signal Ai. If this is logarithmically compressed without averaging, the video signal Vi becomes the video signal Vi shown in FIG.
Is shown. When the averaging process is performed on the video signal Vi as in the comparative example of FIG. 3, the time variation of the averaged video signal ￣Vi becomes relatively severe as shown in FIG. In other words, if an image contains a signal that has disappeared, that is, a signal whose value is close to zero or close to zero, the geometric mean result will be zero or close to zero, so the effect of reducing flicker by smoothing is Not very much.

On the other hand, when the averaging process is performed on the detection signal Ai before the logarithmic compression as in the present embodiment, FIG.
As shown in FIG. 7A and FIG. 7B, relatively large time fluctuations of the detection signal Ai are significantly reduced. This is because in the arithmetic averaging as in the present embodiment, the arithmetic averaging result is not zero or close to zero, but is raised. In other words, even if the detected signal Ai at a certain phase is zero or close to zero, if the detected signal of the neighboring frame is relatively high, the value is increased to the value of the neighboring frame. If the averaged detected signal ￣Ai is logarithmically compressed by the logarithmic compressor 7, the detected signal Ai at or near zero at a certain time phase becomes a sufficiently large value by the averaging process. .

This operation is generally known as the following arithmetic mean and geometric mean inequality.

[0038]

(Equation 5)

The detection signal Ai is an amplitude signal, and is therefore a positive number. Since the myocardium is pulsating and not all the signal groups are equal, the detection signal Ai is not detected as in this embodiment. The averaging process for the detected signal before logarithmic compression, that is, the arithmetic averaging, is higher than the averaging process for the video signal after logarithmic compression, as in the comparative example, that is, the geometric averaging. The effect can be realized,
As a result, flicker can be effectively reduced.

It is needless to say that the present invention is not limited to the above-described embodiment, and that various modifications can be obtained without departing from the scope of the invention. For example, in a conventional ultrasonic diagnostic apparatus having a video signal-based smoothing filter as shown in FIG. 3, an antilog converter 13 is provided after a logarithmic compressor 7 as shown in FIG. A detected signal Ai (amplitude information) before logarithmic compression is reproduced from the signal, and the reproduced detected signal Ai is reproduced.
The smoothing filter 6 is applied to i. After that, by providing the logarithmic compressor 14 after the smoothing filter 6 in order to return to the video signal again, the same effect as in the above-described embodiment can be obtained. Note that the number of bits required for the averaging process operation should be extended more than in the case of the video signal base in order to secure the dynamic range of the amplitude.

[0041]

According to the present invention, it is possible to reduce the flicker caused by attenuating a component having a relatively small temporal variation by a high-pass filter in a low-pass filter. Can be. In addition, this low-pass filter processing is performed not on the video signal after detection and after logarithmic compression but on the detection signal after detection and before logarithmic compression, so that it is more effective. The flicker can be reduced. This is because the low-pass filter processing for the video signal functions substantially as a geometric mean, whereas the low-pass filter processing for the detection signal functions substantially as an arithmetic mean. In the number domain, as is well known as the Schwartz equation, the geometric mean ≦ arithmetic mean holds,
This is because the arithmetic averaging has a greater effect of raising the amplitude at a low amplitude, and the temporal fluctuation of the amplitude, that is, the temporal fluctuation of the luminance is reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an ultrasonic diagnostic apparatus according to a preferred embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a smoothing filter 6 of FIG.

FIG. 3 is a block diagram showing a comparative example of the ultrasonic diagnostic apparatus according to the embodiment.

FIG. 4 is a supplementary diagram for explaining effects of the present embodiment, and is a diagram showing definitions of frame numbers.

FIGS. 5A and 5B are supplementary diagrams for explaining the effect of the present embodiment. FIG. 5A is a detection signal Ai after detection and before logarithmic compression.
FIG. 3B is a diagram showing a time waveform of the video signal Vi after detection and logarithmic compression.

FIG. 6 is a supplementary diagram for explaining the effect of the present embodiment, and is a diagram showing a time waveform of a video signal ￣Vi averaged by the smoothing filter of the comparative example of FIG. 3;

FIGS. 7A and 7B are supplementary diagrams for explaining the effect of the present embodiment. FIG. 7A is a diagram showing a time waveform of a detection signal ￣Ai averaged by the smoothing filter of the embodiment, and FIG. The figure which shows the time waveform of the video signal Vi after logarithmically compressing the averaged detection signal (DELTA) Ai of a).

FIG. 8 is a block diagram showing a configuration of a modified example of the ultrasonic diagnostic apparatus of the present embodiment.

FIG. 9 is a block diagram showing a configuration of a conventional ultrasonic diagnostic apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Ultrasonic probe, 2 ... Transmission system, 3 ... Reception system, 4 ... Fixed noise removal filter, 5 ... Detector, 6 ... Smoothing filter, 7 ... Logarithmic compressor, 8 ... Display unit, 9 ... System controller, 10 ... Console, 11 ... Filter characteristic selection switch, 12 ... ROI setting device, 13 ... Anti-log converter, 14 ... Logarithmic compressor, 21 ... Filter controller, 22 ... Multiplication coefficient ROM, 23, 24, 25 ... Frame memory, 26 , 27, 28, 29 ... multiplier, 30 ... adder.

Claims (4)

[Claims] A scanning unit configured to scan a cross section of an object with an ultrasonic wave; a receiving unit configured to obtain a reception signal based on an output signal of the scanning unit; A first filter for attenuating small components, an image generator for generating an ultrasonic image based on a received signal passing through the first filter, and a display for displaying the ultrasonic image. In the ultrasonic diagnostic apparatus, the image generation unit may include an amplitude extraction unit configured to extract amplitude information included in a reception signal that has passed through the first filter unit, and a filter configured to filter an output signal of the amplitude extraction unit and vary with time. Second filter means for attenuating relatively large components of
Compression means for compressing the dynamic range of the output signal of the second filter means. 2. The ultrasonic diagnostic apparatus according to claim 1, wherein an output signal of said amplitude extracting means is substantially arithmetically averaged in said second filter means. 3. A means for scanning a cross section of an object with an ultrasonic wave to obtain a received signal, and a high-pass filter means for filtering the received signal to attenuate a component having a relatively small temporal variation. Detection means for detecting an output signal of the high-pass filter means, and a low-pass filter means for filtering an output signal of the detection means to attenuate a component having a relatively large temporal variation. A compression unit that compresses a dynamic range of an output signal of the low-pass filter unit; and a display unit that displays an image related to the cross section based on an output signal of the compression unit. Ultrasound diagnostic equipment. 4. A cross section of an object is scanned by an ultrasonic wave, a received signal obtained by the scanning is filtered to attenuate a component having a relatively small temporal variation, the filtered received signal is detected, and the detected signal is detected. In an ultrasonic diagnostic apparatus that compresses the dynamic range of the above and displays an image relating to the cross section based on the compressed detection signal, the detection signal before compression is filtered to attenuate a component having a relatively large temporal variation. An ultrasonic diagnostic apparatus characterized by the above-mentioned.