JP2002143161A - Ultrasonic diagnostic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correctly detect and display a blood stream speed concerning an ultrasonic diagnostic device provided with the function of an ultrasonic Doppler method for obtaining blood stream information in a living body. SOLUTION: The device is provided with a first frequency analyzer 8a for detecting a first Doppler frequency by frequency-analyzing a high frequency component extracted by a high-pass filter 7, a second frequency analyzer 8b for detecting a second Doppler frequency by frequency-analyzing a low-frequency component extracted by a low-pass filter 11 and a subtracter 9 for obtaining a difference between the first and the second Doppler frequencies which are respectively detected by the first and the second frequency analyzers 8a and 8b.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrasonic diagnostic apparatus having an ultrasonic Doppler function for obtaining clutter information by removing clutter using a high-pass filter.

[0002]

2. Description of the Related Art Conventionally, there has been known an ultrasonic diagnostic apparatus which obtains information inside a subject (particularly, a living body) using ultrasonic waves and provides the information for diagnosis. Object, in particular, a function for knowing the blood flow velocity. Heretofore, in a commonly used ultrasonic diagnostic apparatus, only velocity information at one point in a living body can be obtained using an ultrasonic pulse. However, in recent years, a two-dimensional ultrasonic Doppler method capable of two-dimensionally displaying a blood flow velocity in color and observing the velocity distribution in a living body at a glance has been put to practical use.

In the two-dimensional ultrasonic Doppler method, in order to display a blood flow velocity distribution, blood flow velocities of a plurality of sample volumes located on a transmitting and receiving ultrasonic beam are detected and calculated, and a beam direction is calculated. Are sequentially changed to obtain two-dimensional speed information. This speed information is obtained by performing quadrature detection on echo signals obtained on a certain scanning line using reference signals having phases different from each other by 90 degrees, removing high-frequency components, and then performing A / D conversion.
It is obtained by converting and performing digital signal processing. In order to obtain velocity information on one scanning line, transmission / reception is repeated, for example, 10 times, the Doppler frequency f _{d is} obtained from the phase change amount of the echo signal at each depth, and the blood flow velocity v is expressed by the following relational expression. Has been gained.

[0004]

(Equation 1)

Here, f ₀ is the transmission frequency of the ultrasonic wave, c is the speed of sound in the subject, and θ is the angle between the blood cell and the ultrasonic beam.

The received echo signal contains, in addition to a Doppler signal indicating the speed of blood flow as a motion reflector, a clutter signal having a larger amplitude level than that of a clutter caused by a tissue or a blood vessel wall. The amplitude ratio between the clutter signal and the blood flow signal is about 20 to 30 dB inside the heart, and about 35 to 40 dB in the middle and large blood vessels of the abdomen. Generally, the band of the clutter signal is about several hundred Hz, which is lower than the Doppler signal due to blood flow reflection, and is cut by a high-pass filter.

However, in this type of apparatus for displaying a two-dimensional velocity distribution, an analog filter cannot be used in principle because a Doppler signal of a specific depth cannot be continuously obtained. For this reason, the received signal is A / D converted to obtain a sample data string for each depth, and then processed by a digital high-pass filter. Such a high-pass filter is also called a wall filter. By the way, the order of the high-pass filter is limited by the number of scans in the same direction, and therefore cannot be made too high. Usually, a filter of about the second to fourth order is used. Therefore, it is difficult to obtain a steep cutoff characteristic, and it is not possible to sufficiently remove a clutter signal having an excessive amplitude from a heart wall, a diaphragm, or the like. For this reason, in the conventional apparatus, the input signal is constantly monitored, and when the amplitude is excessive, the color display is suppressed because it is not the blood flow reflection but the reflection from the tissue region. That is, the average amplitude value of the Doppler signal is obtained directly from the output of the A / D converter, and when the average amplitude value is larger than a certain limit value, the flow velocity is forcibly set to zero. Usually, this limit value is set to a value sufficiently larger than the average amplitude of the Doppler signal due to blood flow reflection.

[0008]

However, the following problems have been pointed out in the above-mentioned conventional apparatus. One problem is that even if clutter components are removed by a high-pass filter, an error occurs in the detection speed due to the relative speed between the blood vessel and the probe. That is, FIG.
As shown in ( ₁₎ , when the velocity of the blood cell flowing in the blood vessel is V ₁ and the angle between the moving direction of the blood cell and the beam is θ ₁ , the blood flow velocity to be detected is V ₁ cos θ ₁ . However, due to the pulsation of the blood vessel, the same velocity component as that of the pulsation of the blood vessel is applied to the blood flow flowing through the blood vessel. When the beam direction component of the instantaneous velocity and V _2, the actual blood flow velocity is detected becomes V ₁ cosθ ₁ + V ₂ becomes a composite of both. That detection rate is biased by ₂ minutes V.
When this is observed on the frequency axis, it becomes as shown in FIG. Assuming that the center frequency of the Doppler signal due to the movement of the blood vessel wall is f _CL and the center frequency of the Doppler signal due to the true blood cell movement is f _d ,
The center frequency of the Doppler signal by the blood cell motion that is actually observed is shifted by f _CL, a f _d + f _CL. This error is not negligible, especially when the blood flow velocity is low.

Further, in the above-mentioned conventional apparatus, the following problem arises because the discrimination between the reflection by the blood flow and the reflection by the tissue is simply performed by the magnitude of the amplitude value of the Doppler signal. That is, in the case of a blood vessel having a relatively large blood flow such as the aorta, the average amplitude of the blood flow signal takes a large value and may exceed the limit value of the average amplitude for discrimination from tissue reflex. As a result, the original blood flow may be erroneously recognized as a tissue, and a portion where the blood flow exists in the diagnostic image may lose color display.

The present invention has been made in view of the above circumstances, and has as its object to provide an ultrasonic diagnostic apparatus capable of correctly detecting and displaying a blood flow velocity.

[0011]

An ultrasonic diagnostic apparatus according to the present invention that achieves the above object transmits an ultrasonic pulse into a subject and receives reflected ultrasonic waves that have returned after being reflected within the subject. A plurality of times, a signal obtained during this repetition is detected, and a high-pass filter for extracting a high-frequency component of the detected signal in an ultrasonic diagnostic apparatus for detecting a Doppler frequency corresponding to a blood flow by frequency analysis A first Doppler frequency detecting means for detecting a first Doppler frequency by frequency-analyzing a high-frequency component extracted by the high-pass filter, a low-pass filter for extracting a low-frequency component of a signal after detection, and a low-pass filter Second Doppler frequency detection means for detecting a second Doppler frequency by frequency-analyzing the low-frequency component extracted by
Calculating means for calculating a Doppler frequency corresponding to a blood flow based on the first Doppler frequency detected by the first Doppler frequency detecting means and the second Doppler frequency detected by the second Doppler frequency detecting means. It is characterized by having.

Here, the calculating means may calculate a Doppler frequency corresponding to a blood flow by calculating a difference between the first Doppler frequency and the second Doppler frequency.

The present invention has the following effects by the above configuration.

That is, by analyzing the frequency of the Doppler signal subjected to the high-pass filtering by the first Doppler frequency detecting means, a Doppler frequency in which the Doppler frequency due to the blood flow reflection and the Doppler frequency due to the motion of the blood vessel wall are synthesized is obtained. . The low-pass filtered Doppler signal is subjected to frequency analysis by the second Doppler frequency detecting means, whereby a Doppler frequency due to the motion of the blood vessel wall is obtained. Therefore, for example, by subtracting this by a subtractor, a Doppler frequency based only on blood flow reflection can be obtained.

Further, in the ultrasonic diagnostic apparatus of the present invention, a level obtaining means for obtaining the level of the low frequency component extracted by the low-pass filter, and whether the level obtained by the level obtaining means is equal to or lower than a predetermined level. It is preferable that the apparatus further comprises a selection unit for selectively selecting a difference obtained by the subtraction unit and a fixed value.

In this case, the calculation of the average amplitude value of the Doppler signal is performed based on the signal filtered by the low-pass filter. Therefore, the average amplitude value of the Doppler signal due to only tissue reflection is always compared with the limit value to perform discrimination. By doing so, it is avoided that the reflection due to the blood flow having a large signal amplitude is mistaken for the reflection due to the tissue.

[0017]

Embodiments of the present invention will be described below.

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

At the tip of the probe 1, vibrators made of piezoelectric elements as ultrasonic transducers are arranged in an array. The transmission circuit 2 emits ultrasonic waves of 2 to 3 MHz at the transmission repetition frequency of, for example, 4 KHz through the probe 1 in the direction a in the subject (not shown) ten times, for example. The reflected wave reflected by the blood flow in the subject and shifted by Doppler is received by the same probe 1, converted into an electric signal, and sent to the receiving circuit 3. After receiving and amplifying the output of each oscillator in the receiving circuit 3 and amplifying it, the quadrature detection circuit 4
, And subjected to quadrature detection by reference signals having phases different from each other by 90 °. The output of the quadrature detection circuit 4 is passed through a low-pass filter 5 to pass a signal component in the Doppler frequency band, so that I and Q components of the Doppler signal are obtained. Further, the Doppler signal is quantized by the A / D converter 6 to become digital data. At this time, the Doppler signal is 10 for each depth.
Data strings. Next, the ten data strings at each depth are provided to the high-pass filter 7 and the low-pass filter 11. Here, as shown in FIG. 4, the cutoff frequencies of the high-pass filter 7 and the low-pass filter 11 have the same value, and are set to, for example, 100 Hz.

Therefore, the low frequency clutter component is removed from the Doppler signal supplied to the high-pass filter 7. The output is given to the first frequency analyzer 8a, and the Doppler frequency due to blood flow reflection is calculated. At this time, as shown in FIG. 4, assuming that the Doppler frequency due to true blood flow reflection is f _d and the Doppler frequency due to blood pulsation is f _CL , a frequency of f _d + f _CL is obtained.

Meanwhile, Doppler signal applied to the low-pass filter 11, as shown in FIG. 4, the clutter component is extracted, Doppler frequency f _CL by the output given vessel wall motion in a second frequency analyzer 8b Is obtained.

[0022] By further subtractor 9, the output f _d + f _CL from the output f _CL of the second frequency analyzer 8b is subtracted true Doppler frequency f _d of the first frequency analyzer 8a is obtained. After performing the same processing for all the sample points in the a direction in this way, the same processing is further repeated in the b direction and the c direction.

The analysis result of the Doppler frequency is sent to the display device 15 via the selection circuit 10 as speed data, and is converted into RGB data in order to apply a color according to the absolute value and direction of the speed, and is converted into RGB data. The image is synthesized with the monochrome tomographic image data of the image processing circuit 14 and is synthesized and displayed on a color monitor (not shown).

On the other hand, the output of the low-pass filter 11 is supplied to an average amplitude calculation circuit 12, and the average amplitude value of the Doppler signal due to tissue reflection is obtained by the following equation.

[0025]

(Equation 2)

Here, I (n) and Q (n) are outputs of the low-pass filter 11, and are obtained by tissue reflection obtained from a specific sample point on the beam in the n-th scan in the same direction. In-phase and quadrature components of the Doppler signal, and N is the number of scans in the same direction. Comparison circuit 1
The average amplitude value and a preset limit value are input to 3, and the two are compared. Based on the comparison result, a selection signal for selecting one of the two-input selection circuit 10 is selected. Generate That is, when the average amplitude value is smaller than the limit value, the output of the subtractor 9 is selected, and when the average amplitude value is larger, the fixed value (here, 0) is selected. As a result, when the average amplitude value of the Doppler signal due to the tissue reflection exceeds the limit value, the flow velocity becomes a fixed value (here, 0), and the color display is suppressed.

[0027]

As described above, according to the present invention,
A true blood flow velocity in which an error due to the movement of the blood vessel wall is corrected is obtained, and therefore, even with a low-speed blood flow, the velocity can be detected with higher accuracy than the conventional apparatus.

Further, if the display of the flow velocity is selected according to the level of the low frequency component extracted by the low-pass filter, the reflection by the blood flow having a large signal amplitude is erroneously recognized as the reflection by the tissue. Thus, the blood flow and the tissue can be discriminated more accurately than the conventional device.

[Brief description of the drawings]

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

FIG. 2 is a schematic diagram showing a state of blood flow velocity detection by ultrasonic waves.

FIG. 3 is an explanatory diagram of an error of a detected blood flow velocity.

FIG. 4 is a diagram illustrating characteristics of a high-pass filter and a low-pass filter.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 probe 2 transmission circuit 3 reception circuit 4 quadrature detection circuit 5 low-pass filter 6 A / D converter 7 high-pass filter 8 a first frequency analyzer 8 b second frequency analyzer 9 subtractor 10 selection circuit 11 low-pass filter 12 Average amplitude calculation circuit 13 Comparison circuit 14 Tomographic image processing circuit 15 Display device

Claims (2)

[Claims] 1. A method according to claim 1, wherein the transmitting of the ultrasonic pulse into the subject and the receiving of the reflected ultrasonic wave reflected and returned in the subject are repeated a plurality of times, and a signal obtained during the repetition is detected. Further, in an ultrasonic diagnostic apparatus for detecting a Doppler frequency corresponding to a blood flow by frequency analysis, a high-pass filter for extracting a high-frequency component of a signal after detection, and a frequency analysis of the high-frequency component extracted by the high-pass filter A first Doppler frequency detecting means for detecting a first Doppler frequency, a low-pass filter for extracting a low-frequency component of the signal after detection, and a second by performing frequency analysis on the low-frequency component extracted by the low-pass filter. Second Doppler frequency detecting means for detecting the Doppler frequency of the first, and the first Doppler frequency detected by the first Doppler frequency detecting means
An ultrasonic diagnostic apparatus comprising: a calculating means for calculating a difference between the Doppler frequency of the second Doppler frequency and the second Doppler frequency detected by the second Doppler frequency detecting means. 2. A level obtaining means for obtaining a level of a low-frequency component extracted by said low-pass filter, and said subtracting means according to whether or not the level obtained by said level obtaining means is a predetermined level or less. 2. The ultrasonic diagnostic apparatus according to claim 1, further comprising a selection unit for selectively selecting a difference to be obtained and a fixed value.