JP2010246640A - Ultrasonic diagnostic apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To execute an appropriate pulse compressing process considering frequency-dependent attenuation of biological tissue in an ultrasonic diagnostic apparatus for transmitting chirp waves and executing the pulse compression process for received signals. <P>SOLUTION: A process of correcting the frequency characteristics is executed for the received signals based on the attenuation coefficient α output from an attenuation coefficient determining part 20 in a correction part 18, and the received signals after the correction are subjected to the pulse compression process. By acquiring evaluation values on the image after the pulse compression while tentatively changing the attenuation coefficient, the best evaluation value is specified among the evaluation values, and a frequency characteristic correction function to be used for the real ultrasonic diagnosis is set based on the specified attenuation coefficient. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an ultrasonic diagnostic apparatus, and more particularly to an ultrasonic diagnostic apparatus having a pulse compression circuit.

  Recently, in the field of ultrasonic diagnosis, a pulse compression technique is used for the purpose of improving the transmission power. In a typical pulse compression technique, a chirp wave is transmitted to a living body, and a received signal obtained by receiving a reflected wave from the living body is subjected to pulse compression processing. The chirp wave is a frequency modulation (FM) wave, and is a wave that temporally changes from the first frequency (start frequency) to the second frequency (end frequency). On the other hand, the pulse compression process at the time of reception is a process (convolution operation) for compressing the signal component stretched on the time axis.

  A living tissue has a property of being attenuated as the frequency increases. This is called frequency-dependent attenuation, and the degree is expressed by an attenuation coefficient [dB / cm / MHz]. This attenuation coefficient is a constant independent of depth if it is a homogeneous structure. The above chirp wave is also affected by this property. That is, even if the total sound pressure of the transmitted chirp wave is constant (flat), when the received chirp wave is observed, the sound pressure is not constant, and a tendency that the sound pressure decreases as the frequency component becomes higher is recognized. If a received signal affected by such frequency-dependent attenuation is sent to the pulse compression circuit as it is, proper pulse compression cannot be performed and the compressed waveform becomes dull and side lobes are generated. This is a major factor in image quality degradation.

  Patent Document 1 discloses a technique for dealing with the problem. That is, the received signal is corrected before pulse compression to compensate for frequency dependent attenuation. However, how to determine the correction characteristics is not specifically described, and it can be pointed out that appropriate correction cannot be performed according to individual living tissues (that is, according to the subject). If the frequency-dependent attenuation coefficient for a living tissue is known, an appropriate correction function can be derived therefrom. However, a conventional ultrasonic diagnostic apparatus has a circuit for automatically determining the frequency-dependent attenuation coefficient. Absent.

Japanese Patent Laid-Open No. 2-107987

  An object of the present invention is to provide an ultrasonic diagnostic apparatus capable of measuring characteristics of a living tissue in a subject to be diagnosed, particularly a frequency-dependent attenuation coefficient.

  Another object of the present invention is to optimize the operating conditions of the ultrasonic diagnostic apparatus using the actually measured frequency-dependent attenuation coefficient.

  Another object of the present invention is to measure the frequency-dependent attenuation coefficient using the chirp wave transmission technique and the received signal pulse compression technique as they are, and the measured frequency-dependent attenuation coefficient is used as the frequency attenuation before the pulse compression. It is to make use of the correction.

  An ultrasonic diagnostic apparatus according to the present invention transmits a chirp wave, which is a frequency-modulated wave, to a subject, receives a reflected wave from the subject, and outputs a received signal; and the received signal In order to compensate the frequency-dependent attenuation of the living tissue, a correction processing unit that performs dynamic correction processing according to the depth of the reception point, and pulse compression processing is applied to the received signal after the correction processing A pulse compression processing unit, an image forming unit that forms an ultrasonic image based on the received signal after the pulse compression processing, and a plurality of correction characteristics corresponding to a plurality of frequency-dependent attenuation coefficients are sequentially set in the correction processing unit While obtaining a plurality of pulse compression processing results by operating the pulse compression processing unit on a trial basis and determining the best processing result from among the results, frequency dependence on the living tissue in the subject Estimate damping factor A coefficient estimator that, characterized in that it comprises a.

  According to the above configuration, using the existing chirp wave transmission technique and pulse compression technique, the frequency-dependent attenuation coefficient (the corresponding information is sufficient) for the living tissue in the subject is automatically calculated. Is possible. In other words, using the phenomenon (problem) that side lobes occur in the result of chirp wave pulse compression due to frequency-dependent attenuation, trial setting of the attenuation coefficient and trial frequency correction based on it are repeated to compress the pulse. Among the results, the attenuation coefficient at the time when the best result is obtained is regarded as the actual attenuation coefficient for the living tissue in the subject. If the actual attenuation coefficient is known, a frequency correction function can be determined therefrom, and in particular, the slope of the frequency correction function can be dynamically changed according to the depth of the reception point.

  The correction processing unit is preferably composed of a dynamic filter having a function of dynamically changing the filter characteristics, but another filter or the like may be separately provided. The evaluation of the pulse compression processing result can be performed from various viewpoints, but in any case, it is desirable that the degree of compression after pulse compression or the amount of sidelobe components can be quantified. If the pulse compression process is not performed properly, the average brightness (or the sum of the brightness values) is calculated under the circumstances where the sharpness of the image deteriorates and each brightness point is blurred and an increase in average brightness is recognized. It can be used as an evaluation value. In that case, it is desirable to use the average luminance as an evaluation value after applying a normalization process for aligning the peak luminance after pulse compression between trial corrections. Alternatively, the degree of blur may be evaluated from the spatial frequency or dispersion value of the image. In order to reduce the amount of coefficient calculation, and in order to optimize the correction result at the site of interest, a region of interest may be set and only an image in the region of interest may be set as an evaluation target. If the attenuation coefficient of the target biometric coefficient can be found, the actual ultrasonic diagnosis (image formation by a combination of chirp transmission and pulse compression) can be performed on the subject using the attenuation coefficient as it is. The process of determining the attenuation coefficient is positioned as a preliminary investigation step before actual ultrasonic diagnosis. The preliminary investigation process may be performed for each subject or may be performed for each diagnosis site. Alternatively, the attenuation coefficient may be obtained periodically by interruption processing.

  Preferably, the coefficient estimation unit includes an evaluation unit that individually evaluates the plurality of pulse compression processing results to obtain an evaluation value, and the evaluation value is a value indicating a sidelobe content degree after pulse compression. is there. Preferably, the evaluation value is a sum of luminance values in the region of interest after the peak level normalization process. Preferably, the image processing apparatus includes a characteristic setting unit that sets a correction characteristic suitable for the subject with respect to the correction processing unit based on the frequency-dependent attenuation coefficient estimated by the coefficient estimation unit. Preferably, the correction characteristic is a function whose slope changes according to the depth of the reception point.

  It should be noted that the biological tissue is not strictly uniform, and it can be said that the attenuation coefficient changes depending on the depth (or due to the difference in tissue properties). Therefore, a plurality of regions of interest may be set in series from a short distance to a long distance, and an attenuation coefficient may be obtained for each depth (depth category). In that case, the frequency correction can be performed more finely according to the depth of the reception point. Similarly, a plurality of regions of interest may be set two-dimensionally on the scanning plane, and an attenuation coefficient may be obtained for each region. In any case, if an attenuation coefficient is obtained, the frequency correction process before pulse compression can be performed using the attenuation coefficient, so that the image quality of the ultrasonic image can be improved. Once the attenuation coefficient is obtained, filtering can be performed using the attenuation coefficient during normal operation without pulse compression, and the information can be used for automatic image quality adjustment such as brightness and contrast. Furthermore, it can be used for measuring the tissue properties itself.

  According to the present invention, it is possible to measure the characteristics of a living tissue in a subject to be diagnosed, particularly a frequency-dependent attenuation coefficient. Alternatively, according to the present invention, the operating conditions of the ultrasonic diagnostic apparatus can be optimized using the actually measured frequency-dependent attenuation coefficient. Alternatively, according to the present invention, the frequency dependent attenuation coefficient is measured using the chirp wave transmission technique and the received signal pulse compression technique as they are, and the measured frequency dependent attenuation coefficient is corrected to the frequency attenuation before the pulse compression. You can make use of it.

It is a conceptual diagram which shows suitable embodiment of the ultrasonic diagnosing device which concerns on this invention. 3 is a flowchart showing an operation example in the ultrasonic diagnostic apparatus shown in FIG. 1. It is a figure for demonstrating determination of an evaluation value, and creation of an evaluation value function. It is a figure for demonstrating determination of the attenuation coefficient based on an evaluation value function. It is a figure which shows an example of a transmission chirp wave. It is a figure which shows the reception chirp wave with a frequency dependent attenuation | damping. It is a figure for demonstrating the side lobe which arises after pulse compression. It is a figure which shows the function (filter characteristic) for correct | amending frequency dependence attenuation | damping. It is a figure which shows the reception chirp wave after performing frequency correction. It is a figure which shows the received signal after performing frequency correction and performing the pulse compression process. It is a figure which shows the inclination change according to the depth of the attenuation | damping correction function in the ultrasonic diagnosing device shown in FIG.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the drawings.

  FIG. 1 shows a preferred embodiment of an ultrasonic diagnostic apparatus according to the present invention, and FIG. 1 is a block diagram showing the overall configuration thereof. The ultrasonic diagnostic apparatus shown in FIG. 1 is an apparatus that is used in the medical field, obtains a reception signal by transmitting / receiving ultrasonic waves to / from a living body, and forms an ultrasonic image based on the reception signal. . The ultrasonic diagnostic apparatus of this embodiment has a chirp wave transmission function and a pulse compression function.

  The probe 10 is an ultrasonic probe, that is, an ultrasonic transducer. In this embodiment, the probe 10 has a 1D array transducer. The 1D array transducer includes a plurality of transducer elements, and thereby an ultrasonic beam B is formed. By scanning the ultrasonic beam B electronically, a scanning plane S is formed. The scanning plane S corresponds to a two-dimensional data capture area. In FIG. 1, r indicates the depth direction, and θ indicates the electronic scanning direction. As the electronic scanning method, electronic sector scanning, electronic linear scanning, and the like are known. The probe 10 may be provided with a 2D array transducer.

  As shown in FIG. 1, in this embodiment, a region of interest (ROI) 12 is set on the scanning plane S, actually on a tomographic image. In FIG. 1, the region of interest 12 is shown as a rectangular region. The position and size of the region of interest 12 are set by the user using the input unit 32 described later. The region of interest 12 is an area for referring to the received signal when determining an attenuation coefficient, which will be described later. Of course, the entire scanning surface S can be determined as a reference region, or a plurality of regions of interest can be set, and the attenuation coefficient can be determined for each region.

  The transmission unit 14 is a transmission beamformer, and the transmission unit 14 supplies a plurality of transmission signals to a plurality of vibration elements in parallel. Each transmission signal 15 is a chirp wave, that is, a chirp transmission signal in this embodiment. The chirp wave is a wave whose frequency is dynamically changed on the time axis, and a specific example thereof will be described later with reference to FIG. When a transmission pulse is sent out from the probe 10, echoes are generated at each depth in the living body and are received by the probe 10. Thereby, a plurality of reception signals are output from the plurality of vibration elements to the receiving unit 16 in parallel. The receiving unit 16 is a reception beamformer, which performs phasing addition processing on a plurality of reception signals, thereby obtaining a reception signal after phasing addition, that is, beam data. However, since a chirp wave is transmitted in the present embodiment, each component constituting the received signal corresponds to the chirp wave. A waveform of a reception signal obtained by receiving an echo from a certain reception point will be described later with reference to FIG.

  The reception signal output from the reception unit 16 is sent to the pulse compression unit 22. In the present embodiment, a correction unit 18 is provided before the pulse compression unit 22. The correction unit 18 is configured as a reception dynamic filter for compensating for frequency dependent attenuation. In the present embodiment, the correction unit 18 functions in order to improve or eliminate the problem caused by the collapse of the chirp wave depending on the frequency as a problem inherent to the chirp wave. The frequency characteristic correction function set in the correction unit 18 is determined based on the attenuation coefficient α, and the attenuation coefficient α is output from the attenuation coefficient determination unit 20.

  Incidentally, the reception signal in the region of interest 12 may be temporarily stored in the memory 17, and the temporary setting of the attenuation constant and the acquisition of the evaluation value may be repeatedly performed while repeatedly reading the reception signal in the region of interest from the memory 17.

  The pulse compression unit 22 performs pulse compression of the received signal by performing a convolution operation on the time axis. The pulse compression process may be performed on the frequency axis.

  The image forming unit 24 is configured by a digital scan converter in this embodiment, that is, applies coordinate conversion processing and interpolation processing to an input received signal, thereby forming a B-mode image that is a two-dimensional tomographic image. . Of course, an ultrasonic image other than the B-mode image may be configured. The image data is sent to the display unit 26. An ultrasonic image is displayed on the display unit 26. On the other hand, data of an image formed by the image forming unit 24 is output to the evaluation unit 28.

  The evaluation unit 28 is a means for evaluating the correction result of the frequency characteristic in the correction unit 18, and in this embodiment, the sum of luminance values in the region of interest is obtained as the evaluation value on the premise of normalization processing. By obtaining the evaluation value while temporarily setting (changing) the attenuation coefficient, the evaluation function A (α) can be obtained and provided to the attenuation coefficient determination unit 20. The attenuation coefficient determination unit 20 specifies the best evaluation value from the evaluation function, and determines the attenuation coefficient that is actually used as the attenuation coefficient at the time when the evaluation value is obtained.

  The control unit 30 is a controller that controls the operation of each component shown in FIG. An input unit 32 is connected to the control unit 30. The input unit 32 includes an operation panel including a keyboard and a trackball. Using the input unit 32, the user can set the region of interest at an arbitrary position and in an arbitrary size. In this embodiment, the attenuation coefficient determined by the attenuation coefficient determination unit 20 is sent to the correction unit 18, and a correction function for correcting frequency characteristics is set based on the attenuation coefficient. The form is not limited to that. For example, the reception condition and the image processing condition may be changed based on the determined attenuation coefficient. Further, since the attenuation coefficient itself represents the tissue property, the attenuation coefficient can be used for measurement of the tissue property. In addition, the operating conditions of the reception dynamic filter may be determined at the time of normal ultrasonic diagnosis in which pulse compression processing is not performed using the determined attenuation coefficient.

  FIG. 2 is a flowchart showing an operation example of the apparatus shown in FIG. The flowchart shown in the figure includes a preliminary investigation step S101 and an ultrasonic diagnosis execution step S109. The former is a step of determining the attenuation coefficient of the living tissue in advance, and the latter is a step of actually performing ultrasonic diagnosis of the subject.

  In S102, a B-mode tomographic image is first displayed on the screen, and a region of interest is set by the user on this tomographic image. Of course, the region of interest may be set with a default position and size, or the entire scanning plane may be set as the evaluation target without setting the region of interest. In S103, the first attenuation coefficient is set in order to obtain the evaluation value while setting the attenuation coefficient α on a trial basis. In S104, provisional correction is executed in the correction unit shown in FIG. 1 using the set attenuation coefficient. That is, the same process as the normal correction process is performed, but the correction is positioned as a temporary correction in the sense of correction performed in the preliminary survey process. In S105, a temporary pulse compression process is executed in the pulse compression unit shown in FIG. At the same time, it is desirable to apply a normalization process that aligns the peak produced by pulse compression between the settings of each attenuation coefficient before or after that.

  In S106, an image portion within the region of interest in the formed image is cut out and sent to the evaluation unit. In the present embodiment, the sum of luminance values in the region of interest is used as the evaluation value, and such sum is calculated in S106. In S107, it is determined whether or not the currently set attenuation coefficient starts from the lowest value and reaches the highest value. If not, the processes after S103 are repeatedly executed while increasing the attenuation coefficient in sequence. In S108, an attenuation coefficient evaluation function is constructed based on a plurality of evaluation values corresponding to a plurality of attenuation coefficients, and the lowest peak value is searched for specifically as described later by referring to the profile. Thus, it is determined that the attenuation coefficient α1 actually used is the attenuation coefficient at the time when the peak value is generated. As described above, after the attenuation coefficient is determined in the preliminary investigation step S101, the ultrasonic diagnosis is executed in S109. In this ultrasonic diagnosis, the above-described chirp transmission and pulse compression processing are executed. In this case, a frequency characteristic correction function based on an attenuation coefficient that is optimal for a living tissue is applied before the pulse compression, and the pulse compression processing is performed. It will be. As a result, the problem of the side lobe described in the prior art can be solved or reduced, and the image quality of the ultrasonic image can be remarkably improved.

  FIG. 3 shows two steps executed in S106. In S201, the sum of squares of the luminance value of each pixel in the region of interest is calculated. Here, x and y indicate the x address and y address of each pixel, α indicates the attenuation coefficient value set at that time, and the luminance value B at a certain pixel is defined by these three parameters. Is done. The evaluation value A (α) is obtained by obtaining the sum of squares of the luminance values in the region of interest. In S202, an evaluation function is constituted by the evaluation value A (α) obtained for each α.

An example is shown in FIG. The horizontal axis represents the temporarily set attenuation coefficient α, and the vertical axis represents the evaluation value A (α). When such an evaluation value function is obtained, the lowest peak value A (α) _min is specified, and the corresponding attenuation coefficient α1 is specified. The attenuation coefficient α1 is regarded as an attenuation coefficient used in actual ultrasonic diagnosis, and the frequency characteristic correction function in the correction unit is determined according to the attenuation coefficient α1.

  FIG. 5 shows a transmission chirp wave generated by simulation. The start frequency in this transmission chirp wave is 3 MHz, and the end frequency is 7 MHz. The amplitude is constant along the time axis.

  FIG. 6 shows a simulation result of the received chirp wave. The horizontal axis is the time axis, and the vertical axis represents the amplitude E. As shown in the figure, according to the frequency-dependent attenuation characteristic of the living tissue, the amplitude is particularly small as the frequency component is high, that is, the attenuation is large (see reference numeral 100).

  When the pulse compression process is applied to the received chirp wave subjected to such frequency-dependent attenuation as it is, a waveform after pulse compression as shown in FIG. 7 is obtained. Reference numeral 101 denotes a received signal (RF signal) after pulse compression, and reference numeral 102 denotes an envelope of the received signal. The horizontal axis is the time axis, and the vertical axis represents the amplitude. As indicated by reference numerals 103 and 104, unnecessary components, that is, side lobes are generated on both sides of the peak after pulse compression, and such side lobes cause deterioration in image quality.

  Therefore, it is desired to apply a frequency characteristic correction function as shown in FIG. The horizontal axis represents the frequency, and the vertical axis represents the filtering level or the correction level. By raising the higher frequency components, it is possible to obtain a corrected received chirp wave as shown in FIG. That is, the reception chirp wave shown in FIG. 9 is a reception chirp wave after the frequency characteristic correction function shown in FIG. 8 is applied. Yes. This is also a simulation result.

  FIG. 10 shows the result of performing pulse compression processing on such a corrected received chirp wave. Reference numeral 106 indicates a received signal (RF signal), and reference numeral 107 indicates an envelope thereof. As indicated by reference numerals 108 and 109, unnecessary components on both sides of the peak after pulse compression are considerably suppressed. Therefore, the image quality can be improved.

  Waveform correction based on the above simulation is performed on the premise of reception from a certain depth. In actual ultrasonic diagnosis, echoes from reception points at each depth are continuous. Will be observed. Therefore, it is necessary to dynamically change the frequency characteristic correction function according to the depth of the reception point. In addition, since the attenuation coefficient is different for each living tissue, it is necessary to dynamically change the frequency characteristic correction function based on the actual attenuation coefficient after investigating the attenuation coefficient in advance as described above. Such a change is shown in FIG. As shown in the figure, the frequency characteristic correction function F is dynamically varied according to the depth of the reception point. That is, the inclination is dynamically changed. A small inclination is set in a shallow region, and a large inclination is set in a deep portion. However, the attenuation coefficient α, which is one parameter that defines the function F, is a constant value regardless of the depth, and such a value is determined by actual measurement in the above-described preliminary investigation process.

  Therefore, according to the apparatus of the present embodiment, instead of using a uniform attenuation coefficient, the measured attenuation coefficient is used as it is, and both the existing chirp transmission and pulse compression techniques are used as they are, that is, the ultrasonic diagnostic apparatus. Since the attenuation coefficient can be obtained in consideration of actual characteristics, the image quality can be significantly improved as compared with the conventional method. In addition, since the function is changed according to the depth, good correction can be realized at each depth, that is, the image quality can be improved from a shallow region to a deep region. Since the information in the region of interest is referred to when determining the attenuation coefficient, it is possible to improve the image quality particularly in the region of interest. Therefore, if it is desired to optimize the image quality over the entire shallow region to deep region, a plurality of regions of interest may be determined for each depth, and an attenuation coefficient may be calculated for each to reflect the change in the function. . In the above-described embodiment, the sum of the luminance values in the region of interest is set as the evaluation value on the assumption that the peak value after pulse compression is standardized, but other information other than such an evaluation value is used. It is also possible to evaluate the image quality or the degree of occurrence of side lobes. For example, since various known techniques for evaluating the sharpness of an image have been established, the image quality may be evaluated using such techniques, and the side lobe component itself in the received signal on the time axis may be used. You may make it make it an evaluation object. In any case, it is possible to determine an optimum attenuation coefficient by obtaining an index indicating how accurately the pulse compression processing is performed with reference to the signal after pulse compression.

  10 probe, 12 region of interest, 14 transmission unit, 16 reception unit, 18 correction unit, 20 attenuation coefficient determination unit, 22 pulse compression unit, 24 image forming unit, 28 evaluation unit

Claims (5)

A transmitting / receiving unit that transmits a chirp wave, which is a frequency-modulated wave, to a subject, receives a reflected wave from the subject, and outputs a received signal;
A correction processing unit that performs a dynamic correction process according to the depth of the reception point in order to compensate the frequency-dependent attenuation of the living tissue for the received signal;
A pulse compression processing unit that applies a pulse compression process to the received signal after the correction process;
An image forming unit that forms an ultrasonic image based on the received signal after the pulse compression processing;
A plurality of pulse compression processing results are obtained by experimentally operating the pulse compression processing unit while sequentially setting a plurality of correction characteristics corresponding to a plurality of frequency-dependent attenuation coefficients in the correction processing unit. A coefficient estimator for estimating a frequency dependent attenuation coefficient for the living tissue in the subject by determining the best processing result;
An ultrasonic diagnostic apparatus comprising: The apparatus of claim 1.
The coefficient estimation unit includes an evaluation unit that individually evaluates the plurality of pulse compression processing results to obtain an evaluation value,
The ultrasonic diagnostic apparatus according to claim 1, wherein the evaluation value is a value indicating a degree of sidelobe content after pulse compression. The apparatus of claim 2.
The ultrasonic diagnostic apparatus according to claim 1, wherein the evaluation value is a sum of luminance values in the region of interest after the peak level normalization process. The device according to any one of claims 1 to 3,
An ultrasonic diagnostic apparatus comprising: a characteristic setting unit that sets correction characteristics suitable for the subject with respect to the correction processing unit based on the frequency-dependent attenuation coefficient estimated by the coefficient estimation unit. The apparatus of claim 4.
The ultrasonic diagnostic apparatus according to claim 1, wherein the correction characteristic is a function whose inclination changes according to a depth of the reception point.

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