JP2013180035A - Ultrasonograph - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an improvement technique related to filter processing along the array direction of multiple frames.SOLUTION: A large number of fixed echoes exist in an area shallower than a deep area including a heart in an ultrasonic image. An HPF processing part 20 performs filter processing by applying a high-pass filter, where characteristics are set in accordance with a depth in a frame, to frame data of the depth. The high-pass filter can be achieved by a digital filter, for example. Characteristics of the high-pass filter are adjusted by allowing a filter setting part 22 to set a filter coefficient, etc., of the digital filter. That is, the filter setting part 22 controls a filter coefficient in the HPF processing part 20 to set a higher offset level as a depth is larger.

Description

  The present invention relates to an ultrasonic diagnostic apparatus, and more particularly to a technique for improving the image quality of an ultrasonic image.

  The ultrasonic diagnostic apparatus is widely used for diagnosis of organs, blood vessels, fetuses, and the like, and plays an important role in, for example, heart diagnosis. By using the ultrasonic diagnostic apparatus, for example, it is possible to observe a beating heart with a moving ultrasonic image.

  In the ultrasound diagnosis of the heart, the chest wall, ribs, pericardium, etc. are present on the shallower side (probe side) than the heart, so fixed echoes (fixed echoes) resulting from these presences are present in the ultrasound image. Occur. In ultrasonic diagnosis focusing on the heart, fixed echo is a factor that degrades the image quality of an ultrasonic image. Therefore, techniques for reducing the fixed echo have been conventionally proposed.

  For example, in Patent Document 1, by applying a high-pass filter (HPF) along the arrangement direction of a plurality of frames to frame data obtained by transmitting and receiving ultrasonic waves, that is, by HPF processing between frames, There has been proposed a technique for reducing fixed echoes in which variations in the arrangement direction (time direction) of a plurality of frames are smaller than those of the heart.

  However, with simple interframe HPF processing, it is difficult to completely remove only fixed echoes and keep the echoes from the heart completely, and there are some residual fixed echoes and loss of echoes from the heart. This causes flickering in the video. Further, in the HPF processing between frames, processing equivalent to weighted addition between a plurality of frames is performed, so that a heart valve or the like that moves fast in the moving image looks double-triple, and the heart (myocardium) The border appears blurred.

  Therefore, for example, Patent Document 2 proposes a technique for further performing LPF processing (low-pass filter processing) between frames after the HPF processing between frames in order to suppress the flicker. That is, flickering is reduced by performing smoothing (smoothing) between frames by LPF processing. However, when LPF processing between frames is simply performed on the entire image, another problem such as the image being blurred as a whole occurs.

  Further, for example, in Patent Document 3, in order to suppress the above-described flickering and blurring, the weight of frame data after HPF processing is increased in a shallow region where many fixed echoes exist, and HPF processing is performed in a deep region where a heart exists. A technique for increasing the weight of the previous frame data and weighting and adding the frame data before and after the HPF process has been proposed. As a result, fixed echoes are reduced in shallow regions, and heart flickering and blurring are suppressed in deep regions. However, there remains a problem in terms of circuit scale, such as the need for a circuit configuration for weighted addition.

JP-A-8-107896 JP 2000-139909 A Japanese Patent Laying-Open No. 2005-288021

  In view of the background art described above, the inventor of the present application has repeatedly researched and developed a technique for improving the image quality of an ultrasonic image including a moving diagnostic object such as a heart. In particular, we focused on filter processing along the arrangement direction of multiple frames.

  The present invention has been made in the course of its research and development, and an object thereof is to realize an improved technique related to filter processing along the arrangement direction of a plurality of frames.

  An ultrasonic diagnostic apparatus suitable for the above object includes a probe that transmits and receives ultrasonic waves, a transmission and reception unit that obtains an ultrasonic reception signal by controlling the probe, and frame data obtained based on the ultrasonic reception signal. A filter processing unit configured to perform filtering along the arrangement direction of the plurality of frames; and an image forming unit configured to form an ultrasonic image based on the filtered frame data, wherein the filter processing unit has a depth in the frame. A high-pass filter having an offset level set according to the above is applied to the frame data at that depth to perform filtering.

  In the ultrasonic diagnostic apparatus, a high-pass filter (high-pass filter) in which an offset level is set according to the depth in the frame is applied to the frame data at that depth. The offset of the high-pass filter is to boost the gain characteristic of the high-pass filter, and the offset level is, for example, the gain value of the filter at the lower limit (for example, 0 Hz) of the frequency band to be processed by the high-pass filter. Defined by

  The offset level can be adjusted to a desired level relatively easily by changing the filter coefficient in a digital filter, for example. Therefore, according to the ultrasonic diagnostic apparatus, in improving the image quality of the ultrasonic image by the filtering process along the arrangement direction of the plurality of frames, the characteristics of the high-pass filter are adjusted relatively easily according to the depth. It becomes possible.

  In a preferred embodiment, the filter processing unit applies a high-pass filter in which an offset level higher than that in a shallow region is set to frame data in a deep region including a moving diagnostic object. .

  In a preferred embodiment, the filter processing unit applies a high-pass filter in which a higher offset level is set as it is deeper in the frame and closer to the characteristics of the all-pass filter as it is deeper.

  In a preferred embodiment, the image processing apparatus further includes a post-processing unit that applies a low-pass filter or a median filter along the arrangement direction of a plurality of frames to the frame data processed by the filter processing unit. .

  In a preferred embodiment, when applying the low-pass filter, the post-processing unit applies a low-pass filter in which an offset level is set according to the depth in the frame to the frame data at that depth. It is characterized by.

  In a preferred embodiment, when applying the median filter, the post-processing unit applies the median filter limitedly to frame data in a shallow region different from a deep region including a moving diagnostic object. And

  In a preferred embodiment, for the frame data processed by the filter processing unit, by confirming the change of the frame data along the arrangement direction of a plurality of frames at each position in the frame, the processing of the post-processing unit at that position It further has a judgment part which judges whether it is required.

  In a preferred embodiment, the filter processing unit applies a digital high-pass filter in which a filter coefficient is set according to a frame rate of frame data and a depth in the frame to the frame data.

  According to the present invention, an improved technique related to filter processing along the arrangement direction of a plurality of frames is realized. For example, according to a preferred aspect of the present invention, the characteristics of the high-pass filter can be adjusted relatively easily according to the depth.

1 is a diagram illustrating an overall configuration of an ultrasonic diagnostic apparatus that is preferable in the practice of the present invention. It is a figure which shows the specific example of a digital filter. It is a figure which shows the frequency characteristic of HPF comprised with a 3 tap FIR filter. It is a diagram showing a setting example of the filter coefficient k ₀ corresponding to the depth. It is a figure which shows the frequency characteristic of HPF comprised with a 2 tap FIR filter. It is a figure which shows the frequency characteristic of LPF comprised with a 2-tap FIR filter. It is a figure which shows the specific example of a median filter. It is a figure which shows the specific example of a determination part. It is a figure which shows the change of the frequency characteristic of HPF by a frame rate. It is a figure which shows the change of the frequency characteristic of LPF by a frame rate. It is a diagram showing a setting example of the filter coefficient k ₀ corresponding to the depth and frame rate (HPF). It is a diagram showing a setting example of the filter coefficient k ₀ corresponding to the depth and frame rate (LPF).

  FIG. 1 is an overall configuration diagram of an ultrasonic diagnostic apparatus (present ultrasonic diagnostic apparatus) suitable for implementing the present invention. The probe 10 transmits an ultrasonic wave to a diagnostic region including a tissue to be diagnosed and receives an ultrasonic wave reflected from the diagnostic region. The probe 10 includes a plurality of vibration elements that transmit and receive ultrasonic waves, and transmission of the plurality of vibration elements is controlled by the transmission / reception unit 12 to form a transmission beam. Further, the plurality of vibration elements receive the ultrasonic waves reflected from the diagnostic region, and signals obtained thereby are output to the transmission / reception unit 12, and the transmission / reception unit 12 forms a reception beam.

  The transmission / reception unit 12 outputs a transmission signal corresponding to each of the plurality of vibration elements included in the probe 10, thereby forming an ultrasonic transmission beam and scanning the transmission beam. Further, the transmission / reception unit 12 forms a reception beam corresponding to the transmission beam to be scanned by performing a phasing addition process or the like on the reception signal obtained from each of the plurality of vibration elements included in the probe 10, and receives the reception beam. Echo data (received signal) obtained along the beam is output.

  The transmission / reception unit 12 obtains frame data constituting a frame corresponding to the two-dimensional plane by scanning the ultrasonic beam (transmission beam and reception beam corresponding thereto) in the two-dimensional plane and collecting echo data. . Then, the transmission / reception unit 12 repeats scanning of the ultrasonic beam in the two-dimensional plane to obtain frame data over a plurality of frames. The frame data obtained in this way is stored, for example, in a memory or the like, read out from the memory or the like, and processed in the HPF processing unit 20 or the like at the subsequent stage.

  This ultrasonic diagnostic apparatus is suitable for diagnosis of a moving tissue such as a heart, for example, and has a function of improving the image quality of an ultrasonic image including a diagnosis target such as a heart. That is, fixed echoes (fixed echoes) associated with the chest wall, ribs, pericardium, etc. existing on the shallower side (probe 10 side) than the heart are reduced to improve the image quality of the ultrasound image.

  The HPF (High Pass Filter) processing unit 20 filters frame data obtained over a plurality of frames in order to reduce fixed echo. The HPF processing unit 20 applies a high-pass filter (high-pass filter) to the frame data along the arrangement direction of the plurality of frames, that is, by performing HPF processing between the frames, the arrangement direction of the plurality of frames (time direction). ) To reduce fixed echoes with less variation compared to the heart.

  In the ultrasonic image, there are many fixed echoes in a shallower region than a deep region including the heart. Therefore, the HPF processing unit 20 applies a high-pass filter whose characteristics are set according to the depth in the frame to the frame data at that depth, and performs a filtering process. The high-pass filter can be realized by a digital filter, for example, and the filter setting unit 22 adjusts the characteristics of the high-pass filter by setting the filter coefficient of the digital filter.

  FIG. 2 is a diagram illustrating a specific example of the digital filter. The frame data is filtered along the arrangement direction of a plurality of frames. That is, with respect to frame data at a certain position (coordinate) in the frame, the frame data at the same position is sequentially filtered over a plurality of frames.

FIG. 2 <A> shows an FIR filter (3-tap FIR filter) configured with 3 taps. Frame data over a plurality of frames are successively input to the 3-tap FIR filter. In FIG. 2 <A>, x _n−1 , x _n , and x _{n + 1} are frame data at the same position (coordinates) in the frame, and are frame data obtained from three consecutive frames.

When one frame of frame data is input to the 3-tap FIR filter, the one frame of frame data is stored in the frame memory 1. Next, when new frame data of one frame is input, the frame data stored in the frame memory 1 is stored in the subsequent frame memory 2, and the newly input frame data is stored in the previous frame. Stored in the memory 1. In this way, each time new frame data of one frame is inputted successively, the frame data is successively shifted from the preceding frame memory 1 to the succeeding frame memory 2. In FIG. 2 <A> a frame data x _n is the frame of interest, x _n-1 is a frame data obtained from the previous frame than x _n, x _{n + 1,} from the x _n Is also frame data obtained from the next frame.

The 3-tap FIR filter multiplies each of three frame data obtained from three consecutive frames by a filter coefficient, and further adds the three data after multiplication. That is, in FIG. 2 <A>, each of the frame data x _n−1 , x _n , and x _{n + 1} is multiplied by the filter coefficients k ₋₁ , k ₀ , and k ₁ , and further multiplied. The latter three pieces of data are subjected to addition processing, and the result y _n = k ₋₁ x _n−1 + k ₀ x _n + k ₁ x _{n + 1} is output as a result of the filtering process on the frame of interest.

FIG. 2 <B> shows an FIR filter (2-tap FIR filter) configured with two taps. Also in the case of the 2-tap FIR filter, frame data over a plurality of frames are successively input, and one frame of frame data is stored in the frame memory. In FIG. 2 <B>, x _n is the frame data of the frame of interest. Further, x _n and x _{n + 1} are frame data at the same position in the frame, and are frame data obtained from two consecutive frames.

Even in the case of the 2-tap FIR filter, each of frame data x _n and x _{n + 1} is multiplied by filter coefficients k ₀ and k ₁ , and the two further multiplied data are added. As a result of the filtering, the result y _n = k ₀ x _n + k ₁ x _{n + 1} is output.

  In this ultrasonic diagnostic apparatus, for example, a high-pass filter is realized by the 3-tap FIR filter or 2-tap FIR filter shown in FIG. 2, and the characteristics of the high-pass filter are adjusted by adjusting the filter coefficient. In FIG. 2, a 2-tap and 3-tap FIR filter is shown as a specific example of the digital filter, but a filter having four or more taps may be used. An IIR filter may be used instead of the FIR filter.

In the 3-tap FIR filter of FIG. 2, the filter coefficient is symmetric (k ₋₁ = k ₁ ), the gain characteristic (gain characteristic) with respect to frequency is monotonically increasing, and the gain in the highest band is 0 [dB]. When filter coefficients satisfying the following conditions (1-1) and (1-2) are set, a high-pass filter (HPF) having the characteristics shown in FIG. 3 is obtained.

FIG. 3 is a diagram illustrating frequency characteristics of an HPF configured with a 3-tap FIR filter (FIG. 2). In FIG. 3, the horizontal axis indicates the frequency, and Fs is the frame rate, that is, the sampling frequency of the frame data. The vertical axis represents the gain of the filter. Further, k ₋₁ , k ₀ , and k ₁ in FIG. 3 are filter coefficients of the 3-tap FIR filter (FIG. 2).

  According to the frequency characteristics shown in FIG. 3, the HPF offset level changes according to the filter coefficient. The HPF offset is to boost the gain characteristic of the HPF. In FIG. 3, the gain value of the filter at 0 Hz, which is the lower limit of the frequency band to be processed by the HPF, is defined as the offset level.

In FIG. 3, when the filter coefficient k ₀ regarding the center tap is 0.5, the offset level is minimized to 0, and the suppression effect on the low frequency side, that is, the suppression effect of the fixed echo is the largest. Further, as the filter coefficient k ₀ increases from 0.5, the offset level increases and the fixed echo suppression effect gradually decreases. When the filter coefficient k ₀ is 1.0, the offset level becomes 1 and the maximum, the low-frequency side suppression effect, that is, the fixed echo suppression effect is lost, and the characteristics of an all-pass filter that outputs the input signal as it is are obtained. .

In this ultrasonic diagnostic apparatus, the HPF offset level is set according to the depth of the frame data in the frame. Specifically, the filter setting unit 22 in FIG. 1 controls the filter coefficient k ₀ in the HPF processing unit 20 so that a higher offset level is set as the depth increases.

FIG. 4 is a diagram illustrating a setting example of the filter coefficient k ₀ according to the depth. 4, the horizontal axis represents the depth, and the vertical axis represents the value of the filter coefficient k _0. The depth is a depth within the frame of the frame data processed by the filter.

In the setting example shown in FIG. 4, the value of the filter coefficient k ₀ is set to p (0.5 ≦ p <1.0) in a shallow portion from the depth 0 (closest to the probe 10) to the depth d1. The value of the filter coefficient k ₀ is increased as the depth increases from the depth d1 to the depth d2, and the value of the filter coefficient k ₀ is set to q (p <q ≦ 1.0) in a deep part after the depth d2. ing. The depth d1 and smoothly changed the value of the filter coefficient k ₀ in the vicinity thereof may be varied smoothly the value of the filter coefficient k ₀ even in the depth d2 and the vicinity thereof. Further, it may be changed if necessary the value of the filter coefficient k ₀ in a shallow portion from the depth 0 depth d1, optionally the value of filter coefficients k ₀ even in the deep portion of the depth d2 later Can be changed. Also, the position of the depth d1 and a depth d2 by appropriately adjusting, it is also possible and to slow to abrupt changes in accordance with the depth of the filter coefficients k _0.

Further, when it is important to reduce the fixed echo reduction effect in a shallow region, the characteristic of the filter coefficient k ₀ shown in FIG. 4 is shifted in the direction of deepening as a whole, or the characteristic is reduced as a whole to k ₀ . What is necessary is just to shift to the direction where a value becomes small. Of course, both shifts may be mixed. Also, when emphasizing the effect of suppressing flickering and blurring, the characteristics shown in FIG. 4 are shifted in the direction of decreasing overall, or the characteristics are shifted in the direction of increasing the value of k ₀ as a whole. You can do it. Of course, both shifts may be mixed.

  If a 2-tap FIR filter is used instead of the 3-tap FIR filter, the circuit configuration and the like can be further simplified. For example, in the 2-tap FIR filter of FIG. 2, the gain characteristics (gain characteristics) with respect to the frequency are monotonically increasing, and the following conditions (2-1) and (2-2) are set, where the gain in the highest band is 0 [dB]. When the filter coefficient to be satisfied is set, a high-pass filter (HPF) having the characteristics shown in FIG. 5 is obtained.

FIG. 5 is a diagram showing frequency characteristics of an HPF configured with a 2-tap FIR filter (FIG. 2). As in the case of FIG. 3, also in FIG. 5, the horizontal axis indicates the frequency, and Fs is the frame rate, that is, the sampling frequency of the frame data. The vertical axis represents the gain of the filter. Further, k ₀ and k ₁ in FIG. 5 are filter coefficients of the 2-tap FIR filter (FIG. 2). Also in the frequency characteristics shown in FIG. 5, the HPF offset level changes according to the filter coefficient.

That is, when the gain value of the filter at 0 Hz, which is the lower limit of the frequency band to be processed by the HPF, is defined as the offset level, also in FIG. 5, when the filter coefficient k ₀ is 0.5, the offset level is minimized to 0. Therefore, the suppression effect on the low frequency side, that is, the fixed echo suppression effect is the largest. Further, as the filter coefficient k ₀ increases from 0.5, the offset level increases and the fixed echo suppression effect gradually decreases. When the filter coefficient k ₀ is 1.0, the offset level becomes 1 and the maximum, the low-frequency side suppression effect, that is, the fixed echo suppression effect is lost, and the characteristics of an all-pass filter that outputs the input signal as it is are obtained. .

Also in the case of the 2-tap FIR filter, the HPF offset level is set according to the depth of the frame data in the frame. That is, the filter coefficient k ₀ is controlled by the filter setting unit 22 in FIG. 1 so that a higher offset level is set as the depth increases. In the control, the setting example of the filter coefficient k ₀ described with reference to FIG. 4 can be applied.

  By using the high-pass filter (high-pass filter) described with reference to FIGS. 2 to 5, a relatively simple configuration, that is, a complicated circuit configuration such as weighted addition processing (for example, see Patent Document 3). Without the need, it is possible to achieve both reduction of fixed echo and suppression of flickering and blurring of an image.

  Returning to FIG. 1, the frame data of a plurality of frames subjected to the interframe HPF processing in the HPF processing unit 20 is detected in the detection processing unit 30 and then sent to the post-processing unit 40. The detection processing unit 30 executes a known detection process.

  Although the reduction in fixed echo and the suppression of image flickering and blurring have already been realized by the processing in the HPF processing unit 20, the post-processing unit 40 can further suppress the image flickering in the ultrasonic diagnostic apparatus. . The post-processing unit 40 applies a low-pass filter or a median filter to the frame data along the arrangement direction of the plurality of frames.

  When the post-processing unit 40 uses a low-pass filter (LPF), the low-pass filter can be realized by the 3-tap FIR filter or the 2-tap FIR filter shown in FIG. Of course, a filter having four or more taps may be used, or an IIR filter may be used instead of the FIR filter.

  For example, in the 2-tap FIR filter of FIG. 2, the gain characteristic (gain characteristic) with respect to the frequency is monotonously decreased and the gain of the DC component (frequency 0 Hz component) is 0 [dB] (3-1) (3 -2) is set, a low-pass filter (LPF) having the characteristics shown in FIG. 6 is obtained.

FIG. 6 is a diagram showing the frequency characteristics of an LPF composed of a 2-tap FIR filter (FIG. 2). In FIG. 6, the horizontal axis indicates the frequency, and Fs is the frame rate, that is, the sampling frequency of the frame data. The vertical axis represents the gain of the filter. Further, k ₀ and k ₁ in FIG. 6 are filter coefficients of the 2-tap FIR filter (FIG. 2).

  According to the frequency characteristics shown in FIG. 6, the offset level of the LPF changes according to the filter coefficient. The LPF offset is to increase the gain characteristic of the LPF. In FIG. 6, the gain value of the filter at Fs / 2, which is the upper limit of the frequency band to be processed by the LPF, that is, a half of the sampling frequency, is defined as the offset level.

In FIG. 6, when the filter coefficient k ₀ is 0.5, the offset level is minimized to 0, and the suppression effect on the high frequency side, that is, the smoothing effect is the largest. By increasing the smoothing effect in the arrangement direction of a plurality of frames, the flicker suppression effect is also increased. On the other hand, as the filter coefficient k ₀ increases from 0.5, the offset level increases and the smoothing effect gradually decreases. When the filter coefficient k ₀ is 1.0, the offset level becomes 1 and the maximum, the high-frequency side suppression effect, that is, the smoothing effect is eliminated, and the characteristics of the all-pass filter that outputs the input signal as it is are obtained.

In this ultrasonic diagnostic apparatus, the offset level of the LPF is set according to the depth of the frame data in the frame. Specifically, the filter coefficient k ₀ in the post-processing unit 40 is controlled by the filter setting unit 42 in FIG. 1 so that a higher offset level is set as the depth increases. In the control, the setting example of the filter coefficient k ₀ described with reference to FIG. 4 can be applied.

When the post-processing unit 40 uses a median filter, for example, a median filter having a configuration shown in FIG. 7 is used. FIG. 7 is a diagram illustrating a specific example of the median filter. Frame data over a plurality of frames is successively input to the median filter. In FIG. 7, x _n is the frame data of the frame of interest. Further, x _n−1 , x _n , and x _{n + 1} are frame data at the same position (coordinates) in the frame, and are frame data obtained from three consecutive frames.

When one frame of frame data is input to the median filter, the one frame of frame data is stored in the frame memory 1. Next, when new frame data of one frame is input, the frame data stored in the frame memory 1 is stored in the subsequent frame memory 2, and the newly input frame data is stored in the previous frame. Stored in the memory 1. In this way, each time new frame data of one frame is inputted successively, the frame data is successively shifted from the preceding frame memory 1 to the succeeding frame memory 2. In FIG. 7, x _n-1 is the frame data obtained from the previous frame than x _n, x _{n + 1} is a frame data obtained from one after frame than x _n.

The median filter selects and outputs an intermediate value from three frame data obtained from three consecutive frames. That is, in FIG. 7, data that is an intermediate value among x _n−1 , x _n , and x _{n + 1} is selected and output as the processing result y _{n in the} frame of interest.

  The median filter can suppress flickering while suppressing blurring of the image. However, it is preferable to apply the median filter limitedly to a shallow region in the frame, rather than applying the median filter to the entire image. In other words, it is desirable not to apply a median filter to a deep region where the heart exists so as to prevent bleeding from occurring in a heart valve or the like. For example, a depth threshold may be provided, and the median filter may not be applied to frame data at a position deeper than the threshold.

  Further, the determination unit 50 in FIG. 1 may determine whether or not the processing in the post-processing unit 40 is necessary. In this case, the determination unit 50 confirms the change of the frame data along the arrangement direction of a plurality of frames at each position in the frame for the frame data processed by the HPF processing unit 20 and obtained through the detection processing unit 30. By doing so, it is determined whether or not the processing of the post-processing unit 40 is required at that position.

FIG. 8 is a diagram illustrating a specific example of the determination unit 50. Frame data over a plurality of frames is sequentially input to the determination unit 50. In FIG. 8, x _n−1 , x _n , and x _{n + 1} are frame data at the same position (coordinates) in the frame, and are frame data obtained from three consecutive frames.

When one frame of frame data is input to the determination unit 50, the one frame of frame data is stored in the frame memory 1. Next, when new frame data of one frame is input, the frame data stored in the frame memory 1 is stored in the subsequent frame memory 2, and the newly input frame data is stored in the previous frame. Stored in the memory 1. In this way, each time new frame data of one frame is inputted successively, the frame data is successively shifted from the preceding frame memory 1 to the succeeding frame memory 2. 8, a frame data x _n is the frame of interest, x _n-1 is the frame data obtained from the previous frame than x _n, x _{n + 1} is after one than x _n This is frame data obtained from the frame.

Average value calculation unit, an average value A _n of the absolute value before and after the frame data of the frame of interest is calculated by the following equation. M in the following equation is the number of frames before or after the frame of interest, and M = 1 in the specific example of FIG.

Then, the determination processing section, the absolute values for the frame data of the frame of interest, compares the average value A _n calculated in the average value calculating unit, based on the following condition, for the frame data of the frame of interest, flickering It is determined whether or not suppression processing is required.

In the condition (5-1) (5-2), T H is the threshold for flicker determination. For example, when the condition (5-1) is satisfied, it is determined that the flicker suppression process is necessary for the frame data _xn of the frame of interest. In place of the condition (5-1), it may be utilized standardized conditions (5-2) by the mean value A _n.

Returning to FIG. 1, the post-processing unit 40 performs the flicker suppression process on the frame data of the target frame determined to require the flicker suppression process by the determination unit 50, and the attention determined that the flicker suppression process is not necessary. The flicker suppression process is not executed for the frame data of the frame. When the flicker process is executed, for example, the LPF described with reference to FIG. 6 or the median filter described with reference to FIG. 7 is applied to perform the flicker suppression process. Further, the post-processing unit 40 of FIG. 1, as a result of flickering suppression processing for a frame data of the frame of interest, may output the average value A _n obtained in (4) below.

  The logarithmic compression processing unit 60 performs a known logarithmic compression process on the frame data of a plurality of frames obtained from the post-processing unit 40. In this way, the frame data of a plurality of frames subjected to logarithmic compression processing is sent to the display unit 70, and for example, an ultrasonic image representing a moving heart as a moving image is displayed on the display unit 70. Note that the post-processing unit 40 may be provided after the logarithmic compression processing unit 60.

  As described above, according to the present ultrasonic diagnostic apparatus, the HPF processing unit 20 can reduce fixed echoes and suppress image flickering and blurring, and the post-processing unit 40 can further suppress image flickering. In the processing, the HPF processing unit 20 uses a high-pass filter whose characteristic is set according to the depth in the frame, and the post-processing unit 40 is set with the characteristic according to the depth in the frame. A low-pass filter is used. That is, the characteristics of the high-pass filter and the low-pass filter are adjusted according to the depth in the frame. When a high-pass filter or a low-pass filter is realized by a digital filter, it is desirable to consider the frame rate of frame data in addition to the adjustment according to the depth.

FIG. 9 is a diagram showing changes in the frequency characteristics of the HPF depending on the frame rate, and shows how the frequency characteristics of the HPF configured by the 3-tap FIR filter (FIG. 2) change according to the frame rate. In FIG. 9, the horizontal axis indicates the frequency, and the vertical axis indicates the gain of the filter. Note that Fs is a frame rate (frame frequency), that is, a sampling frequency of frame data. Further, the frequency characteristics shown in FIG. 9 are characteristics obtained by each frame rate Fs with the filter coefficients k ₋₁ , k ₀ , and k ₁ of the 3-tap FIR filter of FIG. 2 set to −0.25, 0.50, and −0.25, respectively. .

On the other hand, FIG. 10 is a diagram showing changes in the frequency characteristics of the LPF depending on the frame rate, and shows how the frequency characteristics of the LPF composed of the 3-tap FIR filter (FIG. 2) change according to the frame rate. . In FIG. 10, the horizontal axis indicates the frequency, and the vertical axis indicates the filter gain. Note that Fs is a frame rate (frame frequency), that is, a sampling frequency of frame data. Also, the frequency characteristics shown in FIG. 10 are characteristics obtained by the respective frame rates Fs with the filter coefficients k ₋₁ , k ₀ , and k ₁ of the 3-tap FIR filter of FIG. 2 being 0.25, 0.50, and 0.25, respectively.

  As shown in FIGS. 9 and 10, in the digital filter, if the filter coefficient remains fixed, the frequency characteristic changes according to the size of the frame rate Fs. That is, as the frame rate Fs increases (becomes higher), the frequency characteristics are stretched in the frequency axis direction and the cutoff frequency tends to increase.

  Therefore, in this ultrasonic diagnostic apparatus, for example, the filter coefficient of the digital filter illustrated in FIG. 2 is adjusted according to the magnitude of the frame rate Fs set by a user operation or the like.

When the number of taps is fixed, for example, the frequency characteristics of the HPF configured by the 3-tap FIR filter of FIG. 2 are as illustrated in FIG. FIG. 3 shows that the HPF offset level increases as the filter coefficient k ₀ increases from 0.5. And the cutoff frequency of HPF becomes low, so that an offset level rises. That is, the cutoff frequency of HPF can be lowered by increasing the offset level.

  On the other hand, as described with reference to FIG. 9, as the frame rate Fs increases, the frequency characteristics of the HPF tend to be stretched in the frequency axis direction to increase the cutoff frequency. That is, the cutoff frequency tends to increase as the frame rate Fs increases.

  Therefore, in order to suppress the tendency of the cutoff frequency to increase as the frame rate Fs increases, the filter coefficient is adjusted in such a direction that the offset level is increased and the cutoff frequency decreases as the frame rate Fs increases. As a result, it is possible to perform control so that the cutoff frequency of the HPF does not vary as much as possible according to the variation of the frame rate Fs. The HPF filter coefficient used in the HPF processing unit 20 in FIG. 1 is adjusted by the filter setting unit 22.

When the number of taps is fixed, for example, the frequency characteristics of the LPF configured by the 2-tap FIR filter of FIG. 2 are as illustrated in FIG. FIG. 6 shows that the offset level of the LPF increases as the filter coefficient k ₀ increases from 0.5. The cutoff frequency of the LPF increases as the offset level increases. That is, the cutoff frequency of the LPF can be increased by increasing the offset level.

  On the other hand, as described with reference to FIG. 10, as the frame rate Fs increases, the frequency characteristics of the LPF tend to be stretched in the frequency axis direction and the cutoff frequency tends to increase. That is, the cutoff frequency tends to increase as the frame rate Fs increases.

  Therefore, in order to suppress the tendency of the cutoff frequency to increase as the frame rate Fs increases, the filter coefficient is adjusted in the direction in which the cutoff level decreases as the frame rate Fs increases. As a result, it is possible to control the cut-off frequency of the LPF so as not to fluctuate as much as possible according to the fluctuation of the frame rate Fs. Note that the filter coefficient of the LPF used in the post-processing unit 40 in FIG. 1 is adjusted by the filter setting unit 42.

  Furthermore, in addition to the filter coefficient, the number of filter taps may be adjusted. For example, in FIG. 9, when the frequency characteristic with a frame rate of 30 Hz is effective for reducing fixed echo and suppressing flickering and blurring of an image, even when the frame rate is 40 Hz and 20 Hz, the frequency characteristic approaches 30 Hz. Thus, the filter coefficient and the number of taps are adjusted. Specifically, for example, when the frame rate is increased and changed from 30 Hz to 40 Hz, the cut-off frequency at 40 Hz is not increased (so as to approach the cut-off frequency at 30 Hz), and the frequency characteristics at 40 Hz. The filter coefficient and the number of taps are changed so as not to be gradual (to approach the frequency characteristics in the case of 30 Hz).

  Also in FIG. 10, for example, when the frequency characteristic with a frame rate of 30 Hz is effective in suppressing flickering, the filter coefficient and the number of taps so as to approach the frequency characteristic of 30 Hz even when the frame rate is 40 Hz or 20 Hz. Is adjusted.

Furthermore, in the HPF, in order to achieve both control of the filter coefficient according to the depth and control of the filter coefficient according to the frame rate, the setting example illustrated in FIG. 11 is referred to. For example, when the characteristics of the filter coefficient k ₀ corresponding to the depth are set at the reference frame rate (for example, 30 Hz), if the frame rate is increased (for example, 40 Hz), the graph shown in FIG. The characteristic of the filter coefficient k ₀ of the HPF shown in the figure is shifted in the direction of becoming generally shallower, or the characteristic is shifted in the direction of increasing the value of k ₀ as a whole. Of course, both shifts may be mixed. On the other hand, when the frame rate is low (for example, is the 20 Hz), or to shift the overall Deeper direction characteristics of the filter coefficient k ₀ of the HPF shown in FIG. 11, or, entirely k ₀ and the characteristics The value is shifted in the direction of decreasing. Of course, both shifts may be mixed.

On the other hand, in the LPF, in order to achieve both control of the filter coefficient according to the depth and control of the filter coefficient according to the frame rate, the setting example shown in FIG. 12 is referred to. For example, when the characteristics of the filter coefficient k ₀ corresponding to the depth are set at the reference frame rate (for example, 30 Hz), when the frame rate is increased (for example, 40 Hz), FIG. The characteristic of the filter coefficient k ₀ of the LPF shown is shifted in the direction of becoming deeper as a whole, or the characteristic is shifted in the direction of decreasing the value of k ₀ as a whole. Of course, both shifts may be mixed. On the other hand, when the frame rate is lowered (for example, 20 Hz), the characteristics of the LPF filter coefficient k ₀ shown in FIG. Shift in a direction that increases the value of ₀ . Of course, both shifts may be mixed.

  As a result, the HPF processing unit 20 achieves reduction of fixed echoes and suppression of image flickering and blurring, and further suppresses image flickering in the post-processing unit 40 while suppressing changes in characteristics due to changes in the frame rate. Can do.

  As mentioned above, although preferred embodiment of this invention was described, embodiment mentioned above is only a mere illustration in all the points, and does not limit the scope of the present invention. The present invention includes various modifications without departing from the essence thereof.

  10 probe, 12 transmission / reception unit, 20 HPF processing unit, 40 post-processing unit, 50 determination unit.

Claims (8)

A probe for transmitting and receiving ultrasound,
A transmission / reception unit that obtains an ultrasonic reception signal by controlling the probe; and
A filter processing unit for filtering the frame data obtained based on the ultrasonic reception signal along the arrangement direction of a plurality of frames;
An image forming unit that forms an ultrasonic image based on the filtered frame data;
Have
The filter processing unit applies a high-pass filter, in which an offset level is set according to the depth in the frame, to the depth frame data, and performs filter processing.
An ultrasonic diagnostic apparatus. The ultrasonic diagnostic apparatus according to claim 1,
The filter processing unit applies a high-pass filter in which an offset level higher than a shallow region is set to frame data of a deep region including a moving diagnostic object.
An ultrasonic diagnostic apparatus. The ultrasonic diagnostic apparatus according to claim 1 or 2,
The filter processing unit applies a high-pass filter that is set closer to the characteristics of the all-pass filter as the deeper in the frame, the higher the offset level is set.
An ultrasonic diagnostic apparatus. The ultrasonic diagnostic apparatus according to any one of claims 1 to 3,
A post-processing unit that applies a low-pass filter or a median filter along the arrangement direction of a plurality of frames to the frame data processed by the filter processing unit,
An ultrasonic diagnostic apparatus. The ultrasonic diagnostic apparatus according to claim 4,
In applying the low-pass filter, the post-processing unit applies a low-pass filter whose offset level is set according to the depth in the frame to the frame data of the depth.
An ultrasonic diagnostic apparatus. The ultrasonic diagnostic apparatus according to claim 4,
In applying the median filter, the post-processing unit applies the median filter in a limited manner to frame data in a shallow region different from a deep region including a moving diagnostic object.
An ultrasonic diagnostic apparatus. The ultrasonic diagnostic apparatus according to claim 4,
For the frame data processed by the filter processing unit, whether the post-processing unit needs to be processed at each position by checking the change of the frame data along the arrangement direction of a plurality of frames at each position in the frame. A determination unit for determining whether or not
An ultrasonic diagnostic apparatus. The ultrasonic diagnostic apparatus according to any one of claims 1 to 7,
The filter processing unit applies a digital high-pass filter in which a filter coefficient is set according to a frame rate of frame data and a depth in the frame to the frame data.
An ultrasonic diagnostic apparatus.

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