JP2024011497A - Ultrasonic imaging device and reflection wave noise removal method - Google Patents

Abstract

To provide an ultrasonic imaging device which can generate highly accurate image information of a junction interface of an object by generating pixelizing information with a waveform from which noise is removed.SOLUTION: An ultrasonic imaging device 100 of acquiring a reflection wave of a junction interface of an object by irradiating the object formed by being laminated in multiple layers with the ultrasonic wave and generating an image of the junction interface based on the signal intensity of the reflection wave comprises: a matching processing unit 56 which corrects a deviation of a phase of the second reflection wave from the second irradiation point in the object with respect to the first reflection wave from the first irradiation point in the object; and a pixelizing information generation processing unit (averaging processing unit 57) which removes noise from the first reflection wave on the basis of the first reflection wave and the second reflection wave after phase correction and generates pixelizing information of a portion that returns an interface echo in the junction interface on the basis of the signal intensity of the interface echo indicating a waveform from the junction interface of the first reflection wave from which the noise is removed.SELECTED DRAWING: Figure 1

Description

The present invention relates to an ultrasound imaging device and a method for removing noise from reflected waves.

An ultrasonic imaging device irradiates an object to be inspected (subject) with ultrasonic waves from a probe, receives the reflected waves, and images the target interface. In an ultrasonic inspection apparatus that repeatedly sends and receives ultrasonic waves to an object to be inspected, a method has been proposed to sharpen the object waveform by averaging similar reflected waves to remove noise.

Patent Document 1 describes an ultrasonic transmitter that repeatedly transmits ultrasonic waves to an object to be inspected, and an ultrasonic wave transmitter that repeatedly receives ultrasonic waves to be inspected that have been transmitted from the ultrasonic transmitter and propagated through the object to be inspected. A repetition interval setting unit that sets a repetition interval between the ultrasound transmission by the ultrasound reception unit and the ultrasound transmission unit, the repetition interval setting unit being configured to shift the repetition interval by a fixed amount each time the ultrasound transmission unit repeats transmission of the ultrasound. A repetition interval setting unit that increases or decreases the frequency by increments, and an averaging process that averages the ultrasound to be inspected that has been repeatedly received by the ultrasound receiver by synchronizing the timing at which the ultrasound transmitter starts transmitting the ultrasound. , and the shift amount is a value that satisfies the condition that the index representing the absolute value of the ratio of the reverberation component superimposed on the ultrasound to be inspected before and after averaging by the averaging processing section is minimum or below a predetermined value. An ultrasonic inspection apparatus is disclosed.

JP2013-29396A

In the technique disclosed in Patent Document 1, the influence of jitter (fluctuation in the time axis direction) is not taken into account. In an ultrasonic imaging device, an object is irradiated with ultrasonic waves and reflected waves thereof are obtained, but these reflected waves have a phase shift due to jitter. If averaging is performed in a state where a phase shift has occurred, the signal strength of the necessary waveform may be weakened. As a result, the problem arises that defects that should be found are overlooked.

The present invention has been made in view of the above-mentioned problems, and is an ultrasonic image capable of generating highly accurate image information of a joint interface of an object by generating pixelated information using a waveform from which noise has been removed. The object of the present invention is to provide a device and a method for removing noise from reflected waves.

In order to achieve the above object, the ultrasonic imaging device of the present invention irradiates an object made of multiple layers with ultrasonic waves, obtains reflected waves from the joint interface of the object, and detects the reflected waves. An ultrasonic imaging device that generates an image of the bonding interface based on signal intensity, the ultrasound imaging device generating an image of the bonding interface based on a signal intensity, wherein a matching processing unit that corrects a phase shift of the second reflected wave, and a matching processing unit that removes noise from the first reflected wave based on the first reflected wave and the second reflected wave after phase correction; a pixelated information generation processing unit that generates pixelated information of a portion of the bonded interface that returns the interface echo based on the signal strength of an interface echo indicating a waveform from the bonded interface among the removed first reflected waves; It is characterized by having the following. Other aspects of the present invention will be explained in the embodiments described below.

According to the present invention, by generating pixelated information using a waveform from which noise has been removed, highly accurate image information of a joint interface of a multilayered object can be generated.

FIG. 1 is a diagram showing the configuration of an ultrasound imaging device according to the present embodiment. FIG. 3 is a diagram showing an interface echo, which is a reflected wave, and noise. It is an irradiation point arrangement diagram, and is a diagram showing the arrangement of a first irradiation point and a second irradiation point. FIG. 3 is a diagram illustrating an example of averaging processing. It is a figure showing an example of matching processing. 3 is a flowchart illustrating noise removal processing of reflected waves. FIG. 3 is a diagram showing waveform data after averaging processing. FIG. 3 is a diagram showing an image before averaging processing. FIG. 3 is a diagram showing an image after averaging processing.

Embodiments for implementing the present invention will be described in detail with reference to the drawings as appropriate.
FIG. 1 is a diagram showing the configuration of an ultrasound imaging apparatus 100 according to this embodiment. FIG. 2 is a diagram showing an interface echo, which is a reflected wave, and noise. The ultrasonic imaging device 100 is used to find defects at the bonding interface of a multilayered object such as a multilayered semiconductor.

The ultrasound imaging apparatus 100 includes a control section 50, a scanner section 70 (mechanical section), and an ultrasound probe 20. The ultrasound imaging device 100 irradiates ultrasonic waves to irradiation points set at predetermined intervals in an inspection range of a target object via a probe, obtains the reflected waves, and selects objects to be inspected from among the reflected waves. Extracts the interface echo showing the waveform of the bonded interface, converts the signal intensity of the interface echo to a positive integer value (0 to 255), and generates pixelization information. An image of the bonding interface is generated based on the generated pixelated information of the irradiation point to find defects.

However, as shown in the waveform data 31 of FIG. 2, noise such as electrical noise may be superimposed on the reflected wave in addition to surface echoes and interface echoes. Since these noises affect the generation of pixelated information of interface echoes, it is necessary to remove them as much as possible.

Returning to FIG. 1, the ultrasound probe 20 includes an encoder 21 that detects the scanning position of the ultrasound probe 20, and a piezoelectric element 22 that mutually converts electrical signals and ultrasound signals. The piezoelectric element 22 is a single focus type ultrasonic sensor.

The control unit 50 generates ultrasonic waves based on pixelated information generated by a scanning control unit 51 that controls the scanning position of the ultrasound probe 20, a transmission/reception control unit 52 that controls transmission and reception of ultrasound waves, and an averaging processing unit 57. The image generation unit 53 generates an image, and the phase difference of the second reflected wave from the second irradiation point on the subject 15 with respect to the first reflected wave from the first irradiation point on the subject 15 (object). A matching processing unit 56 that corrects the deviation removes noise from the first reflected wave based on the first reflected wave and the second reflected wave after phase correction, and the noise is removed from the first reflected wave from which the noise has been removed. an averaging processing unit 57 (pixelated information generation processing unit) that generates pixelated information of a portion of the bonded interface that returns an interface echo based on the signal strength of the interface echo indicating a waveform from the bonded interface; and a storage unit 58; It includes a transmitting/receiving section 60 and the like. Note that the first irradiation point and the second irradiation point will be described later with reference to FIG. 3. The averaging process will also be explained later.

Although not shown, the transmitter/receiver 60 includes a transmitter, an amplifier that amplifies the received signal received by the ultrasound probe 20, an A/D converter that converts the received signal from an analog signal to a digital signal, and a transmitter that amplifies the received signal received by the ultrasound probe 20. It is equipped with a signal processing section etc. that performs signal processing.

The scanning control section 51 is connected to a mechanical control device 77 so as to be capable of inputting and outputting. The scan control unit 51 controls the scanning position of the ultrasonic probe 20 using a mechanical control device 77 , an Receive scanning position information.

The output side of the mechanical control device 77 is connected to an X-axis scanner 71, a Y-axis scanner 72, and a Z-axis scanner 73. The output side of the encoder 21 of the ultrasonic probe 20 is connected to the mechanical control device 77 . The mechanical control device 77 detects the scanning position of the ultrasound probe 20 based on the output signal of the encoder 21, and detects the scanning position of the ultrasound probe 20 using the X-axis scanner 71, Y-axis scanner 72, and Z-axis scanner 73. control so that The mechanical control device 77 receives control instructions for the ultrasound probe 20 from the scan control unit 51 and also responds with scanning position information of the ultrasound probe 20 .

The piezoelectric element 22 has electrodes attached to both sides of a piezoelectric film, and is made of zinc oxide (ZnO), ceramics, fluorine-based copolymer, or the like. The piezoelectric element 22 transmits ultrasonic waves from the piezoelectric film when a voltage is applied between both electrodes. Furthermore, the piezoelectric element 22 converts the echo wave (received wave) received by the piezoelectric film into a received signal that is a voltage generated between the two electrodes.

Water 11 is poured into the water tank 10, and a subject 15 is placed submerged in the water 11. The water 11 in the water tank 10 is a propagation medium necessary for efficiently propagating the ultrasonic waves emitted from the opening surface of the lower end of the ultrasonic probe 20 (ultrasonic probe) into the interior of the subject 15. It is a liquid substance. The test object 15 is, for example, a wafer, a semiconductor package including a multilayer structure (or a laminated structure), or the like.

The ultrasonic probe 20 is immersed in the water 11 filled in the water tank 10 and is disposed so as to face the upper part of the subject 15 in the Z direction at a predetermined distance.

The scanner section 70 allows the ultrasound probe 20 to be freely moved in the XYZ directions. For example, the ultrasound probe 20 scans the subject 15 in the X-axis direction from the starting point (one end point) to the end point (the other end point) at a predetermined speed while irradiating the subject 15 with ultrasonic waves. When the ultrasonic probe 20 reaches the end point, the probe is moved a predetermined amount in the Y-axis direction and scans in the X-axis direction from the viewpoint to the end point in the opposite direction at a predetermined speed.

Based on this movement, the ultrasonic probe 20 scans a predetermined measurement range on the surface of the subject 15, transmits ultrasonic waves, and generates reflected echo waves at a plurality of preset measurement points within the measurement range. can be received, and defects in the internal structure included in the measurement range can be visualized and inspected.

As shown in FIG. 1, the noise removal processing of the reflected waves in this embodiment is performed in two stages: processing in the matching processing section 56 (matching processing) and processing in the averaging processing section 57 (averaging processing). The feature is that it is processed by

In the averaging processing unit 57 in FIG. 1, noise is removed by an averaging method (details will be described later) in which reflected waves obtained from multiple irradiation points are added and divided by the number of added irradiation points (noise removal step). ). Although the noise is generated randomly and temporally uncorrelated in each reflected wave, the interface echoes are acquired at the same time. Therefore, when these reflected waves are added, the interface echo becomes n times (n is the number of added irradiation points) and the noise becomes smaller. Then, by averaging these results, the interface echo becomes clearer.

FIG. 3 is an irradiation point arrangement diagram, which shows the arrangement of the first irradiation point and the second irradiation point. In Figure 3, the irradiation points for which pixelated information is sought (pixelated irradiation points) are indicated by ● as the first irradiation points, and the other irradiation points (non-pixelated irradiation points) are indicated by ○ as the second irradiation points. Showing. In this embodiment, addition and averaging processing is performed as follows by averaging processing.

The control unit 50 selects and sets one or more second irradiation points to be used for the averaging process from among those adjacent to the first irradiation point. The reason why the effect of sharpening can be obtained by using adjacent irradiation points is that the reflected waves to be added have similar properties. That is, in the ultrasound imaging apparatus 100, the scanning pitch of the scanner is often set to be smaller than the focal size of the ultrasound. As a result, the difference in acquired reflected waves due to a difference in one pitch is very small. If the reflected waves to be added have similar properties, they will not cancel each other out by addition. On the other hand, since temporally random noise is reduced by addition, the effect of sharpening can be obtained. In the embodiment, when P0 is the first irradiation point in FIG. 3, one or more second irradiation points used for the averaging process are selected from P1 to P8. Note that the larger the number of selected pixels, the clearer the pixelated information can be obtained, but the longer the processing time, so the selection should be made in consideration of the overall processing time.

FIG. 4 is a diagram showing an example of averaging processing (noise removal processing). For example, the averaging process using the first irradiation point (P0) and eight adjacent second irradiation points (P1 to P8) is performed as follows. The reflected waves from P0 and P1 to P8 are stored in the storage unit 58 (see FIG. 1) as time-axis values (time based on the reflected wave acquisition time) and signal strength time series data (time, signal strength). Remember. Add the signal strength of the time series data of P0 and the signal strength of the time series data of P1 to P8 at the same time, and the number of irradiation points (the number of the first irradiation points and the number of the second irradiation points) averaged by dividing by 9, which is the sum of the numbers of . This processing is applied to the signal intensities at all times in the time-series data, noise is removed, the interface echo of the reflected wave at the first irradiation point is made clearer, and the signal intensities are obtained.

The averaging process in this embodiment has been described above, but in actual processing, time factors are taken into consideration. In other words, it is necessary to take measures against the influence of jitter, which is fluctuation in the time axis direction. In the reflected waves obtained by irradiating an object with ultrasonic waves, minute fluctuations occur in the waveform in the time axis direction. In other words, these minute fluctuations on the time axis are unique to each reflected wave, so if the reflected wave from the pixelated irradiation point is used as the reference, the reflected wave from the non-pixelated irradiation point used for averaging processing will result in a phase shift. If averaging processing is performed with a phase shift occurring, a phenomenon will occur in which the waveforms of interface echoes for which pixelated information should be generated are not added properly.

FIG. 5 is a diagram showing an example of matching processing. The waveform data 32 is a waveform before performing matching processing, and the waveform data 33 is a waveform after performing matching processing. In this embodiment, a countermeasure is taken using the matching process shown in FIG. The reflected wave from the pixelated irradiation point is used as a template signal, and the phase of the reflected wave from the non-pixelated irradiation point is matched to the template signal. Specifically, the peak of the interface echo from the target joint interface in the reflected wave from the non-pixelated irradiation point targeted for averaging processing is the peak of the interface echo from the target joint interface in the template signal. Adjust the position of the time axis so that the parts match, and eliminate the phase shift.

However, although this method can be used when the signal strength of the interface echo from the target joint interface is sufficiently large, it is difficult to extract the target interface echo when the signal strength is small. Therefore, in that case, the surface echo that can be extracted first among the reflected waves is used. In other words, the time axis position is adjusted so that the peak part of the surface echo in the averaged signal matches the peak part of the surface echo in the reflected wave from the non-pixelated irradiation point targeted for averaging processing, and the phase Eliminate misalignment.

In the matching process, whether to use the interface echo from the target joint interface or the surface echo is determined depending on whether the signal intensity of the interface echo is equal to or higher than a preset threshold. If the signal strength of the interface echo is greater than or equal to a preset threshold, matching is performed using the interface echo, and if the signal strength of the interface echo is lower than the preset threshold, matching is performed using the surface echo. Perform processing. The threshold value is set to an appropriate value through experiments. Note that, as shown in FIG. 1, the matching process is performed as preprocessing of the averaging process.

FIG. 6 is a flowchart illustrating noise removal processing of reflected waves. The control unit 50 first determines the number and position of the second irradiation points, and selects the second irradiation points (processing S51). The control unit 50 determines whether the processing of all the first irradiation points has been completed (processing S52), and if the processing of all the first irradiation points has not been completed (processing S52, No), The process advances to process S53, and if the process for all the first irradiation points has been completed (process S52, Yes), the process advances to process S57.

The control unit 50 aligns the probe with the irradiation position, transmits and receives ultrasonic waves to and from the target object (process S53), and stores the reflected waves in the storage unit 58 as time series data (process S54). Then, the control unit 50 performs matching processing on the first reflected wave and the second reflected wave (processing S55), then performs averaging processing (processing S56), and returns to processing S52.

In processing S57, image generation processing is performed at all first irradiation points. Specifically, pixelization information is generated by converting the signal intensity of the sharpened interface echo into a positive integer value. These processes are repeated over the entire inspection range of the object, and the joint interface is imaged.

In process S54, when the object is irradiated with ultrasound and all the second irradiation points are located in front of the first irradiation point, the time point when the ultrasonic transmission and reception processing for the first irradiation point is completed. In this step, matching processing is performed on the second irradiation point, and then averaging processing is performed.

Note that if the second irradiation point is also located after the first irradiation point, matching processing and averaging processing are performed when the ultrasound transmission and reception processing for all related second irradiation points is completed. It turns out.

FIG. 7 is a diagram showing waveform data 31A after averaging processing. FIG. 7 shows the results of performing matching processing and averaging processing on the waveform data 31 shown in FIG. 2. Signals of surface echoes and interface echoes originating from the sample (originating from the subject 15) will not disappear even if the averaging process is performed. On the other hand, noise components that are not derived from the sample and occur randomly over time are averaged and their intensity is reduced.

FIG. 8 is a diagram showing an image before averaging processing. FIG. 9 is a diagram showing an image after averaging processing. In the image shown in Figure 8, which was generated without performing matching processing or averaging processing, the defect is visualized within the field of view, but because the surrounding noise level is high compared to the brightness value of the defective part, visibility is poor. Contrast becomes low.

On the other hand, in the image generated by performing the matching processing and averaging processing shown in Fig. 9, the signal of the defect does not disappear and the surrounding noise level is reduced, so higher visibility and contrast can be obtained. is completed.

As described above, the ultrasound imaging apparatus 100 of this embodiment has the following features.
(1) Ultrasonic imaging device 100 that irradiates a multi-layered object with ultrasonic waves, obtains reflected waves from the joint interface of the object, and generates an image of the joint interface based on the signal strength of the reflected waves. and the phase of the second reflected wave from the second irradiation point (for example, P1 to P8) on the target object with respect to the first reflected wave from the first irradiation point (for example, P0) on the target object. A matching processing unit 56 that corrects the deviation of the first reflected wave and removes noise from the first reflected wave based on the first reflected wave and the second reflected wave after phase correction, and A pixelated information generation processing unit (e.g., averaging processing unit 57) that generates pixelated information of a portion of the bonded interface that returns an interface echo based on the signal intensity of the interface echo indicating a waveform from the bonded interface. It is characterized by Thereby, in the ultrasound imaging device, by generating pixelized information using a waveform from which noise has been removed, it is possible to generate highly accurate image information of a joint interface of a multilayered object.

(2) In (1), if the signal intensity of the interface echo of the first reflected wave is greater than or equal to a threshold value indicating a predetermined value, the matching processing unit 56 performs Processing is performed to match the phase of the peak part of the interface echo of the second reflected wave with the phase of the peak part, and if the signal intensity of the interface echo of the first reflected wave does not have a magnitude greater than the threshold value, the first It is preferable to perform processing to match the phase of the peak portion of the surface echo of the second reflected wave with the phase of the peak portion of the surface echo of the second reflected wave.

(3) In (2), the pixelated information generation processing section adds and averages the first reflected wave and the second reflected wave that has undergone phase correction.

(4) In (1), the matching processing unit 56 sets the second irradiation point by selecting one or more irradiation points at a predetermined position from among the irradiation points adjacent to the first irradiation point. (For example, see process S51, FIG. 3).

(5) Ultrasonic imaging device 100 that irradiates a multi-layered object with ultrasonic waves, obtains reflected waves from the joint interface of the object, and generates an image of the joint interface based on the signal strength of the reflected waves. A method for removing noise from a reflected wave in which a first reflected wave from a first irradiation point on an object and a second reflected wave from a second irradiation point on the object are matched. A phase correction step (for example, process S55, see FIG. 5) that corrects the phase shift of the second reflected wave with respect to the reflected wave of and a noise removal step (for example, process S56, see FIG. 4) of removing noise from the first reflected wave based on the first reflected wave.

Note that the present invention is not limited to the embodiments described above, and includes various modifications. Further, the embodiments described above have been described in detail to explain the present invention in an easy-to-understand manner, and the present invention is not necessarily limited to having all the configurations described. Further, it is possible to add, delete, or replace a part of the configuration of each embodiment with other configurations.

10 Water tank 11 Water 15 Subject (object)
20 Ultrasonic probe 31, 31A, 32, 33 Waveform data 50 Control section 51 Scanning control section 52 Transmission/reception control section 53 Image generation section 56 Matching processing section 57 Averaging processing section (pixelized information generation processing section)
60 Transmission/reception section 70 Scanner section (mechanical section)
71 X-axis scanner 72 Y-axis scanner 73 Z-axis scanner 100 Ultrasonic imaging device

In order to achieve the above object, the ultrasonic imaging device of the present invention irradiates an object made of multiple layers with ultrasonic waves, obtains reflected waves from the joint interface of the object, and detects the reflected waves. An ultrasonic imaging device that generates an image of the bonding interface based on signal intensity, the ultrasound imaging device generating an image of the bonding interface based on a signal intensity, wherein a matching processing unit that corrects a phase shift of the second reflected wave, and a matching processing unit that removes noise from the first reflected wave based on the first reflected wave and the second reflected wave after phase correction; a pixelated information generation processing unit that generates pixelated information of a portion of the bonded interface that returns the interface echo based on the signal strength of an interface echo indicating a waveform from the bonded interface among the removed first reflected waves; , the matching processing unit is configured to match the interface echo of the first reflected wave when the signal intensity of the interface echo of the first reflected wave is greater than or equal to a threshold value indicating a predetermined value. processing to match the phase of the peak part of the interface echo of the second reflected wave with the phase of the peak part of the interface echo, and the signal intensity of the interface echo of the first reflected wave does not have a magnitude greater than the threshold value. In this case, processing is performed to match the phase of the peak portion of the surface echo of the second reflected wave with the phase of the peak portion of the surface echo of the first reflected wave . Other aspects of the present invention will be explained in the embodiments described below.

Claims (8)

An ultrasonic imaging device that irradiates a multi-layered object with ultrasonic waves, obtains a reflected wave from a joint interface of the object, and generates an image of the joint interface based on the signal strength of the reflected wave. There it is,
a matching processing unit that corrects a phase shift of a second reflected wave from a second irradiation point on the target object with respect to a first reflected wave from the first irradiation point on the target object;
Noise is removed from the first reflected wave based on the first reflected wave and the second reflected wave after phase correction, and a waveform of the noise-removed first reflected wave from the bonding interface. An ultrasonic imaging device comprising: a pixelated information generation processing unit that generates pixelated information of a portion of the bonding interface that returns the interface echo based on the signal intensity of the interface echo indicating the interface echo. The ultrasound imaging device according to claim 1,
The matching processing unit is
When the signal intensity of the interface echo of the first reflected wave is greater than or equal to a threshold value indicating a predetermined value, the second reflection is in phase with the peak portion of the interface echo of the first reflected wave. Performing processing to match the phase of the peak part of the interface echo of the wave,
If the signal intensity of the interface echo of the first reflected wave does not have a magnitude equal to or greater than the threshold value, the surface echo of the second reflected wave is in phase with the peak of the surface echo of the first reflected wave. An ultrasound imaging device characterized by performing processing to match the phases of peak parts of echoes. The ultrasound imaging device according to claim 2,
The ultrasound imaging apparatus is characterized in that the pixelated information generation processing section averages the first reflected wave and the second reflected wave after the phase correction of the first reflected wave is performed. The ultrasound imaging device according to claim 1,
The matching processing unit sets the second irradiation point by selecting one or more irradiation points at predetermined positions from among irradiation points adjacent to the first irradiation point. Video equipment. An ultrasonic imaging device that irradiates a multi-layered object with ultrasonic waves, obtains a reflected wave from a joint interface of the object, and generates an image of the joint interface based on the signal strength of the reflected wave. A method for removing noise from reflected waves,
The first reflected wave from the first irradiation point on the target object and the second reflected wave from the second irradiation point on the target object are matched by a matching process. a phase correction step for correcting a phase shift of the reflected wave;
and a noise removal step of removing noise from the first reflected wave based on the first reflected wave and the second reflected wave whose phase has been corrected by the phase correction step. How to remove noise from reflected waves. The reflected wave noise removal method according to claim 5,
If the signal intensity of the interface echo indicating the waveform of the first reflected wave from the joint interface is greater than or equal to a threshold value indicating a predetermined value, in the matching process, the interface echo of the first reflected wave is performing processing to match the phase of the peak portion of the interface echo of the second reflected wave with the phase of the peak portion of the second reflected wave;
If the signal intensity of the interface echo of the first reflected wave does not have a magnitude equal to or greater than the threshold value, in the matching process, the phase of the peak part of the surface echo of the first reflected wave is A method for removing noise from a reflected wave, comprising performing processing to match the phases of the peak portions of the surface echoes of the reflected wave. The reflected wave noise removal method according to claim 6,
In the noise removal step, the first reflected wave and the second reflected wave whose phase has been corrected by the matching process are averaged. Removal method. The reflected wave noise removal method according to claim 5,
In the matching process, the second irradiation point is set by selecting one or more irradiation points at a predetermined position from among irradiation points adjacent to the first irradiation point. noise removal method.

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