Patent Publication Number: US-2020297319-A1

Title: Ultrasound diagnostic apparatus and method of controlling the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 2019-0030525, filed on Mar. 18, 2019 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     The disclosure relates to an ultrasound diagnostic apparatus and a method of controlling the same. 
     2. Description of the Related Art 
     Ultrasound diagnostic apparatuses operate to irradiate an ultrasound signal generated from an ultrasound probe transducer to a target site inside an object through the surface of the object and noninvasively acquire tomographic images or blood stream images of soft tissues using information about an ultrasound signal (an ultrasound echo signal) reflected from the object. 
     The ultrasound diagnostic apparatus has advantages in that it is compact and inexpensive, is displayable in real time, and has high safety compared to X-ray diagnostic devices due to having no risk of exposure to X-rays or the like, and thus are widely used for cardiac, breast, abdominal, urinary, and obstetrical diagnoses. 
     On the other hand, tissues of a human body have an elasticity, and a lesion tissue may be detected on the basis of the elasticity of the tissue. The ultrasound diagnostic apparatus may measure the elasticity of the tissue and image the elasticity. In detail, the ultrasound diagnostic apparatus may calculate the elasticity by estimating the velocity of a shear wave, and generate an elasticity image of the shear wave. 
     However, when diagnosing obese patients, severe reverberation occurs due to a fat layer, which causes difficulty in accurately measuring the elasticity. 
     SUMMARY 
     Therefore, it is an object of the disclosure to provide an ultrasound diagnostic apparatus capable of accurately measuring the elasticity even in the presence of reverberation by improving the performance of shear wave observation, and a method of controlling the same. 
     It is another object of the disclosure to provide an ultrasound diagnostic apparatus capable of accurately measuring the elasticity by setting the interval of tracking pulses for shear wave observation to be wide when a region of interest (ROI) is set wide, and a method of controlling the same. 
     Additional aspects of the disclosure will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the disclosure. 
     Therefore, it is an aspect of the disclosure to provide a method of controlling an ultrasound diagnostic apparatus, the method including: transmitting a push pulse to a region of interest (ROI) of a target object to induce a shear wave; adjusting a position of a focal point to which a plurality of tracking pulses are transmitted, on the basis of a position of the ROI; transmitting the plurality of tracking pulses to the ROI; receiving ultrasound echo signals reflected from the ROI in response to the plurality of tracking pluses; estimating a velocity of the shear wave velocity associated with the ROI on the basis of the ultrasound echo signals; generating a shear wave elasticity image on the basis of the velocity of the shear wave; and outputting the shear wave elasticity image on a display. 
     The method may further include setting the ROI in a radial form, wherein the transmitting of the plurality of tracking pulses includes radially transmitting the plurality of tracking pulses to the ROI in the radial from. 
     The adjusting of the position of the focal point may include moving the focal point into the ROI in response to movement of the ROI. 
     The receiving of the ultrasound echo signals may include setting sets of multi-reception scan lines, each set corresponding to a respective one of the plurality of tracking pulses, wherein the estimating of the velocity of the shear wave may include selectively performing signal processing on the multi-reception scan lines. 
     The estimating of the velocity of the shear wave may include: grouping reception scan lines positioned at a same relative position in each of the sets of the multi-reception scan lines to generate a plurality of groups; estimating a plurality of velocities of the shear wave each corresponding to a respective one of the plurality of groups; and determining a final velocity of the shear wave on the basis of the plurality of the velocities of the shear wave. 
     The determining of the final velocity of the shear wave may include determining an average value of the plurality of velocities of the shear wave or a weighted average value obtained using a reliability measurement index (RMI) on each of the plurality of velocities of the shear wave as the final shear wave. 
     The estimating of the velocity of the shear wave may include: selecting some reception scan lines from the multi-reception scan lines; and estimating the velocity of the shear wave on the basis of ultrasound echo signals received along the selected some reception scan lines. 
     The selecting of the reception scan lines may include selecting reception scan lines adjacent to each of the plurality of tracking pulses from the sets of the multi-reception scan lines. 
     The selecting of the reception scan lines may include selecting reception scan lines having a positional error smaller than a predetermined value. 
     The estimating of the velocity of the shear wave may further include estimating an arrival time of the shear wave on each of the multi-reception scan lines, wherein the selecting of the reception scan lines may include selecting reception scan lines except for a reception scan line in which the shear wave has a minimum arrival time and a reception scan line in which the shear wave has a maximum arrival time. 
     The outputting of the shear wave elasticity image may include displaying an elasticity, a depth, and a reliability measurement index (RMI). 
     The transmitting of the plurality of tracking pulses may include transmitting the plurality of tracking pulses in an interleaving method. 
     The estimating of the velocity of the shear wave may include: detecting a displacement of a tissue at a plurality of sampling points of each of the multi-reception scan lines; estimating an arrival time of the shear wave on each of the multi-reception scan lines on the basis of the displacement of the tissue; and estimating the velocity of the shear wave on the basis of a distance between the multi-reception scan lines and a difference between the arrival times of the shear wave on the multi-reception scan lines. 
     It is another aspect of the disclosure to provide an ultrasound diagnosis apparatus including: an ultrasound probe configured to transmit a push pulse to a region of interest (ROI) of a target object, transmit a plurality of tracking pulses to the ROI for observing a shear wave that is induced by the push pulse, and receive ultrasound echo signals reflected from the ROI in response to the plurality of tracking pluses; a controller configured to adjust a position of a focal point to which the plurality of tracking pulses are transmitted, on the basis of a position of the ROI, estimate a velocity of the shear wave velocity associated with the ROI on the basis of the ultrasound echo signals, generate a shear wave elasticity image on the basis of the velocity of the shear wave; and a display on which the shear wave elasticity image is output. 
     The controller may set the ROI in a radial form, and control the ultrasound probe to radially transmitting the plurality of tracking pulses to the ROI in the radial from. 
     The controller may move the focal point into the ROI in response to movement of the ROI. 
     The controller may arrange sets of multi-reception scan lines, each set corresponding to a respective one of the plurality of tracking pulses, and may selectively perform signal processing on the multi-reception scan lines. 
     The controller may group reception scan lines positioned at a same relative position in each of the sets of the multi-reception scan lines to generate a plurality of groups, may estimate a plurality of velocities of the shear wave each corresponding to a respective one of the plurality of groups, and may determine a final velocity of the shear wave on the basis of the plurality of the velocities of the shear wave. 
     The controller may determine an average value of the plurality of velocities of the shear wave or a weighted average value obtained using a reliability measurement index (RMI) on each of the plurality of velocities of the shear wave as the final shear wave. 
     The controller may select some reception scan lines from the multi-reception scan lines, and estimate the velocity of the shear wave on the basis of ultrasound echo signals received along the selected some reception scan lines. 
     The controller may select reception scan lines adjacent to each of the plurality of tracking pulses from the sets of the multi-reception scan lines. 
     The controller may select reception scan lines having a positional error smaller than a predetermined value. 
     The controller may estimate an arrival time of the shear wave on each of the multi-reception scan lines, and select reception scan lines except for a reception scan line in which the shear wave has a minimum arrival time and a reception scan line in which the shear wave has a maximum arrival time. 
     The controller may control the display to display an elasticity, a depth, and a reliability measurement index (RMI). 
     The controller may control the ultrasound probe to transmit the plurality of tracking pulses in an interleaving method. 
     The controller may detect a displacement of a tissue at a plurality of sampling points of each of the multi-reception scan lines, estimate an arrival time of the shear wave on each of the multi-reception scan lines on the basis of the displacement of the tissue, and estimate the velocity of the shear wave on the basis of a distance between the multi-reception scan lines and a difference between the arrival times of the shear wave on the multi-reception scan lines. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects of the disclosure will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  illustrates an ultrasound diagnostic apparatus according to an embodiment. 
         FIG. 2  is a block diagram illustrating a configuration of an ultrasound diagnostic apparatus according to an embodiment. 
         FIG. 3  is a block diagram illustrating a configuration of an ultrasound probe according to an embodiment. 
         FIG. 4  illustrates transmission and reception of ultrasound waves. 
         FIG. 5  is a flowchart schematically showing a method of estimating the shear wave velocity. 
         FIG. 6  illustrates induction of a shear wave by a push pulse. 
         FIG. 7  illustrates propagation of a shear wave. 
         FIG. 8  illustrates an example of a method of detecting a shear wave. 
         FIG. 9  illustrates transmission of a tracking pulse according to another example of a method of detecting a shear wave. 
         FIG. 10  illustrates a method of transmitting a plurality of tracking pulses to widen an observing area. 
         FIG. 11  illustrates radial transmission of a plurality of tracking pulses to suit a region of interest. 
         FIG. 12  illustrates multi-reception scan lines corresponding to a single tracking pulse. 
         FIG. 13  illustrates a plurality of tracking pulses and a sequence of sets of multi-reception scan lines. 
         FIG. 14  illustrates the relationship between the displacement of a tissue and the arrival time of a shear wave. 
         FIG. 15  illustrates the positional error of multi-reception scan lines. 
         FIG. 16  illustrates the error of the shear wave arrival time on each of the multi-reception scan lines. 
         FIG. 17  illustrates reception scan lines positioned at the same relative location in the sets of the multi-reception scan lines. 
         FIG. 18  illustrates the shear wave arrival times for reception scan lines positioned at the same relative location. 
         FIG. 19  is a flowchart showing a method of controlling an ultrasound diagnostic apparatus, which describes a method of estimating the shear wave velocity by grouping reception scan lines. 
         FIG. 20  illustrates a wave front graph for describing the method of estimating the shear wave velocity shown in  FIG. 19 . 
         FIG. 21  is a flowchart showing a method of controlling an ultrasound diagnostic apparatus according to another embodiment, which describes a method of estimating the shear wave velocity by selecting some reception scan lines. 
         FIGS. 22 and 23  show wave front graphs for describing the method of estimating the shear wave velocity shown in  FIG. 21 . 
         FIGS. 24 and 25  illustrate the intervals between a plurality of tracking pulses. 
         FIGS. 26 and 27  show the result of the elasticity measurement according to the related art. 
         FIG. 28  shows the result of the elasticity measurement by the method of controlling the ultrasound diagnostic apparatus according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Like numerals refer to like elements throughout the specification. Not all elements of embodiments of the present disclosure will be described, and description of what are commonly known in the art or what overlap each other in the embodiments will be omitted. 
     It will be further understood that the term “connect” or its derivatives refer both to direct and indirect connection, and the indirect connection includes a connection over a wireless communication network. 
     It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, 
     Although the terms “first,” “second,” “A,” “B,” etc. may be used to describe various components, the terms do not limit the corresponding components, but are used only for the purpose of distinguishing one component from another component. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     Moreover, terms described in the specification such as “part,” “module,” and “unit,” refer to a unit of processing at least one function or operation, and may be implemented by software, a hardware component such as a field-programmable gate array (FPGA) or an application-specific integrated circuit (ASIC), or a combination of software and hardware. 
     Reference numerals used for method steps are just used for convenience of explanation, but not to limit an order of the steps. Thus, unless the context clearly dictates otherwise, the written order may be practiced otherwise. 
     An ‘object’ may include a person or animal, or part of a person or animal. For example, the object may include not only a mass but also organs such as the liver, heart, uterus, brain, breast, abdomen, or blood vessels. In addition, in the specification, the “user” may be a doctor, a nurse, a clinical pathologist, a medical imaging expert, or the like, and may be a technician who develops and repairs a medical device, but is not limited thereto. 
     The term “ultrasound image” and “image of an object” refer to an image of an object obtained using ultrasound waves. 
     Hereinafter, embodiments of the disclosure will be described in detail with reference to the accompanying drawings 
       FIG. 1  illustrates an ultrasound diagnostic apparatus according to an embodiment. 
     Referring to  FIG. 1 , the ultrasound diagnostic apparatus  1  includes an ultrasound probe  100  and a main body  200 . The ultrasound probe  100  may transmit an ultrasound signal to an object to be diagnosed and receive an ultrasound echo signal reflected from the object. The ultrasound probe  100  receives the ultrasound echo signal reflected from the object and converts the ultrasound echo signal into an electrical signal (hereinafter, referred to as an ultrasound signal). 
     The ultrasound probe  100  may be connected to the main body  200  of the ultrasound diagnostic apparatus  1  through a cable  120 , and may receive various signals required for controlling the ultrasound probe P from the main body  200 . In addition, the ultrasound probe  100  may transmit an analog signal or a digital signal corresponding to the ultrasound echo signal to the main body  200 . 
     Meanwhile, the ultrasound probe  100  may be implemented as a wireless probe, and may transmit and receive a signal through a network formed between the probe  100  and the main body  200 . A detailed description of the probe  100  is described below with reference to  FIG. 3 . 
     The main body  200  may include a probe select assembly (PSA) board  250 , a control panel  260 , and a display  280  ( 280 - 1  and  280 - 2 ). The PSA board  250  includes a port connected to the ultrasound probe  100 . The PSA board  250  may activate the ultrasound probe  100  according to a user command input through the control panel  260  and the control of the controller  300 . One end of the cable  120  includes a connector  130  connectable to the port of the PSA board  250 . 
     The control panel  260  is a device that receives a command for operating the ultrasound diagnostic apparatus  1  from a user. The control panel  260  may receive setting information regarding the probe  100 , and receive various control commands related to the operation of the main body  200 . 
     The control panel  260  may include a keyboard. The keyboard may include buttons, switches, knobs, touch pads, trackballs, and the like. In addition, the control panel  260  may include a first display  270 - 1 . The first display  270 - 1  may display a graphic user interface (GUI) for controlling the operation of the ultrasound diagnostic apparatus  1 . The first display  270 - 1  may display related information, such as a menu or an auxiliary image, for optimizing the ultrasound image. 
     The first display  270 - 1  may include a touch panel and receive a user&#39;s touch input on the graphic user interface. The first display  270 - 1  may display a graphic user interface having the same shape as a button included in a keyboard. The user may input a command for controlling the ultrasound diagnostic apparatus  1  through a touch input to the first display  270 - 1 . 
     The second display  270 - 2  may display an ultrasound image. The ultrasound image may be a two-dimensional (2D) ultrasound image or a three dimension (3D) stereoscopic ultrasound image, and various ultrasound images may be displayed according to an operation mode of the ultrasound diagnostic apparatus  1 . In addition, the second display  270 - 2  may display menus, guide items, information about an operation state of the probe  100 , and the like, which are required for the ultrasound diagnosis. 
     The second display  270 - 2  may display a shear wave elasticity image that overlaps or is registered with a reference ultrasound image. 
     The second display  270 - 2  may also include a touch panel and receive a user&#39;s touch input on the graphic user interface. The user may input a command for controlling the ultrasound diagnostic apparatus  1  through a touch input on the second display  270 - 2 . 
     The display  270  may be implemented as various display devices, such as a liquid crystal display (LCD), a light emitting diode (LED), a plasma display panel (PDP), and an organic light emitting diode (OLED). 
       FIG. 2  is a block diagram illustrating a configuration of an ultrasound diagnostic apparatus according to an embodiment. 
     Referring to  FIG. 2 , the ultrasound probe  100  may be a linear array probe, a curved array probe, a phased array probe, or a volume probe. The ultrasound probe  100  is not limited thereto, and may include various probes, such as an endocavity probe, a convex probe, a matrix probe, and/or a 3D probe. 
     The main body  200  of the ultrasound diagnostic apparatus  1  may further include beamformers  281  and  282 , an image processor  290 , and a controller  300 . 
     The beamformer may be divided into a transmission beamformer  281  and a reception beamformer  282 . In obtaining an image using an ultrasound signal, a beamforming technique is used to increase the resolution of the image. The transmission beamformer  281  may apply a transmission pulse to the ultrasound probe  100 . The transmission beamformer  281  may apply an appropriate time delay so that ultrasound signals to be transmitted by a plurality of transducer elements are simultaneously focused at one focal point, and generate a transmission beam. A transducer array  110  may transmit the transmission beam to a target site in the object. 
     In addition, the transmission beamformer  281  may generate a push pulse transmitted along a push line. The push pulse may be irradiated to a region of interest (ROI) R of an object to induce displacement of a tissue and induce shear waves. The displacement of the tissue is used to measure the shear wave velocity, which will be described below. The push pulse may be a focused beam with a relatively high focusing. 
     The ultrasound transmitted to the object may be reflected from the object and may be incident back to the transducer array  110  of the ultrasound probe  100 . The reflected ultrasound signal may be defined as an ultrasound echo signal. 
     The reception beamformer  281  performs analog/digital conversion on the ultrasound echo signal received from the ultrasound probe  100  and performs reception beamforming. The reception beamformer  281  may apply a time delay to the ultrasound echo signals reflected from the focal point and returning to the transducer elements and add up the ultrasound echo signals at the same time. 
     Meanwhile, the beamformers  281  and  282  may be provided in the ultrasound probe  100 . For example, when the ultrasound probe  100  is a wireless probe, the ultrasound probe  100  may include beamformers  281  and  282 . 
     The image processor  290  filters out noise components in a reception beam to improve the image quality of the ultrasound image, performs an envelope detection process for detecting the intensity of the received signal, and generates digital ultrasound image data. 
     The image processor  290  may perform scan conversion to convert scan lines of the digital ultrasound image data such that the digital ultrasound image data is displayed on the display  270 . In addition, the image processor  290  performs image processing on the ultrasound echo signal to generate an A-mode image, a B-mode image, a D-mode image, an E-mode image, an M-mode image, a Doppler image, and/or a 3D ultrasound image. The image processor  290  performs RGB-processing on the ultrasound image data such that the ultrasound image is displayed on the display  270  and transmits the ultrasound image data to the display  270 . 
     In addition, the image processor  290  may perform image processing for displaying various pieces of additional information on the ultrasound image. 
     Although the image processor  290  is illustrated as being separated from the controller  300  in  FIG. 2 , the controller  300  may include the image processor  290 . 
     The display  270  may display the ultrasound image and various types of information processed by the ultrasound diagnostic apparatus  1 . The display  270  may display various graphic user interfaces for adjusting the generated ultrasound image. 
     The controller  300  may control the operation of the ultrasound diagnostic apparatus  1  and the signal flow between internal components of the ultrasound diagnostic apparatus  100 . The controller  300  may include a processor  310  and a memory  320 . The controller  300  may be implemented as a processing board in which the processor  310  and the memory  320  are installed on a circuit board. The processor  310  and the memory  320  may be connected through a bus. The processor  310  may be provided in a single unit or in a plurality of units thereof. 
     The controller  300  may be implemented with a plurality of logic gates or a combination of a general-purpose microprocessor and a memory  320  configured to store a program that may be executed in the microprocessor. 
     The memory  320  refers to a storage medium that stores algorithms and data required for the operation of each component of the ultrasound diagnostic apparatus  1 . The memory  320  may include high-speed random-access memory, a magnetic disk, an SRAM, a DRAM, a ROM, or the like. In addition, the memory  320  may be detachable from the ultrasound diagnostic apparatus  1 . The memory  320  may include a compact flash (CF) card, a secure digital (SD) card, a smart media (SM) card, a multimedia card (MMC), or a memory stick, but is not limited thereto. 
     The controller  300  may be electrically connected to each of the PSA board  250 , the control panel  260 , the display  270 , and the beamformers  281  and  282 , and may generate a control signal to control components of the probe  100  and the main body  200 . 
     A detailed operation of the controller  300  is described below with reference to  FIGS. 5 to 14 . 
       FIG. 3  is a block diagram illustrating a configuration of an ultrasound probe according to an embodiment. 
     Referring to  FIG. 3 , the ultrasound probe  100  may include a transducer array  110 , a cable  120 , a connector  130 , and a probe controller  170 . 
     he transducer array  110  is provided at an end of the ultrasound probe  100 . The transducer array  110  includes an array of a plurality of ultrasound transducer elements. The transducer array  110  generates ultrasound waves while vibrating by a pulse signal or an alternating current applied by the transmission beamformer  281  of the main body  200 . The generated ultrasound is transmitted to a target site inside an object. 
     The ultrasound generated by the transducer array  110  may be transmitted to a plurality of focuses for a plurality of target sites inside the object. That is, the ultrasound may be multi-focused and transmitted to the plurality of target sites. 
     The ultrasound transmitted by the transducer array  110  returns to the transducer array  110  as an ultrasound echo signal reflected from the target site inside the object. Upon arrival of the ultrasound echo signal, the transducer array  110  vibrates at a predetermined frequency corresponding to the frequency of the ultrasound echo signal and outputs an alternating current having a frequency corresponding to the vibration frequency. Accordingly, the transducer array  110  may convert the ultrasound echo signal into a predetermined electrical signal. 
     Referring to  FIG. 4 , the ultrasound probe  100  may transmit a reference pulse  511  to an ROI and receive a first ultrasound echo signal  513  as a result of the reference pulse  511  being reflected from the region of interest. The reference pulse  511  has a beam profile. The width of the beam profile may be adjusted. 
     The ultrasound diagnostic apparatus  1  may generate a first ultrasound image on the basis of the first ultrasound echo signal  513 . The first ultrasound image may be a reference ultrasound image distinguished from a shear wave elasticity image, that represents the position of a tissue before a force is applied to the ROI. The first ultrasound image may be a B-mode image or an M-mode image of the ROI. 
     Meanwhile, the ultrasound probe  100  may induce a shear wave by transmitting a push pulse to an ROI of the object, and may transmit a tracking pulse for observing the shear wave to the ROI of the object and receive an ultrasound echo signal as a result of reflection of the tracking pulse. The ultrasound echo signal obtained by reflection of the tracking pulse may be defined as a second ultrasound echo signal. The controller  300  of the ultrasound diagnostic apparatus  1  may generate a second ultrasound image on the basis of the second ultrasound echo signal. That is, the second ultrasound echo signal may be defined as a shear wave photographing image. 
     The transducer elements included in the transducer array  110  may be selectively activated. By selective activation of the transducer elements, the width of the transmission beam may be adjusted. In addition, the plurality of tracking pulses may be transmitted at a preset interval. 
     The probe controller  170  may include a processor  171  and a memory  172 . The processor  171  of the probe controller  170  may be a general micro-processor, and the memory  172  may store a program that may be executed by the processor  171 . The probe controller  170  transmits and receives data into and from the main body  200  and controls the overall operation of the probe  100 . 
     The ultrasound probe  100  may further include a T/R switch and a beamformer. The T/R switch serves as a switch to control the conversion between an operation of the transducer array  110  irradiating the ultrasound signal and an operation of the transducer array  110  receiving the reflected echo signal. 
     Components included in the probe  100  are not limited to those shown in  FIG. 3 , and may be provided in various combinations. 
       FIG. 5  is a flowchart schematically showing a method of estimating the shear wave velocity. 
     Referring to  FIG. 5 , the ultrasound probe  100  may induce a shear wave by transmitting a push pulse to an ROI of an object ( 410 ). In detail, the ultrasound probe  100  may induce displacement in a tissue in the object by irradiating a focused beam to the object. When the focused beam is irradiated to the object, deformation occurs according to an axial movement of the tissue in the object by the focused beam, so that displacement of the tissue is induced. The tissue displacement may cause a shear wave to be propagated. 
     Thereafter, the ultrasound probe  100  may transmit tracking pulses for observing the shear wave to the ROI of the object, and receive ultrasound echo signals a result of reflection of the tracking pulses ( 420 ). The ultrasound echo signal resulting from reflection of the tracking pulse may be defined as a second ultrasound echo signal. The tracking pulse has a beam profile of a predetermined width. The tracking pulses may be sequentially transmitted to a plurality of points multi-times. Such a shear wave observation method is referred to as an interleaving method. This will be described in detail with reference to  FIG. 10 . 
     Meanwhile, the controller  300  may generate a second ultrasound image on the basis of the second ultrasound echo signal. That is, the second ultrasound echo signal may be defined as a shear wave photographing image. 
     The controller  300  of the ultrasound diagnostic apparatus  1  may detect the displacement of the tissue at a plurality of sampling points in the ROI ( 430 ). Specifically, the controller  300  may set a plurality of transmission scan lines such that a plurality of tracking pulses are transmitted to different positions, and detect the displacement of the tissue at a plurality of sampling points corresponding to the plurality of transmission scan lines. For example, the controller  300  may detect displacement of a tissue by performing cross correlation on the first ultrasound image that is a reference ultrasound image and the second ultrasound image that is a shear wave image. 
     The controller  300  may estimate the time at which the shear wave arrives at the tissue on the basis of the displacement of the tissue ( 440 ). In detail, the controller  300  may estimate the point in time at which the displacement of the tissue is the maximum as the shear wave arrival time. The controller  300  may estimate the shear wave arrival time on each of the plurality of sampling points. 
     The controller  300  may estimate the shear wave velocity on the basis of the distance between the plurality of sampling points and the shear wave arrival times ( 450 ). For example, the controller  300  may calculate the shear wave velocity using a distance between two sampling points located in the traveling direction of the shear wave and the shear wave arrival times for the two sampling points. 
     Hereinafter, a method of estimating the shear wave velocity will be described in more detail. 
       FIG. 6  illustrates induction of a shear wave by a push pulse.  FIG. 7  illustrates propagation of a shear wave. 
     Referring to  FIG. 6 , the ultrasound probe  100  may transmit a push pulse  521  along a push line in a depth direction (Z direction) under the control of the ultrasound diagnostic apparatus  1 . The push pulse may be irradiated to a focal point  520  in an ROI to induce displacement of the tissue and induce a shear wave  530 . The push pulse is a focused beam having a relatively high focusing, and may have beam profiles  521   a  and  521   b  in a narrow width. 
     When the push pulse  521  is transmitted to the focal point  520  in the ROI, the shear wave  530  may be induced. That is, when a force is applied to a tissue of the focal point  520  in the depth direction (Z direction) by the push pulse  521 , the tissue of the focal point  520  moves in the depth direction (Z direction). The distance moved by the tissue in the depth direction (Z direction) may be defined as a displacement. Since tissues of an object has a certain degree of elasticity, and adjacent tissues are organically connected, the movement of the tissue located at the focal point  520  also exerts influence on the adjacent tissues. 
     Referring to  FIG. 7 , the movement of the tissue located at the focal point  520  induces displacement of the adjacent tissues. The shear wave  530  may propagate in the X direction, which is a direction perpendicular to the depth direction (Z direction), due to the displacement of the tissues. The shear wave  530  propagates from the focal point  520  of the push pulse  521  to both sides. The shear wave  530  changes its velocity according to the vibrational characteristics of the medium. Therefore, the elasticity of a tissue may be obtained by estimating the shear wave velocity. 
       FIG. 8  illustrates an example of a method of detecting a shear wave.  FIG. 9  illustrates transmission of a tracking pulse according to another example of a method of detecting a shear wave. 
     Referring to  FIG. 8 , the ultrasound probe  100  may transmit a tracking pulse  540  having wide beam profiles  540   a  and  540   b  to an ROI  550  in which the shear wave  530  propagates as a result of displacement of the tissues, and receive an ultrasound echo signal as a result of the tracking pulse  540  being reflected from the ROI  550 . The ultrasound diagnostic apparatus  1  may detect the displacement of the tissues on the basis of the echo signal of the tracking pulse  540 . 
     For example, the controller  300  may detect a displacement of a tissue by performing cross correlation on a first ultrasound image that is a reference ultrasound image and a second ultrasound image that is a shear wave photographing image. In other words, the controller  300  may compare the first ultrasound image (a reference ultrasound image) before application of the push pulse  521  with the second ultrasound image (a shear wave photographing image) after application of the push pulse  521 , so that the degree to which the tissues are moved is detected. 
     In addition, the controller  300  may acquire shear wave photographing images at a high frame rate, and compare successive shear wave photographing image frames, so that displacement of the tissues are detected. 
     However, when measuring the elasticity of the tissue using the shear wave, the propagation of the shear wave needs to be accurately observed to accurately obtain the elasticity. As shown in  FIG. 8 , when the beam profiles  540   a  and  540   b  of the tracking pulse  540  are wide, the observing area is caused to be wide, and thus uniform observation of the ROI  550  is performable, but the observation performance (e.g., signal to noise ratio (SNR)) drops. The tracking pulse  540  shown in  FIG. 8  is defined as a widely focused transmission beam (a widely focused transmission Tx beam). 
     In addition, when using the tracking pulse  540  having wide beam profiles  540   a  and  540   b , the elasticity may not be accurately measured in an environment where reverberation exists. That is, since the transmission beam having a large width may include a lot of disturbances, the accuracy of the elasticity measurement may be reduced. 
     As shown in  FIG. 9 , when a tracking pulse Tx 1  having narrow beam profiles  560   a  and  560   b  is used, a considerably high SNR may be obtained. The tracking pulse Tx 1  shown in  FIG. 9  may be defined as a tightly focused transmission beam (a tightly focused Tx beam). Using a narrow transmission beam may increase the accuracy of elasticity measurements even in an environment having reverberation. However, the tracking pulse Tx 1  having narrow beam profiles  560   a  and  560   b  causes the observing area to be narrowed. Therefore, in order to widen the observing area of a tracking pulse Tx 1 , an interleaving or interrogation scheme is used. 
     A tightly focused tracking pulse Tx 1  is transmitted into the ROI. The position of a focal point to which the tracking pulse Tx 1  is transmitted may be adjusted on the basis of the ROI. That is, the focal point of the tracking pulse Tx 1  may be moved into the ROI in response to movement of the ROI. At least one of the depth (Z direction) and the position in transverse direction (X direction) of the focal point of the tracking pulse Tx 1  may be adjusted. 
     The beam profiles  560   a  and  560   b  of the tightly focused tracking pulse Tx 1  are set to be smaller than the width of the ROI. That is, as shown in  FIG. 9 , the beam width of the tracking pulse Tx 1  in the X direction at the focal point is set to be smaller than the width of the ROI. 
       FIG. 10  illustrates a method of transmitting a plurality of tracking pulses to widen an observing area. 
     Referring to the left side drawing of  FIG. 10 , in order to widen the observing area of the tracking pulse Tx 1  having narrow beam profiles  560   a  and  560   b , a plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  may be sequentially transmitted to a plurality of positions and/or focal points in the ROI. That is, after one push pulse  520  is transmitted along a push line  521 , a plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  may be sequentially transmitted along respective transmission scan lines. 
     Before the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  are transmitted to the ROI, the position of the focal point for each of the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  is adjusted to be included in the ROI. In addition, the position of each focal point for each of the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  may be adjusted such that the focal point is moved into the ROI according to movement of the ROI. Since the position of the focal point for each of the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  is adjusted in response to movement of the ROI, the accuracy of the shear wave observation may be improved. 
     The controller  300  of the ultrasound diagnostic apparatus  1  performs sampling on echo signals of each of the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4 . In addition, interpolation may be performed to increase the sampling rate. Such a shear wave observation method is referred to as an interleaving method because sampling is interleaved in time. 
     On the other hand, as the number of times of interleaving increases, the observation area becomes wider, and errors in estimating the shear wave velocity and the elasticity may decrease. In  FIG. 10 , four times of interleaving are illustrated as being performed. That is,  FIG. 10  illustrates transmission of four tracking pulses. 
     In addition, referring to the right-side drawing of  FIG. 10 , a plurality of push pulses  520  may be sequentially transmitted along the push line  521 . The plurality of push pulses  520  may be transmitted to the same focal point or to positions at different depths on the push line  521 . In addition, the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  may be sequentially transmitted to correspond to the respective push pulses  520 . In other words, the ultrasound diagnostic apparatus  1  may transmit a first tracking pulse and transmit a first tracking pulse Tx 1  to observe a shear wave, transmit a second push pulse and transmit a second tracking pulse Tx 2  to observe a shear wave, transmit a third push pulse and transmit a third tracking pulse Tx 3  to observe a shear wave, and then transmit a fourth push pulse and transmit a fourth tracking pulse Tx 4  to observe a shear wave. Such a shear wave observation method is referred to as an interrogation method. 
     As described above, by sequentially transmitting the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  to a plurality of points in the ROI, the observable area may be widened even using a tracking pulse having narrow beam profiles. 
       FIG. 11  illustrates radial transmission of a plurality of tracking pulses to suit a region of interest. 
     Referring to  FIG. 11 , the controller  300  of the ultrasound diagnostic apparatus  1  may set an ROI  550  in a radial shape, a fan shape, or a trapezoidal shape. The controller  300  may control the ultrasound probe  100  to radially transmit the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  to correspond to the ROI  550  that is radially formed. That is, the controller  300  may control the transducer array  110  to radially transmit the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4 . 
     When the ultrasound probe  100  is a convex type, the transducer array  110  may be formed in a curved surface. Therefore, with the convex type transducer array  110 , the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  may be transmitted in a radial shape. The controller  300  may selectively activate the transducer elements included in the transducer array  110 . The controller  300  may activate different transducer elements to transmit a plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4 . 
     The ROI  550  may be set to have various sizes or widths. The user may set the ROI  550  using the control panel  260 . Meanwhile, as the depth to which the ultrasound is irradiated increases, the size or width of the ROI  550  may be set larger. The controller  300  may set intervals between the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  to correspond to the ROI  550  that is set. In addition, the controller  300  may set transmission angles of the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  to correspond to the ROI  550 . 
     Setting the intervals between the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  will be described with reference to  FIGS. 24 and 25 . 
       FIG. 12  illustrates multi-reception scan lines corresponding to a single tracking pulse. 
     Referring to  FIG. 12 , the controller  300  of the ultrasound diagnostic apparatus  1  may set multi reception scan lines Rx 1 - 1 , Rx 1 - 2 , Rx 1 - 3 , and Rx 1 - 4  corresponding to a first tracking pulse Tx 1 . In  FIG. 12 , four reception scan lines Rx 1 - 1 , Rx 1 - 2 , Rx 1 - 3 , and Rx 1 - 4  are illustrated. In general, the distance d between the multi-reception scan lines Rx 1 - 1 , Rx 1 - 2 , Rx 1 - 3 , and Rx 1 - 4  is set constant. 
     The first tracking pulse Tx 1  has beam profiles  560   a  and  560   b  with a predetermined width and transmits beams to positions corresponding to the four multi-reception scan lines Rx 1 - 1 , Rx 1 - 2 , Rx 1 - 3 , and Rx 1 - 4 . The transducer array  110  of the ultrasound probe  100  may receive ultrasound echo signals through the four reception scan lines Rx 1 - 1 , Rx 1 - 2 , Rx 1 - 3 , and Rx 1 - 4 . The controller  300  may appropriately delay and sum the ultrasound echo signals received through the four reception scan lines Rx 1 - 1 , Rx 1 - 2 , Rx 1 - 3 , and Rx 1 - 4 . 
     Beamforming using such multi-reception scan lines may reduce the ultrasound image acquisition time and increase the frame rate of the ultrasound image. Shear waves rapidly attenuate while propagating along the tissues, and thus travel a short distance for a short time. Therefore, by sampling the ultrasound echo signals received through the multi-reception scan lines, the shear wave may be easily observed. 
     On the other hand, increasing the number of reception scan lines may be considered as a method of increasing the sampling points. However, setting a large number of reception scan lines in response to a single tracking pulse may increase the width of the transmission beam. As described with reference to  FIG. 8 , when the width of the transmission beam is wide, a large amount of disturbance may be included, which may reduce the SNR and lower the accuracy of the elasticity measurement. 
       FIG. 13  illustrates a plurality of tracking pulses and a sequence of sets of multi-reception scan lines. 
     Referring to  FIG. 13 , a sequence of multi-reception scan lines corresponding to each of the four tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  is illustrated. The controller  300  may set sets B 1 , B 2 , B 3 , and B 4  of the multi-reception scan lines to correspond to the plurality of respective tracking pulses. Specifically, the controller  300  may set a first set B 1  from the first reception scan line to the fourth reception scan line Rx 1 - 1 , Rx 1 - 2 , Rx 1 - 3 , and Rx 1 - 4  corresponding to the first tracking pulse Tx 1 . In addition, the controller  300  may set a second set B 2  from the fifth reception scan line to the eighth reception scan line Rx 2 - 1 , Rx 2 - 2 , Rx 2 - 3 , and Rx 2 - 4  corresponding to the second tracking pulse Tx 2 . Similarly, the controller  300  may set a third set B 3  and a fourth set B 4  of the multi-reception lines corresponding to the third and fourth tracking pulses Tx 3  and Tx 4 , respectively. 
     With reference to the first reception scan line Rx 1 - 1 , as an example of the reception scan lines of the set B 1  corresponding to the first tracking pulse Tx 1 , the first reception scan line Rx 1 - 1  and the second reception scan Rx 1 - 2  are disposed on the left side of the first tracking pulse Tx 1 , and the third reception scan line Rx 1 - 3  and the fourth reception scan line Rx 1 - 4  are disposed on the right side of the first tracking pulse Tx 1 . 
     In general, the distanced between the multi-reception scan lines Rx 1 - 1 , Rx 1 - 2 , Rx 1 - 3 , and Rx 1 - 4  included in the same set is set constant. In addition, the distance d between adjacent sets of multi-reception scan lines is also set constant. For example, the distance d between the fourth reception scan line Rx 1 - 4  and the fifth reception scan line Rx 2 - 1  is set constant. 
     The ultrasound probe  100  may induce the shear wave  530  by transmitting a push pulse to the focal point  520  in the depth direction (Z direction). Since the shear wave  530  travels in the X direction, which is perpendicular to the depth direction (Z direction), the ultrasound probe  100  may transmit the first tracking pulses Tx 1  to the fourth tracking pulse Tx 4  along the X direction. When the shear wave is observed using the interleaving method, transmission beams by the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3  and Tx 4  are sequentially transmitted to the positions of the plurality of reception scan lines Rx 1 - 1  to Rx 4 - 4 . The ultrasound probe  100  may receive ultrasound echo signals as a result of the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  being reflected along the sets B 1 , B 2 , B 3 , and B 4  of multi reception scan lines. 
     The shear wave  530  sequentially reaches the positions of the plurality of reception scan lines Rx 1 - 1  to Rx 4 - 4 . The controller  300  may detect displacement of a tissue corresponding to the position of the reception scan line on the basis of the ultrasound echo signals received along the plurality of reception scan lines Rx 1 - 1  to Rx 4 - 4 . The controller  300  may detect displacement of tissues at a plurality of sampling points for each of the plurality of reception scan lines Rx 1 - 1  to Rx 4 - 4 . 
     In addition, the controller  300  may estimate the arrival time of the shear wave  530  on each of the reception scan lines Rx 1 - 1  to Rx 4 - 4  on the basis of the displacement of the tissue. The controller  300  may estimate the velocity of the shear wave on the basis of the distance between the reception scan lines and a difference between the arrival times of the shear wave on the reception scan lines. 
     For example, the controller  300  may detect displacement of a tissue at a plurality of sampling points of the first reception scan line Rx 1 - 1  and estimate the arrival time of the shear wave  530 . Similarly, the controller  300  may detect displacement of a tissue at a plurality of sampling points of the second reception scan line Rx 1 - 2  and estimate the arrival time of the shear wave  530 . Subsequently, the controller  300  may estimate the velocity of the shear wave  530  on the basis of the distance d between the first reception scan line Rx 1 - 1  and the second reception scan line Rx 1 - 2  and the arrival time of the shear wave on each of the first reception scan line Rx 1 - 1  and the second reception scan line Rx 1 - 2 . 
     On the other hand, the distance d between the first reception scan line Rx 1 - 1  and the second reception scan line Rx 1 - 2  may refer to the distance d between a first sampling point of the first reception scan line Rx 1 - 1  and a second sampling point of the second reception scan line Rx 1 - 2  that are located at the same depth. 
       FIG. 14  illustrates the relationship between the displacement of a tissue and the arrival time of a shear wave. 
     Referring to  FIG. 14 , displacement of a tissue corresponding to the first sampling point of the first reception scan line Rx 1 - 1  is maximum at time t 1 . Therefore, a first shear wave arrival time on the first sampling point of the first reception scan line Rx 1 - 1  may be determined as t 1 . Similarly, displacement of a tissue corresponding to the second sampling point of the second reception scan line Rx 1 - 2  is maximum at time t 2 . Therefore, a second shear wave arrival time on the second sampling point of the second reception scan line Rx 1 - 2  may be determined as t 2 . 
       FIG. 15  illustrates the positional error of multi-reception scan lines. 
     Referring to  FIG. 15 , it can be seen that there is a difference between the ideal position and an actual position of the multi reception scan lines (the first reception scan line Rx 1 - 1  to the fourth reception scan line Rx 1 - 4 ) corresponding to the first tracking pulse Tx 1 . 
     As described above, the controller  300  arranges the first reception scan line Rx 1 - 1  to the fourth reception scan line Rx 1 - 4  on both sides of the first tracking pulse Tx 1 , and sets the distances d between the first reception scan line Rx 1 - 1  to the fourth reception scan line Rx 1 - 4  to be constant. However, since the beam of the first tracking pulse Tx 1  is tightly focused, a positional error occurs between the set multi-reception scan line (dotted line) and the actual multi-reception scan line (solid line). 
     Specifically, the actually generated multi-reception scan lines tends to cluster around the tracking pulse Tx 1 . The first reception scan line Rx 1 - 1  and the second reception scan line Rx 1 - 2  are generated at positions shifted to the right side from the set positions. The third reception scan line Rx 1 - 3  and the fourth reception scan line Rx 1 - 4  are generated at positions shifted to the left side from the set positions. Accordingly, an error occurs between the distance d between the set first reception scan line Rx 1 - 1  and the set second reception scan line Rx 1 - 2  and the distance de between the actual first reception scan line Rx 1 - 1  and the actual second reception scan line Rx 1 - 2 . The positional error of the reception scan lines increases as being distant from the tracking pulse Tx 1 . 
     The related art estimates the velocity of a shear wave using the entire sampling points of the multi-reception scan lines without considering such a positional error of the reception scan lines. Therefore, the estimated shear wave velocity is caused to have an error. 
       FIG. 16  illustrates the error of the shear wave arrival time on each of the multi-reception scan lines. 
     Referring to  FIG. 16 , a wave front graph showing the arrival times of a shear wave measured in a plurality of reception scan lines Rx 1 - 1  to Rx 4 - 4  is illustrated.  FIG. 16  illustrates the points in time when a shear wave arrives at the respective sampling points of the plurality of reception scan lines Rx 1 - 1  to Rx 4 - 4 . The numerical values shown in the wave front graph are illustrative purpose only, without being limited thereto. 
     As described above, when a tightly focused transmission beam is used, a positional error of reception scan lines may occur, and thus an error of shear wave arrival time on a plurality of reception scan lines Rx 1 - 1  to Rx 4 - 4  may also occur. 
     In  FIG. 16 , the shear wave arrival times on the first reception scan line Rx 1 - 1  to the fifth reception scan line Rx 2 - 1  are described. Referring to the sampling points located at a depth of −44 mm, the first shear wave arrival time on the first sampling point of the first receiving scan line Rx 1 - 1  is approximately 3.4 ms, and the second shear wave arrival time on the second sampling point of the second reception scan line Rx 1 - 2  is 3.6 ms, the third shear wave arrival time on the third sampling point of the third reception scan line Rx 1 - 3  is 3.75 ms, the fourth shear wave arrival time on the fourth sampling point of the fourth reception scan line Rx 1 - 4  is 4.1 ms, and the fifth shear wave arrival time on the fifth sampling point of the fifth reception scan line Rx 2 - 1  is 4.5 ms. 
     In addition, a difference value ta 1  between the first shear wave arrival time and the second shear wave arrival time is 0.2 ms, and a difference value ta 2  between the second shear wave arrival time and the third shear wave arrival time is 0.15 ms, a difference value ta 3  between the third shear wave arrival time and the fourth shear wave arrival time is 0.35 ms, and a difference value ta 4  between the fourth shear wave arrival time and the fifth shear wave arrival time is 0.4 ms. As such, it can be seen that a positional error of the reception scan lines has occurred. 
     Various methods may be used to estimate the shear wave velocity. As an example, the velocity of the shear wave may be estimated on the basis of the distance between the first sampling point and the second sampling point and the difference value between the first shear wave arrival time and the second shear wave arrival time. As another example, the velocity of the shear wave may be estimated with respect to each of a plurality of sampling points using a plane equation or a wave equation, and adding up and averaging the velocities of the shear wave with respect to all sampling points. 
     However, the conventional shear wave velocity estimation methods reflecting the values of all the scan lines and sampling points have difficulty in removing the error of the shear wave velocities caused by the positional errors of the reception scan lines. Therefore, the reliability of the estimation result of the shear wave velocity is considerably low. 
     Hereinafter, a method of estimating the shear wave velocity that may remove the positional error of the reception scan line will be described. According to the disclosure, the shear wave velocity may be accurately obtained by selectively performing signal processing on multi-reception scan lines. Signal processing may refer to processing for ultrasound echo signals. 
       FIG. 17  illustrates reception scan lines positioned at the same relative location in the sets of the multi-reception scan lines.  FIG. 18  illustrates the shear wave arrival times on reception scan lines positioned at the same relative location. 
     Referring to  FIG. 17 , when assuming that the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  are transmitted at a constant interval 4d, the interval between the plurality of reception scan lines may also be set at a constant interval d. In addition, the positions of the actually generated reception scan lines may be different from the positions of the set reception scan lines as described above. Therefore, the distance de between the plurality of actually generated reception scan lines may be different from the distance d between the plurality of set reception scan lines. 
     However, the distance 4d between the reception scan lines located at the same relative position in each of the sets B 1 , B 2 , B 3 , and B 4  of the multi reception scan line may be constant. For example, the distance between the first reception scan line Rx 1 - 1  and the fifth reception scan Rx 2 - 1  is 4d. Since the first reception scan line Rx 1 - 1  and the fifth reception scan Rx 2 - 1  have positional errors caused by the first tracking pulse Tx 1  and the second tracking pulse Tx 2 , respectively, the first reception scan line Rx 1 - 1  and the fifth reception scan Rx 2 - 1  have the same relative position. In other words, since the first reception scan line Rx 1 - 1  is a reception scan line arranged on the first position in the first set B 1 , and the fifth reception scan line Rx 2 - 1  is a reception scan line arranged on the first position in the second set B 2 , the first reception scan line Rx 1 - 1  and the second reception scan line Rx 1 - 2  may be considered to have the same relative position. Similarly, the second reception scan line Rx 1 - 2  and the sixth reception scan line Rx 2 - 2  also exist on the same relative position. 
       FIG. 18  is a diagram illustrating reception scan lines located at same relative positions extracted from in the wave front graph of  FIG. 16 . That is,  FIG. 18  illustrates the shear wave arrival times on the first reception scan line Rx 1 - 1 , the fifth reception scan line Rx 2 , the ninth reception scan line Rx 3 - 1 , and the thirteenth reception scan line Rx 4 - 1 , which are reception scan lines arranged at respective first positions in the sets B 1 , B 2 , B 3 , and B 4  of the multi reception scan lines. 
     Referring to the sampling points located at a depth of −44 mm in  FIG. 18 , the first shear wave arrival time on the first sampling point of the first reception scan line Rx 1 - 1  is approximately 3.2 ms, the fifth shear wave arrival time for the fifth sampling point of the fifth reception scan line Rx 2 - 1  is 4.2 ms, the ninth shear wave arrival time for the ninth sampling point of the ninth reception scan line Rx 3 - 1  is 5.2 ms, and the thirteenth shear wave arrival time for the thirteenth sampling point of the thirteenth reception scan line Rx 4 - 1  is 6.2 ms. 
     In addition, a difference tb 1  between the first shear wave arrival time and the fifth shear wave arrival time is 1 ms, and a difference tb 2  between the fifth shear wave arrival time and the ninth shear wave arrival time is 1 ms, and a difference between the ninth shear wave arrival time and the thirteenth shear wave arrival time tb 3  is 1 ms. That is, it can be seen that the difference values between the shear wave arrival times on the reception scan lines at the same relative positions are the same. 
     As such, the ultrasound diagnostic apparatus  1  may remove the error of the shear wave velocity caused by the positional error of the reception scan line by estimating the shear wave velocity using the shear wave arrival times on the reception scan lines at the same relative position. 
       FIG. 19  is a flowchart showing a method of controlling an ultrasound diagnostic apparatus, which describes a method of estimating the shear wave velocity by grouping reception scan lines.  FIG. 20  illustrates a wave front graph for describing the method of estimating the shear wave velocity shown in  FIG. 19 . 
     Referring to  FIG. 19 , the controller  300  of the ultrasound diagnostic apparatus  1  may control the ultrasound probe  100 . The ultrasound probe  100  transmits a push pulse to an ROI of an object to induce a shear wave ( 1710 ). Thereafter, the ultrasound probe  100  transmits a plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  for observing the shear wave to the ROI of the object ( 1720 ). In this case, the intervals between the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  may be adjusted on the basis of the ROI. 
     The ultrasound probe  100  receives ultrasound echo signals reflected from the ROI along sets B 1 , B 2 , B 3 , and B 4  of multi-reception scan lines, each of the sets corresponding to a respective one of the plurality of tracking pulses ( 1730 ). 
     The controller  300  of the ultrasound diagnostic apparatus  1  may detect displacement of tissues at a plurality of sampling points of each of the multi-reception scan lines Rx 1 - 1  to Rx 4 - 4  ( 1740 ). In addition, the controller  300  may estimate the points in time when the shear wave arrives at the plurality of sampling points of each of the multi reception scan lines Rx 1 - 1  to Rx 4 - 4  ( 1750 ).  FIG. 20  illustrates the point in time when the shear wave arrives at the plurality of sampling points of each of the reception scan lines Rx 1 - 1  to Rx 4 - 4 . 
     The controller  300  may group reception scan lines arranged at the same relative position in the respective sets of the multi reception scan lines and generate a plurality of groups ( 1760  and  2010 ). Referring to  FIG. 20 , the first reception scan line Rx 1 - 1 , the fifth reception scan line Rx 2 - 1 , the ninth reception scan line Rx 3 - 1 , and the thirteenth reception scan line Rx 4 - 1  are set to the first group. In addition, the second reception scan line Rx 1 - 2 , the sixth reception scan line Rx 2 - 2 , the tenth reception scan line Rx 3 - 2 , and the fourteenth reception scan line Rx 4 - 2  are set to the second group. 
     The third reception scan line Rx 1 - 3 , the seventh reception scan line Rx 2 - 3 , the eleventh reception scan line Rx 3 - 3 , and the fifteenth reception scan line Rx 4 - 3  are set to the third group, and the fourth reception scan line Rx 1 - 4 , the eighth reception scan line Rx 2 - 4 , the twelfth reception scan line Rx 3 - 4 , and the sixteenth reception scan line Rx 4 - 4  are set to the fourth group. 
     The controller  300  may estimate a plurality of shear wave velocities each corresponding to one of the groups ( 1770 ). The controller  300  may estimate the first shear wave velocity for the first group, the second shear wave velocity for the second group, the third shear wave velocity for the third group, and the fourth shear wave velocity for the fourth group. 
     For example, in the case of the first group, the controller  300  may calculate the first shear wave velocity using the distance between the first sampling point of the first reception scan line Rx 1 - 1  and the fifth sampling point of the fifth reception scan line Rx 2 - 1  and the difference between the shear wave arrival times on the first sampling point and the fifth sampling point. The controller  300  may calculate the first shear wave velocity using the sampling points of the first reception scan line Rx 1 - 1 , the fifth reception scan line Rx 2 - 1 , the ninth reception scan line Rx 3 - 1 , and the thirteenth reception scan line Rx 3 - 1  included in the first group. 
     On the other hand, the controller  300  may calculate the shear wave velocity with respect to each of the plurality of sampling points included in the first group using the plane equation or the wave equation, and calculate the first shear wave velocity by summing and averaging the shear wave velocities. The controller  300  may calculate the shear wave velocity for each group in various ways. 
     The controller  300  may combine the shear wave velocities of the respective groups to obtain the final shear wave velocity ( 1780  and  2020 ). In detail, the controller  300  may determine an average value of the plurality of shear wave velocities v i  as the final shear wave velocity v final , as shown in Equation 1 below. 
     
       
         
           
             
               
                 
                   
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     In addition, the controller  30  may determine a weighted average value using the reliability measurement index (RMI) (r i ) for each of the shear wave velocities v i  as the final shear wave velocity v final  as shown in Equation 2 below. 
     
       
         
           
             
               
                 
                   
                     v 
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     On the other hand, the RMI with respect to the shear wave velocity may be obtained by combining the uniformity of the shear wave propagation, the magnitude of the shear wave displacement, the degree of correlation of the shear wave shape, and the like. In addition, the RMI may be calculated by various known methods. 
     For example, the controller  300  may calculate the first shear wave velocity for the first group in two different methods, and determine the RIM measure index for the first shear wave velocity on the basis of the proportions of the first shear wave velocities calculated by the two different methods. Specifically, the controller  300  may calculate a 1-1 shear wave velocity using a value obtained by averaging the distances from the focal point  520 , to which the push pulse is transmitted, to the respective reception scan lines included in the first group and a value obtained by averaging the shear wave arrival times on the respective reception scan lines included in the first group. In addition, the controller  300  may calculate a 1-2 shear wave velocity using the first reception scan line Rx 1 - 1  and the fifth reception scan line Rx 2 - 1  as described above. The controller  300  may calculate a first shear wave velocity ratio SWV_ratio by dividing the difference between the 1-1 shear wave velocity and the 1-2 shear wave velocity by the 1-1 shear wave velocity. The controller  300  may determine an RMI of the first shear wave velocity on the basis of the first shear wave velocity ratio SWV_ratio. 
     On the other hand, when the shear wave velocity ratio SWV_ratio has a value greater than or equal to 0 and less than 0.5, the controller  300  may determine the value of the RMI to be 1. When the value of the shear wave velocity ratio SWV_ratio is greater than or equal to 0.5, the controller  300  may determine the value of the RMI by Equation 3 below. 
       RMI=−2*SWV_ratio+2   [Equation 3]
 
     The controller  300  may calculate the elasticity of the tissue in the ROI on the basis of the final shear wave velocity and generate a shear wave elasticity image. The controller  300  may control the display  270  to output the shear wave elastic image. The shear wave elasticity image may be displayed to overlap or be registered with a reference ultrasound image. The reference ultrasound image may be a B-mode image. In addition, the controller  300  may control the display  270  to display elasticity, depth, and RMI. 
       FIG. 21  is a flowchart showing a method of controlling an ultrasound diagnostic apparatus according to another embodiment, which describes a method of estimating the shear wave velocity by selecting some reception scan lines.  FIGS. 22 and 23  show wave front graphs for describing the method of estimating the shear wave velocity shown in  FIG. 21 . 
     Referring to  FIG. 21 , the ultrasound probe  100  transmits a push pulse to an ROI of an object to induce a shear wave ( 1810 ), and transmits a plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  for observing the shear wave to the ROI of the object ( 1820 ). In this case, the intervals between the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  may be adjusted on the basis of the ROI. 
     The ultrasound probe  100  receives ultrasound echo signals reflected from the ROI along sets B 1 , B 2 , B 3 , and B 4  of multi-reception scan lines, each of the sets corresponding to a respective one of the plurality of tracking pulses ( 1830 ). The controller  300  may detect displacement of tissues at a plurality of sampling points of each of the multi-reception scan lines Rx 1 - 1  to Rx 4 - 4  ( 1840 ). In addition, the controller  300  may estimate the points in time when the shear wave arrives at the plurality of sampling points of each of the multi reception scan lines Rx 1 - 1  to Rx 4 - 4  ( 1850 ). 
     The controller  300  may select some reception scan lines of the multi-reception scan lines and estimate the shear wave velocity on the basis of ultrasound echo signals received along the selected reception scan lines. The controller  300  may select the reception scan lines on the basis of a predetermined selection type ( 1860 ,  2110 , and  2210 ). 
     Referring to  FIG. 22 , as a first selection type, the controller  300  may select reception scan lines adjacent to the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4 , in the sets B 1 , B 2 , B 3 , and B 4  of the multi reception scan lines. Since each of the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  forms a transmission beam, the controller  300  may select reception scan lines adjacent to the center of the transmission beam. 
     As shown in  FIG. 22 , the second reception scan line Rx 1 - 2  and the third reception scan line Rx 1 - 3  adjacent to the center of the transmission beam of the first tracking pulse Tx 1  are selected, the fifth reception scan line Rx 2 - 2  and the sixth reception scan line Rx 2 - 3  adjacent to the center of the transmit beam of the second tracking pulse Tx 2  are selected, the tenth scan line Rx 3 - 2  and the eleventh reception scan line Rx 3 - 3  adjacent to the center of the transmission beam of the third tracking pulse Tx 3  are selected, and the fourteenth reception scan line Rx 4 - 2  and the fifteenth reception scan line Rx 4 - 3  adjacent to the center of the transmission beam of the fourth tracking pulse Tx 4  are selected. 
     As described in  FIG. 15 , since the reception scan lines adjacent to the tracking pulse have a small positional error, the reception scan lines adjacent to the tracking pulse are selected, so that the error of the final shear wave velocity may be reduced. 
     Referring to  FIG. 23 , as a second selection type, the controller  300  may select reception scan lines except for a reception scan line having the minimum shear wave arrival time and a reception scan line having the maximum shear wave arrival time. On the wave front graph of  FIG. 23 , the reception scan line having the minimum shear wave arrival time is the first reception scan line Rx 1 - 1 , and the reception scan line having the maximum shear wave arrival time is the 16th reception scan line Rx 4 - 4 . Considering the positions of the reception scan lines are shifted to the center due to the transmission beam energy of the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4 , the position of the outermost reception scan lines in the sequence of the plurality of reception scan lines may have the largest error. 
     In addition, as a third selection type (not shown), the controller  300  may select reception scan lines having a positional error smaller than a predetermined value. 
     The controller  300  may estimate the final shear wave velocity on the basis of the shear wave arrival times associated with the selected reception scan lines ( 1870 ,  2120 , and  2220 ). 
     Meanwhile, the shear wave velocity estimation method described in  FIGS. 21, 22, and 23  may be combined with the shear wave velocity estimation method described with reference to  FIGS. 19 and 20 . That is, the selected reception scan lines may be set as a plurality of groups, and the shear wave velocity may be estimated for each group. 
       FIGS. 24 and 25  illustrate the intervals between a plurality of tracking pulses. 
     Referring to  FIG. 24 , a beam profile graph of a plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  is shown. The controller  300  may set the intervals w 1  and w 2  between the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  to be narrow such that flat areas (the area greater than −3 dB) of transmission beams of the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4 , is used. The narrow setting of the intervals w 1  and w 2  between the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  may be applied when a narrow ROI is observed. 
     In this case, since multi-reception scan lines Rx 1 , Rx 2 , Rx 3 , and Rx 4  corresponding to each of the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  are set to match the flat area (the area greater than −3 dB) of the transmission beam, the intervals between the reception scan lines may be set to be narrow. On the other hand, the intervals between the reception scan lines are set to be constant. 
     Referring to  FIG. 25 , the controller  300  may set the intervals between a plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  to be larger than a predetermined interval (for example, an interval of the transmission beam at −3 dB) to use non-flat areas (the area smaller than −3 dB) of the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4 . Even in this case, the beam widths of the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  are tightly set. The wide setting of the intervals w 1  and w 2  between the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  may be applied when observing a wide ROI. The intervals between the plurality of tracking pulses may be set without deviation from a range ROI. 
     For example, in order to observe a shear wave in an ROI larger than a predetermined magnitude using four tracking pulses Tx 1 , Tx 2 , Tx 3  and Tx 4 , the intervals between the four tracking pulses need to be set large. 
     In this case, multi-reception scan lines Rx 1 , Rx 2 , Rx 3 , and Rx 4  corresponding to each of the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  are set to match the non-flat areas of the transmission beam (the area smaller than −3 dB), and thus the intervals between the reception scan lines may be set to be wide. Meanwhile, the intervals between the reception scan lines shown in  FIG. 25  is larger than the intervals between the reception scan lines shown in  FIG. 24 . 
     Although not shown, the controller  300  may set the intervals between the plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , and Tx 4  to be different from each other. The plurality of tracking pulses Tx 1 , Tx 2 , Tx 3 , Tx 4  are transmitted to the ROI on the basis of a preset interval. 
     On the other hand, when the intervals between the plurality of tracking pulses are set to be larger than a predetermined interval as shown in  FIG. 25 , the benefit of the shear wave velocity estimation method described in  FIGS. 19 to 23  may be provided. That is, since the shear wave velocity estimation method according to the disclosure selectively perform signal processing and/or data processing on multi-reception scan lines, the error of the shear wave velocity estimation may be reduced even when observing the shear wave fora wide ROI. 
       FIGS. 26 and 27  show the result of the elasticity measurement according to the related art.  FIG. 28  shows the result of the elasticity measurement by the method of controlling the ultrasound diagnostic apparatus according to the embodiment. 
       FIGS. 26 to 28  illustrate ultrasound diagnosis images of a phantom including a liver and a fat layer. To generate an environment in which reverberation occurs, a phantom with a fat layer of 2 cm is used. On the other hand, the liver phantom has an elasticity value of 13 kPa to 14 kPa. 
     For comparison of the related art and the disclosure, an ROI  550  was set at a position in which the depth of the phantom is about 4 cm to 6 cm, and the elasticity of the ROI  550  was measured. 
     Referring to  FIGS. 26 and 27 , the related art failed to properly measure the elasticity value in an environment in which reverberation strongly occur. In  FIG. 26 , the elasticity value was measured at 23.7 kPa (see  2500 ). In  FIG. 27 , the elasticity value was measured at 34.2 kPa (see  2600 ). As such, the reliability of the elasticity value measured by the related art is very low. 
     On the other hand, referring to  FIG. 28 , it can be seen that the disclosure measured the elasticity value with a higher accuracy even in an environment in which reverberation occurs. That is, when the shear wave velocity estimation method according to the disclosure was used, the elasticity value was measured at 12.6 kPa (see  2700 ). The RMI value was calculated to be 0.5. As such, the elasticity value measured according to the disclosure is significantly close to the elasticity value of the phantom, and the reliability is very high. 
     As described above, according to the disclosed ultrasound diagnostic apparatus and the control method, the tracking pulse with a narrow beam width may improve the shear wave observation performance and may accurately measure the elasticity even in an environment in which reverberation occurs. 
     In addition, according to the disclosed ultrasound diagnostic apparatus and the control method, when the ROI is set wide, intervals between the tracking pulses for the shear wave observation are set to be wide, so that the elasticity may be accurately measured. 
     In addition, according to the disclosed ultrasound diagnostic apparatus and the control method, the shear wave velocity may be accurately obtained by selectively performing signal processing on the multi-reception scan line used to estimate the shear wave velocity. 
     Meanwhile, the disclosed embodiments may be embodied in the form of a recording medium storing instructions executable by a computer. The instructions may be stored in the form of program code and, when executed by a processor, may generate a program module to perform the operations of the disclosed embodiments. The recording medium may be embodied as a computer-readable recording medium. 
     The computer-readable recording medium includes all kinds of recording media in which instructions which may be decoded by a computer are stored, for example, a Read Only Memory (ROM), a Random-Access Memory (RAM), a magnetic tape, a magnetic disk, a flash memory, an optical data storage device, and the like. 
     As is apparent from the above, the ultrasound diagnostic apparatus and the method of controlling the same can improve the performance of shear wave observation and accurately measure the elasticity even in the presence of reverberation by narrowing the beam width of the tracking pulses. 
     The ultrasound diagnostic apparatus and the method of controlling the same can accurately measure the elasticity by setting the interval of tracking pulses for shear wave observation to be wide when a region of interest (ROI) is set wide. 
     In addition, the ultrasound diagnostic apparatus and the method of controlling the same can accurately measure the elasticity by selectively performing signal processing on multiple-reception scan lines used to estimate the shear wave velocity. 
     Although embodiments of the disclosure have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the disclosure. Therefore, embodiments of the disclosure have not been described for limiting purposes.