Abstract:
The present invention aims at providing an ultrasound image diagnostic apparatus capable of shortening the time required for measurement and calculation of an optimal sound velocity and obtaining an ultrasound image having excellent image quality at a tissue or lesion the operator desires to observe. There are provided an element data memory storing element data; a region-of-interest setter setting a region of interest; an optimal sound velocity calculator calculating an optimal sound velocity at the region of interest using the element data corresponding to the region of interest; a reconstruction region setter setting a reconstruction region that contains the region of interest and is larger than the region of interest; and an image reconstructor generating a reconstructed image by reconstructing the ultrasound image in the reconstruction region based on the optimal sound velocity.

Description:
CROSS REFERENCE TO RELATE APPLICATIONS 
       [0001]    This application is a continuation application of International Application No. PCT/JP2013/064038 filed on May 21, 2013, which claims priority under 35 U.S.C. 119(a) to patent application Ser. No. 2012-120011 filed in Japan on May 25, 2012, all of which are hereby expressly incorporated by reference into the present application. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to an ultrasound image diagnostic apparatus that generates an ultrasound diagnostic image for use in diagnosis by imaging an organ and the like in a living body through transmission and reception of ultrasonic waves. 
         [0003]    Conventionally, ultrasound diagnostic apparatuses that use ultrasound images have been put to practical use in the medical field. In general, this type of ultrasound diagnostic apparatus comprises an ultrasound probe having a built-in transducer array and an apparatus body connected to the ultrasound probe. The ultrasound probe transmits ultrasonic waves toward a subject and receives ultrasonic echoes from the subject, and the apparatus body electrically processes the reception signals to generate an ultrasound image. 
         [0004]    When such an ultrasound diagnostic apparatus generates an ultrasound image, the sound velocity in a living body of a subject is assumed to be constant. However, in an actual living body, the sound velocity varies depending on the nature of an organ in the living body and thus there is variability among sound velocities. The variability causes deterioration in image quality of an ultrasound image, such as spatial distortion and decrease in contrast or spatial resolution. 
         [0005]    In recent years, in order to more accurately diagnose a diagnosis site in a subject, a sound velocity value at a given diagnosis site is optimized to minimize distortion of an image, decrease in spatial resolution and the like to thereby improve the image quality. 
         [0006]    For instance, JP 2011-229817 A describes having evaluation means that calculates an evaluation value indicative of a degree of variability of phase information based on plural pieces of phase information obtained with respect to all or a part of plural reception signals after delay processing but before addition processing; and determination means that determines an optimal value of a sound speed parameter value based on the change in the evaluation value occurring with varying sound speed parameter value, thereby obtaining the optimal sound speed value, wherein an ultrasound image is generated using the optimal sound speed value to thereby improve spatial resolution of the resulting ultrasound image. Furthermore, JP 2011-229817 A describes that the optimal sound speed value at a region of interest (ROI) set by the operator is obtained based on the degree of variability of phase information among the plural reception signals after delay processing but before addition processing. 
         [0007]    Owing to the configuration in which a region of interest is set in an ultrasound image and the optimal sound speed value at the region of interest is obtained, it is possible to prevent distortion of the image from occurring in the region of interest, i.e., at a tissue or lesion the operator desires to observe, and improve spatial resolution of the image, while the optimal sound speed value, which is generally obtained through time-consuming calculation, can be obtained in a short time, thus shortening the time required for calculation. 
       SUMMARY OF THE INVENTION 
       [0008]    However, in the case where a region of interest is set in an ultrasound image, a tissue or lesion the operator desires to observe may be displaced from the set region of interest due to movement of the probe or tissue. When a region of interest is set and the optimal sound speed value at the region of interest is obtained to thereby generate an ultrasound image, the ultrasound image is generated with the sound speed value optimal for a tissue or lesion the operator desired to observe in the region of interest, whereas the ultrasound image is generated with a sound speed value different from the sound speed value optimal for the tissue or lesion the operator desires to observe in a region other than the region of interest. Accordingly, when the tissue or lesion the operator desires to observe is displaced from the region of interest, this may adversely affect the image quality. 
         [0009]    The present invention aims at solving the foregoing problems of the prior art and providing an ultrasound image diagnostic apparatus capable of shortening the time required for measurement and calculation of an optimal sound velocity and obtaining an ultrasound image having excellent image quality at a tissue or lesion the operator desires to observe. 
         [0010]    In order to attain the above objects, the present invention provides an ultrasound image diagnostic apparatus, comprising: a transducer array having a plurality of oscillators and adapted to transmit an ultrasonic wave toward a subject, receive an ultrasonic echo from the subject and output element data which is a reception signal of each of the plurality of oscillators; an image generator adapted to generate an ultrasound image based on a sound ray signal produced from the element data output by the transducer array; an element data memory adapted to store the element data that each of the plurality of oscillators of the transducer array has output upon receipt of an ultrasonic echo; a region-of-interest setter adapted to set a region of interest in the ultrasound image generated by the image generator; a region-of-interest element data acquirer adapted to acquire element data corresponding to the region of interest; an optimal sound velocity calculator adapted to calculate an optimal sound velocity at the region of interest using the element data corresponding to the region of interest as acquired; a reconstruction region setter adapted to set a reconstruction region that contains the region of interest and is larger than the region of interest; and an image reconstructor adapted to generate a reconstructed image by reconstructing the ultrasound image in the reconstruction region using the element data stored in the element data memory based on the calculated optimal sound velocity. 
         [0011]    Preferably, the reconstruction region setter sets the reconstruction region by adding a margin region having a preset width to the region of interest set by the region-of-interest setter. 
         [0012]    It is preferable to include an image regenerator adapted to regenerate an ultrasound image using the ultrasound image generated by the image generator and the reconstructed image generated by the image reconstructor. 
         [0013]    Preferably, the image regenerator regenerates an ultrasound image by replacing the reconstruction region of the ultrasound image by the reconstructed image. 
         [0014]    Alternatively, the image regenerator preferably regenerates an ultrasound image by replacing the reconstruction region of the ultrasound image by an image obtained by synthesizing the ultrasound image in the reconstruction region and the reconstructed image. 
         [0015]    Alternatively, the image regenerator preferably regenerates an ultrasound image by replacing the region of interest of the ultrasound image by the reconstructed image in the region of interest and replacing a margin region of the ultrasound image by an image obtained by synthesizing the ultrasound image in the margin region and the reconstructed image in the margin region at a ratio varied such that a proportion of the ultrasound image increases with increasing distance from the region of interest. 
         [0016]    Preferably, the region-of-interest element data acquirer acquires the element data corresponding to the region of interest from the element data stored in the element data memory. 
         [0017]    Alternatively, the region-of-interest element data acquirer preferably acquires element data that the transducer array has output upon transmission of an ultrasonic wave toward the region of interest and receipt of an ultrasonic echo. 
         [0018]    Preferably, the region-of-interest setter sets the region of interest in accordance with an instruction input from an input device. 
         [0019]    Alternatively, the region-of-interest setter preferably extracts a structural region based on a difference in brightness between adjacent pixels of the ultrasound image and sets a region containing the structural region as the region of interest. 
         [0020]    According to the ultrasound image diagnostic apparatus of the invention having the foregoing configuration, it is possible to shorten the time required for measurement and calculation of an optimal sound velocity and obtain an ultrasound image having excellent image quality at a tissue or lesion the operator desires to observe. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a block diagram conceptually showing the configuration of an ultrasound image diagnostic apparatus according to the present invention. 
           [0022]      FIG. 2  is a block diagram conceptually showing the configuration of a reconstructed image generator shown in  FIG. 1 . 
           [0023]      FIG. 3  is a diagram schematically showing a region of interest and reconstruction region as set. 
           [0024]      FIG. 4  is a flowchart for explaining the operation of the ultrasound image diagnostic apparatus shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    An ultrasound image diagnostic apparatus of the invention is described in detail below with reference to a preferred embodiment shown in the accompanying drawings. 
         [0026]      FIG. 1  is a block diagram conceptually showing an example of the configuration of the ultrasound image diagnostic apparatus (hereinafter also referred to as “ultrasound diagnostic apparatus”) of the invention. 
         [0027]    An ultrasound diagnostic apparatus  10  includes an ultrasound probe  12 , a transmission circuit  14  and reception circuit  16  connected to the ultrasound probe  12 , an A/D converter  20 , an image generator  18 , an element data memory  22 , a reconstructed image generator  24 , an image regenerator  26 , a display controller  32 , a monitor  34 , a controller  36 , an operating unit  38  and a storage  40 . 
         [0028]    The ultrasound probe  12  includes a transducer array  42  of type used in general ultrasound diagnostic apparatuses. 
         [0029]    The transducer array  42  comprises a plurality of ultrasound transducers arranged one-dimensionally or two-dimensionally. During the generation of an ultrasound image, these ultrasound transducers transmit an ultrasonic beam according to driving signals supplied from the transmission circuit  14  to them, respectively, and receive an ultrasonic echo from the subject to output reception signals. 
         [0030]    Each of the ultrasound transducers comprises an oscillator (element) composed of a piezoelectric body and electrodes each provided on either end of the piezoelectric body. The piezoelectric body is composed of, for example, a piezoelectric ceramic represented by a PZT (lead zirconate titanate), a piezoelectric polymer represented by PVDF (polyvinylidene fluoride), or a piezoelectric monocrystal represented by PMN-PT (lead magnesium niobate-lead titanate solid solution). 
         [0031]    When a pulsed voltage or a continuous-wave voltage is applied across the electrodes of each oscillator, the piezoelectric body expands and contracts and the oscillator produces a pulsed or continuous ultrasonic wave. The thus produced ultrasonic waves are combined to form an ultrasonic beam. Upon reception of a propagating ultrasonic wave, each oscillator expands and contracts to produce an electric signal, which is in turn output as a reception signal of the ultrasonic wave. 
         [0032]    The transmission circuit  14  including, for example, a plurality of pulsers modifies delay amounts for driving signals in accordance with a transmission delay pattern set based on a sound velocity or a sound velocity distribution as selected according to a control signal from the controller  36  so that ultrasonic waves transmitted from the ultrasound transducers of the transducer array  42  form an ultrasonic beam, and supplies the driving signals to the ultrasound transducers. 
         [0033]    The reception circuit  16  receives element data which is a reception signal each ultrasound transducer of the transducer array  42  outputs upon receipt of an ultrasonic echo from the subject, amplifies the element data associated with each ultrasound transducer, and supplies the element data which is analog to the A/D converter  20 . 
         [0034]    The A/D converter  20  converts the analog element data supplied from the reception circuit  16  into digital element data. The A/D converter  20  supplies the digital element data to a phasing adder  44  of the image generator  18  and also to the element data memory  22 . 
         [0035]    The image generator  18  produces a sound ray signal (reception data) from the digital element data supplied from the A/D converter  20  and generates an ultrasound image using the sound ray signal. 
         [0036]    The image generator  18  includes the phasing adder  44 , a signal processor  46 , a digital scan converter (DSC)  48 , an image processor  50  and an image memory  52 . 
         [0037]    Based on a receiving direction set by the controller  36 , the phasing adder  44  selects one from among plural reception delay patterns set according to a sound velocity or a sound velocity distribution and stored in advance, applies a corresponding delay to each piece of element data in accordance with the selected reception delay pattern, and adds up the pieces of element data to thereby perform reception focusing processing. By this reception focusing processing, reception data (sound ray signal) is produced in which the ultrasonic echo is well focused. 
         [0038]    The phasing adder  44  supplies the reception data to the signal processor  46 . 
         [0039]    The signal processor  46  corrects the reception data produced by the phasing adder  44  in terms of attenuation caused due to distance based on the depth at which the ultrasonic waves are reflected, and then performs envelope detection processing to thereby produce a B-mode image signal which is tomographic image information on a tissue inside the subject. 
         [0040]    The DSC  48  converts the B-mode image signal produced by the signal processor  46  into an image signal compatible with an ordinary television signal scanning mode (raster conversion). 
         [0041]    The image processor  50  performs various types of necessary image processing such as gradation processing on the B-mode image signal entered from the DSC  48  and then outputs the B-mode image signal to the display controller  32  or stores the B-mode image signal in the image memory  52 . 
         [0042]    The display controller  32  causes the monitor  34  to display an ultrasound image based on the B-mode image signal having undergone the image processing performed by the image processor  50 . 
         [0043]    The monitor  34  includes a display device such as an LCD, for example, and displays the ultrasound image under the control of the display controller  32 . 
         [0044]    The element data memory  22  sequentially stores the digital element data output from the A/D converter  20 . The element data memory  22  also stores in association with the above element data information on a frame rate input by the controller  36 . The information on a frame rate includes, for example, the depth at which an ultrasonic wave is reflected, the density of scanning lines, and a parameter representing the range of visual field. 
         [0045]    The reconstructed image generator  24  calculates an optimal sound velocity at a region of interest to reconstruct an image of a region containing the region of interest under the control of the controller  36 . 
         [0046]      FIG. 2  is a block diagram conceptually showing the configuration of the reconstructed image generator  24 . 
         [0047]    As shown in  FIG. 2 , the reconstructed image generator  24  includes a region-of-interest (ROI) setting section  60 , a ROI data acquiring section  62 , an optimal sound velocity calculating section  64 , a reconstruction region setting section  66  and an image reconstructing section  68 . 
         [0048]    The ROI setting section  60  sets a region of interest ROI in accordance with an input by the operator through the operating unit  38 . 
         [0049]      FIG. 3  is a diagram schematically showing an example of an ultrasound image in which the region of interest ROI is set. 
         [0050]    In the ultrasound image shown in  FIG. 3 , a target site P such as a tissue (organ) or lesion the operator desires to observe is captured, and the region of interest ROI is set so as to contain the target site P. Furthermore, a reconstruction region Z containing the region of interest ROI is set as indicated by dashed line in  FIG. 3 . The reconstruction region Z will be described later in detail. 
         [0051]    The ROI setting section  60  supplies information on the set region of interest ROI to the ROI data acquiring section  62  and the reconstruction region setting section  66 . 
         [0052]    The ROI setting section  60  is not limited to the one configured to set a region of interest ROI in accordance with an input from the operating unit  38  but may set a region of interest ROI by analyzing an ultrasound image (B-mode image signal) generated by the image generator  18 . 
         [0053]    For instance, the ROI setting section  60  may extract a structural region based on the difference in brightness between adjacent pixels in an ultrasound image generated by the image generator  18 . Specifically, the ROI setting section  60  may determine positions (pixels) where the difference in brightness between adjacent pixels is equal to or greater than a predetermined value as constituting the edge of a structural region (tissue or lesion), determine and extract a region surrounded by the edge or a region significantly containing the edge as the structural region, namely, an image of the target site P, and set a region containing the image of the target site P and having predetermined shape and size as the region of interest ROI. 
         [0054]    When the region of interest ROI is set by analyzing an ultrasound image, the structural region may be identified as a single region. Alternatively, two or more portions different in continuity of the edge, brightness level or the like may be identified as regions which are determined and extracted as images of separate target sites P, and a region of interest ROI may be set for each of the target sites P. 
         [0055]    The ROI data acquiring section  62  reads out element data for a position corresponding to the region of interest ROI from element data stored in the element data memory  22  based on the information on the region of interest ROI supplied from the ROI setting section  60  and supplies the element data as read out to the optimal sound velocity calculating section  64 . 
         [0056]    The ROI data acquiring section  62  is not limited to the one configured to read out element data for a position corresponding to the region of interest ROI from the element data memory  22 . For instance, the configuration may be made in such a manner that after a region of interest ROI is set, the transducer array  42  transmits ultrasonic waves with a focus on a position corresponding to the set region of interest ROI. In this case, the ROI data acquiring section  62  acquires element data, which has been produced by transmitting ultrasonic waves to the position corresponding to the region of interest ROI and receiving the resulting ultrasonic echoes, as the element data for the position corresponding to the region of interest ROI. 
         [0057]    The optimal sound velocity calculating section  64  calculates an optimal sound velocity at the set region of interest ROI. 
         [0058]    The optimal sound velocity refers to a set sound velocity that allows an ultrasound image to have the highest contrast and/or sharpness when the set sound velocity is caused to vary in many ways and the reception focusing processing is performed based on various sound velocities to thereby generate ultrasound images. The optimal sound velocity may be determined based on, for example, image contrast or spatial frequency dispersion in a scanning direction as described in JP 08-317926 A. 
         [0059]    For instance, the optimal sound velocity calculating section  64  causes a set sound velocity v to vary from 1400 m/s to 1650 m/s in increments of 50 m/s and calculates sharpness values of images of the region of interest ROI with respective set sound velocities v. More specifically, for each set sound velocity v, the optimal sound velocity calculating section  64  performs the reception focusing processing based on the relevant set sound velocity v and using the element data corresponding to the region of interest ROI that is supplied from the ROI data acquiring section  62 , so as to produce a sound ray signal and form an ultrasound image from the sound ray signal, and calculates the sharpness of the image in the region of interest ROI for the relevant set sound velocity v. 
         [0060]    The optimal sound velocity calculating section  64  compares the sharpness values of images generated with the respective set sound velocities v and defines the set sound velocity v allowing the highest sharpness as the optimal sound velocity. 
         [0061]    The optimal sound velocity calculating section  64  supplies the calculated optimal sound velocity to the image reconstructing section  68 . 
         [0062]    The reconstruction region setting section  66  sets a region in which an image is reconstructed, i.e., a reconstruction region Z, using the optimal sound velocity calculated by the optimal sound velocity calculating section  64 . 
         [0063]    The reconstruction region setting section  66  defines a region that contains the region of interest ROI supplied from the ROI setting section  60  and that is larger than the region of interest ROI as the reconstruction region Z. 
         [0064]    As an example, the reconstruction region setting section  66  sets the reconstruction region Z by adding a region having a predetermined width to the edge of the region of interest ROI as shown in  FIG. 3 . Hereinafter, a part of the reconstruction region Z other than the region of interest ROI is called “margin region.” 
         [0065]    The reconstruction region Z is not particularly limited in size but preferably has the margin region that is large enough to cover an area corresponding to movement of the probe or tissue. This enables the target site P to remain in the reconstruction region Z even when the target site P is displaced from the region of interest ROI due to movement of the probe or tissue. 
         [0066]    The reconstruction region setting section  66  supplies information on the set reconstruction region Z to the image reconstructing section  68 . 
         [0067]    The image reconstructing section  68  reconstructs an image of the reconstruction region Z based on the optimal sound velocity. 
         [0068]    The image reconstructing section  68  first reads out element data for a position corresponding to the reconstruction region Z from element data stored in the element data memory  22  based on the information on the reconstruction region Z supplied from the reconstruction region setting section  66 . Next, the image reconstructing section  68  produces a sound ray signal by performing the reception focusing processing on the read element data based on the optimal sound velocity supplied from the optimal sound velocity calculating section  64  and produces a B-mode image signal for the reconstruction region Z from the sound ray signal. The image reconstructing section  68  then performs raster conversion as well as various necessary image processing such as gradation processing on the produced B-mode image signals to thereby generate a reconstructed image. 
         [0069]    The image reconstructing section  68  supplies the generated reconstructed image to the image regenerator  26 . 
         [0070]    As described above, with the configuration in which in generating an ultrasound image, a region of interest is set in an ultrasound image and the optimal sound velocity at the region of interest is obtained, it is possible to improve the image quality at the region of interest, i.e., at a tissue or lesion the operator desires to observe, and obtain the optimal sound velocity value in a short time. 
         [0071]    However, in the case where a region of interest is set in an ultrasound image, a tissue or lesion the operator desires to observe may be displaced from the set region of interest due to movement of the probe or tissue. When a region of interest is set and the optimal sound velocity value at the region of interest is obtained to thereby generate an ultrasound image, the ultrasound image is generated with the sound velocity value optimal for a tissue or lesion the operator desires to observe in the region of interest, whereas the ultrasound image is generated with a sound velocity value different from the sound velocity value optimal for the tissue or lesion the operator desires to observe in a region not within the region of interest. Accordingly, when the tissue or lesion the operator desires to observe is displaced from the region of interest, this may adversely affect the image quality. 
         [0072]    To cope with it, the present invention reconstructs an image in a reconstruction region that contains a region of interest and is larger than the region of interest based on the optimal sound velocity at the region of interest. As a result, even when a tissue or lesion the operator desires to observe is displaced from the set region of interest, the resulting ultrasound image can have excellent image quality at the tissue or lesion the operator desires to observe. 
         [0073]    The image regenerator  26  synthesizes the original ultrasound image with the reconstructed image supplied from the image reconstructing section  68  to regenerate an ultrasound image. 
         [0074]    Specifically, the image regenerator  26  reads out an ultrasound image stored in the image memory  52  while acquiring the reconstructed image supplied from the image reconstructing section  68 , replaces the reconstruction region Z of the ultrasound image by the reconstructed image to thereby generate a new ultrasound image. 
         [0075]    The image regenerator  26  supplies the generated new ultrasound image to the display controller  32 . 
         [0076]    The image regenerator  26  is not limited to the forgoing configuration in which the image of the reconstruction region Z of the ultrasound image is replaced by the reconstructed image. 
         [0077]    For instance, the image regenerator  26  may generate a new ultrasound image by combining a brightness value of each pixel of the image in the reconstruction region Z in the original ultrasound image and a brightness value of the corresponding pixel of the reconstructed image at a predetermined ratio to thereby obtain brightness values of respective pixels, and defining the resulting image as the image in the reconstruction region Z. 
         [0078]    Alternatively, the image regenerator  26  may generate a new ultrasound image by replacing the image in the region of interest ROI by the corresponding part of the reconstructed image and replacing the image in the margin region by an image obtained by synthesizing the original ultrasound image in the margin region and the corresponding part of the reconstructed image. Still alternatively, the image in the margin region may be regenerated by synthesizing the original ultrasound image and the reconstructed image so that the reconstructed image makes up a greater proportion at a side closer to the region of interest and the proportion of the original ultrasound image increases with increasing distance from the region of interest. 
         [0079]    The controller  36  controls the components in the ultrasound diagnostic apparatus  10  in accordance with an instruction entered by the operator through the operating unit  38 . 
         [0080]    When the operator enters an input for setting a region of interest ROI through the operating unit  38 , the controller  36  supplies to the ROI setting section  60  information on the region of interest ROI as input through the operating unit  38 . 
         [0081]    The operating unit  38  is provided for the operator to perform input operations and may be composed of, for example, a keyboard, a mouse, a track ball, and/or a touch panel. 
         [0082]    The operating unit  38  also includes an input device to be used by the operator to input information on the region of interest ROI. 
         [0083]    The storage  40  stores, for example, an operation program and may be constituted by a recording medium such as a hard disk, a flexible disk, an MO, an MT, a RAM, a CD-ROM, or a DVD-ROM. 
         [0084]    The phasing adder  44 , the signal processor  46 , the DSC  48 , the image processor  50 , the display controller  32  and the reconstructed image generator  24  are constituted by a CPU and operation programs for causing the CPU to perform various kinds of processing, but they may be each constituted by a digital circuitry. 
         [0085]    While in this embodiment, the image reconstructing section  68  generating a reconstructed image and the image generator  18  generating an ultrasound image are provided separately, the present invention is not limited thereto and the image generator  18  may double as the image reconstructing section. 
         [0086]    To be more specific, the configuration may be made so that after the optimal sound velocity calculating section  64  calculates the optimal sound velocity at the region of interest ROI and the reconstruction region setting section  66  sets the reconstruction region Z, in the image generator  18 , the phasing adder  44  reads out element data corresponding to the reconstruction region Z and performs the reception focusing processing based on the optimal sound velocity at the region of interest ROI to thereby produce a sound ray signal; the signal processor  46  produces a B-mode image signal from the sound ray signal; the DSC  48  performs raster conversion on the B-mode image signal; and the image processor  50  performs the image processing to generate the reconstructed image. 
         [0087]    Next, the operation of the ultrasound diagnostic apparatus  10  is described more specifically with reference to a flowchart of  FIG. 4 . 
         [0088]    When the operator holds the ultrasound probe  12  in contact with a surface of the subject and starts measurement, the transducer array  42  transmits ultrasonic beams in accordance with driving signals supplied from the transmission circuit  14 , receives ultrasonic echoes from the subject and outputs reception signals (element data). 
         [0089]    The reception circuit  16  amplifies the analog element data and supplies the amplified analog element data to the A/D converter  20 . The A/D converter  20  converts the received analog element data into digital element data and supplies the digital element data to the phasing adder  44  of the image generator  18  and also to the element data memory  22 . 
         [0090]    The phasing adder  44  of the image generator  18  performs the reception focusing processing on the element data to produce reception data (sound ray signal) and supplies the reception data to the signal processor  46 . The signal processor  46  processes the sound ray signal to produce a B-mode image signal. The DSC  48  performs raster conversion on the B-mode image signal, and the image processor  50  performs the image processing on the converted B-mode image signal to thereby generate an ultrasound image. The generated ultrasound image is stored in the image memory  52  and also displayed on the monitor  34  under the control of the display controller  32  (S 102 ). 
         [0091]    Next, the operator operates the operating unit  38  as referring to the displayed ultrasound image to input an instruction for setting a region of interest ROI. The ROI setting section  60  sets the region of interest ROI in accordance with the instruction input from the operating unit  38  (S 104 ). 
         [0092]    Once the region of interest ROI is set, the ROI data acquiring section  62  reads out element data for a position corresponding to the set region of interest ROI from the element data memory  22  based on the information on the region of interest ROI and supplies the element data to the optimal sound velocity calculating section  64  (S 106 ). 
         [0093]    The optimal sound velocity calculating section  64  causes a set sound velocity v to vary from 1400 m/s to 1650 m/s in increments of 50 m/s, performs the reception focusing processing based on the relevant set sound velocity v and using the element data for the position corresponding to the region of interest ROI, so as to produce a sound ray signal and form an ultrasound image from the sound ray signal, and calculates the sharpness of the image in the region of interest ROI for the relevant set sound velocity v (S 108  to S 114 ). 
         [0094]    The optimal sound velocity calculating section  64  compares sharpness values of images formed with the respective set sound velocities v and defines the set sound velocity v allowing the highest sharpness as the optimal sound velocity V (S 116 ). 
         [0095]    On the other hand, the reconstruction region setting section  66  sets a reconstruction region Z by adding a margin region having a predetermined width to the edge of the set region of interest ROI (S 118 ). 
         [0096]    The image reconstructing section  68  reads out the element data for the position corresponding to the reconstruction region Z and performs the reception focusing processing based on the optimal sound velocity V so as to produce a sound ray signal and generate an image in the reconstruction region Z (reconstructed image) from the sound ray signal (S 120 ). 
         [0097]    The image regenerator  26  reads out the ultrasound image from the image memory  52 , replaces the reconstruction region Z of the ultrasound image by the reconstructed image to thereby generate a new ultrasound image (S 122 ). 
         [0098]    Thus, the ultrasound diagnostic apparatus  10  of the invention calculates the optimal sound velocity at the set region of interest ROI and reconstructs the image in the reconstruction region Z covering a broader area than the region of interest ROI. As a result, even when a tissue or lesion the operator desires to observe is displaced from the set region of interest due to movement of the probe or tissue, the resulting ultrasound image can have excellent image quality at the tissue or lesion the operator desires to observe. 
         [0099]    The present invention is basically as described above. 
         [0100]    While the present invention has been described in detail above, the invention is not limited to the above embodiment and various modifications and improvements may be made without departing from the spirit of the invention.