Patent Publication Number: US-10765405-B2

Title: Ultrasound imaging pickup apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to an ultrasound imaging technology that takes images inside of a test object using ultrasound waves. 
     BACKGROUND ART 
     The ultrasound imaging technology is a technology that takes images of the inside of a test object such as a human body noninvasively using ultrasound waves (inaudible sound waves, that is, sound waves whose frequencies are typically 20 kHz or higher). 
     As a transmission method for transmitting an ultrasound beam from an ultrasound probe to a test object such as a human body, there are two kinds of transmission methods, and one is a dispersing-type transmission method in which an ultrasound beam that disperses in a fan shape is transmitted, and another is a focusing-type transmission method in which the transmit focus of an ultrasound beam is disposed inside of a test object, and the ultrasound beam is converged on the focus. 
     Because the transmission/reception of ultrasound waves by an ultrasound image pickup apparatus is performed by means of an array with an aperture of a finite diameter, the transmission or reception is affected by the diffractions of the ultrasound waves caused by the edge of the aperture, therefore it is difficult to improve the resolution in the direction of an azimuthal angle. The above problem can be solved if an array of an infinite length can be prepared, but in actuality it is impossible to prepare an array of an infinite length. Therefore, in order to improve the resolution in the direction of an azimuthal angle, channel domain phasing technologies have been widely studied in recent years, with the result that new phasing schemes such as an adaptive beamformer and aperture synthesis have been extensively reported. 
     The aperture synthesis will be briefly explained. First, by respectively giving delay times to reception signals received by plural elements included in an ultrasound probe, the delayed reception signals are virtually focused on a certain point, and then a phased signal is obtained by adding these delayed reception signals. The aperture synthesis is performed by synthesizing this phased signal and one phased signal or more obtained regarding the same point through other one or more transmissions/receptions, and by superimposing these signals on each other. 
     In the aperture synthesis, because phased signals obtained by an ultrasound probe through the transmission/reception to or from different directions regarding a certain point can be superimposed on each other, it can be expected that the improvement of the resolution of a point image and the robustness against the inhomogeneity of the point image are provided. In addition, because processing gain can be increased owing to the superimposing processing, the number of transmissions of ultrasound waves can be reduced in comparison with the number of usual transmissions of ultrasound waves, the aperture synthesis can be also applied to high-speed imaging. 
     Patent Literature 1 relates to an ultrasound diagnostic apparatus, and discloses a technology in which aperture synthesis is performed using an improved virtual source method in ultrasound imaging in which focusing-type transmission is performed. To put it concretely, the aperture synthesis is performed under the assumption that a focus is a virtual source in an area where the energy of an ultrasound beam is converged on a focus (an area A shown in FIG. 2 of Patent Literature 1), while the aperture synthesis is performed under the assumption that a spherical wave is irradiated from the end of a probe in areas which are adjacent to the area A and in which the energy of the ultrasound beam disperses (areas B and C). 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. Hei 10 (1998)-277042 
     SUMMARY OF INVENTION 
     Technical Problem 
     The focusing-type transmission method has smaller errors between delay times even in the case where the divergence angle of transmission is large in comparison with the dispersing-type transmission method. Therefore, in the focusing-type transmission method, because the divergence angle of the transmitted ultrasound wave can be set large, a larger number of reception scanning lines (assemblies of points at which phased signals are calculated) can be set in comparison with in the dispersion-type transmission method. It becomes possible to speedily image a wider imaged area with a fewer number of transmissions by setting a many number of reception scanning lines. Furthermore, in transmission aperture processing, more phased signals can be synthesized in the case of a large number of reception scanning lines being set than in the case of a small number of reception scanning lines being set even if the same number of transmissions are performed in both cases, and advantageous effects such as the improvement of resolution can be obtained. 
     As shown in Patent Literature 1, delay times are calculated in the irradiation area of a transmission beam (in an area where ultrasound energy is converged) using the virtual source method, and delay times are calculated under the assumption that a spherical wave is irradiated from the end of a probe outside of the irradiation area of the transmission beam (in areas where the energy of the ultrasound beam disperses), which makes it possible to obtain phased signals even at points outside of the irradiation area of the transmission beam. Therefore, reception scanning lines can be set even outside of the irradiation area of the transmission beam. 
     However, in the case where delay times at points on a reception scanning line outside of the irradiation area of the transmission beam are calculated using the waveform of a spherical wave which is considered to be irradiated from the end of the probe according to the technology disclosed in Patent Literature 1, the waveform of the spherical wave used for calculation of the delay times have to be switched from the waveform of a spherical wave irradiated from the left part of the edge of the probe to the waveform of a spherical wave irradiated from the right part of the edge of the probe or vice versa in the vicinity of the depth of a transmit focus. Owing to this switching, there arises a problem in that a curve representing the variation between delay times in the direction of the depth along the reception scanning line becomes discontinuous in the vicinity of the depth of the transmit focus. The discontinuity of the variation between the delay times in the vicinity of the depth of the transmit focus incurs the discontinuity of the pixel values of a generated ultrasound image in the vicinity of the depth of the transmit focus, so that an artifact is generated in the vicinity of the depth of the transmit focus. 
     One of the objects of the present invention is to execute reception beamforming that does not generate discontinuity in the vicinity of the depth of the transmit focus even if reception scanning lines are disposed outside of the irradiation area of a focusing-type transmission beam. 
     Solution to Problem 
     In a first embodiment of the present invention, a discontinuity extracting unit detects the degree of discontinuity showing the discontinuity of the wave fronts of reception signals received by plural ultrasound elements or the discontinuity of the wave fronts of phased signals. If there is an area where the degree of discontinuity is larger than a predefined value, a delay time generating unit for discontinuity elimination changes delay times in the area where the discontinuity is generated. 
     An ultrasound image pickup apparatus according to a second embodiment of the present invention includes: an ultrasound element array in which plural ultrasound elements are arranged in a predefined direction; a transmission beamformer that makes at least some of the plural ultrasound elements transmit a focusing-type transmission beam to the imaged area of a test object; and a reception beamformer that delays reception signals output by the plural ultrasound elements, which receive ultrasound waves from the test object, by delay times to phase the reception signals, and outputs phased signals after adding the delayed and phased reception signals. The reception beamformer includes: a scanning line setting unit that sets reception scanning lines not only inside of the irradiation area of the focusing-type transmission beam but also outside of the irradiation area of the focusing-type transmission beam; and a delay time calculation unit that calculates delay times at predefined points on the reception scanning lines. The delay time calculation unit calculates delay times on the reception scanning lines outside of the irradiation area in a shallow area on the shallow side of the transmit focus of the transmission beam on the basis of the waveform of a diffracted wave from one end of the plural ultrasound elements that irradiate the transmission beam, and calculates delay times on the reception scanning lines outside of the irradiation area in a deep area on the deep side of the transmit focus of the transmission beam on basis of the waveform of a diffracted wave from the other end. Furthermore, the delay time calculation unit includes a delay time generating unit for discontinuity elimination for generating delay times that connects the delay times in the shallow area and the delay times in the deep area in the vicinity of the transmit focus. 
     Advantageous Effects of Invention 
     According to the present invention, reception beamforming that does not generate discontinuity in the vicinity of the depth of a transmit focus can be executed even if reception scanning lines are disposed outside of the irradiation area of a focusing-type transmission beam. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing the configuration of the reception beamformer of an ultrasound image pickup apparatus of a first embodiment. 
         FIG. 2  is a block diagram showing the configuration of the reception beamformer of an ultrasound image pickup apparatus of a second embodiment. 
         FIGS. 3( a ) and ( b )  are a perspective view and a block diagram of the ultrasound image pickup apparatus of the second embodiment respectively. 
         FIG. 4  is an explanatory diagram showing the relationship between the irradiation range  32  of the transmission beam and the reception scanning lines of the second embodiment. 
         FIG. 5( a )  is an explanatory diagram showing that a reception scanning line  31  is divided into areas A to C depending on the positional relationship between the reception scanning line  31  and the irradiation area  32  of a transmission beam, and (b) is a graph showing the curves of delay times calculated from the wave fronts in the respective areas A to C. 
         FIG. 6  is a graph showing the curves of delay times calculated from the wave fronts and examples of the shapes of approximating curves  91  and  92  that connect the curves of delay times. 
         FIG. 7( a )  is an explanatory diagram for explaining beamforming by means of a dispersing-type transmission beam, and (b) is an explanatory diagram for explaining beamforming by means of a focusing-type transmission beam. 
         FIG. 8  is an explanatory diagram showing the shapes of wave fronts located inside of and outside of the irradiation area  32  of the focusing-type transmission beam. 
         FIG. 9  is a graph showing discontinuous curves  82  calculated from wave fronts and a curve  81  that continuously connects these discontinuous curves. 
         FIG. 10  is a block diagram showing a configuration example for realizing the operation of a delay time generating unit  114  for discontinuity elimination of the second embodiment in a software-based way using a processor  301  and a memory  302 . 
         FIG. 11( a )  is a block diagram showing a register configuration for realizing the operation of the delay time generating unit  114  for discontinuity elimination of the second embodiment in a hardware-based way, (b) is a block diagram showing an example for configuring the delay time generating unit  114  for discontinuity elimination using a register and an interpolation circuit, and (c) is a block diagram showing an example for configuring the delay time generating unit  114  for discontinuity elimination using plural registers. 
         FIG. 12  is a block diagram showing the configuration of the reception beamformer of an ultrasound image pickup apparatus of a fourth embodiment. 
         FIGS. 13( a ) and ( b )  are explanatory diagrams showing the operation of a detection unit  113   a  of the fourth embodiment that detects the degree of discontinuity. 
         FIG. 14  is a flowchart showing the operation of an optimal coefficient setting unit  113   b  of the fourth embodiment. 
         FIG. 15( a )  is an explanatory diagram showing an example of a register  151  that configures an optimal coefficient setting unit  113 B of the fourth embodiment and that is disposed in the delay time generating unit  114  for discontinuity elimination, and (b) is an explanatory diagram showing a register  151  that configures an optimal coefficient setting unit  113   b  of the fourth embodiment. 
         FIG. 16  is an explanatory diagram showing an image data and the position of the depth of a transmit focus used in a fifth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an ultrasound image pickup apparatus of one embodiment according to the present invention will be explained. 
     First Embodiment 
     An ultrasound image pickup apparatus of a first embodiment will be explained with reference to  FIG. 1 . 
     The ultrasound image pickup apparatus of the first embodiment includes: a reception beamformer  108  that delays reception signals, which are received by plural ultrasound elements  105 , by delay times at respective predefined points on reception scanning lines, phases the delayed reception signals, and then adds these signals to get phased signals  12 ; a discontinuity extracting unit  113 ; and a delay time generating unit  114  for discontinuity elimination. The discontinuity extracting unit  113  detects the degree of discontinuity showing the discontinuity of the wave fronts of the phased signals  12 . If there is an area where the degree of discontinuity is larger than a predefined value, the delay time generating unit  114  for discontinuity elimination changes delay times in the area where the discontinuity is generated. 
     As mentioned above, in the first embodiment, whether or not there is an area where the discontinuity of phased signals is generated owing to the discontinuity of delay times is detected from the degree of the discontinuity of the phased signals. If there is an area where discontinuity is generated, the discontinuity of the phased signal can be controlled by changing delay times in the area. Therefore, even in the case where a focusing-type transmission beam is transmitted and reception scanning lines are disposed outside of the irradiation area of the transmission beam, reception beamforming that does not generate discontinuity in the vicinity of the depth of the transmit focus of the focusing-type transmission beam can be executed. 
     In addition, using image data generated from phased signals, the discontinuity extracting unit  113  can detect discontinuity in the image data. 
     Although  FIG. 1  includes configurations other than the configurations described above, these configurations are the same as configurations used in a second embodiment, and therefore descriptions about them has been omitted above. 
     Second Embodiment 
     An ultrasound image pickup apparatus of a second embodiment will be explained with reference to  FIG. 2 ,  FIGS. 3( a ) and ( b ) .  FIG. 2  is a block diagram showing a part of the apparatus,  FIG. 3( a )  is a perspective view of the apparatus, and  FIG. 3( b )  is a block diagram showing the schematic configuration of the entirety of the apparatus. 
     As shown in  FIG. 2 ,  FIGS. 3( a ) and ( b ) , the ultrasound image pickup apparatus of the second embodiment includes: an ultrasound element array  101  in which plural ultrasound elements  105  are arranged in a predefined direction; a transmission beamformer  104  that makes at least apart ( 201 ,  202 , and  203 ) of the plural ultrasound elements  105  transmit a focusing-type transmission beam to the imaged area of a test object  100 ; a reception beamformer  108  that delays reception signals output by the plural of ultrasound elements  105 , which receive ultrasound waves from the test object  100 , by delay times to phase the reception signals, and outputs phased signals after adding the delayed and phased reception signals. 
     The reception beamformer  108  includes: a scanning line setting unit  116  that sets reception scanning lines  31  not only inside of the irradiation area  32  of the focusing-type transmission beam but also outside of the irradiation area  32  of the focusing-type transmission beam as shown in  FIG. 4 ; and a delay time calculation unit  112  that calculates delay times at predefined points on the reception scanning lines  31 . 
     The delay time calculation unit  112  calculates delay times in an area B (refer to  FIG. 4 ) on reception scanning lines outside of the irradiation area  32 . To put it concretely, as shown in  FIGS. 5( a ) and ( b ) , delay times (shown by a curve  72 ) are calculated on the basis of the wave shape of a diffracted wave irradiated from one end  105   a  of the plural ultrasound elements  105 , which irradiates the transmission beam, in a shallow area B 1  on the shallow side of the transmit focus  33  of the transmission beam. In a deep area B 2  on the deep side of the transmit focus  33 , delay times (shown by a curve  73 ) are calculated on the basis of the wave shape of a diffracted wave irradiated from the other end  105   b.    
     The delay times (shown by the curve  72 ) of the area B 1  and the delay times (shown by the curve  73 ) of the area B 2  are discontinuous with each other as shown in  FIG. 5 . Therefore, as shown in  FIG. 6 , the delay time calculation unit  112  includes a delay time generating unit  114  for discontinuity elimination that generates delay times (shown by a curve  91  or a curve  92 ) that connect the delay times (shown by the curve  72 ) in the shallow area B 1  in the vicinity of the depth of the transmit focus  33  and the delay times (shown by the curve  73 ) in the deep area B 2 . 
     In this way, the delay time generating unit  114  for discontinuity elimination solves a problem in that the curve  72  of the delay times and the curve  73  of the delay times calculated from the diffracted waves in the vicinity of the depth of the transmit focus  33  become discontinuous with each other. With this, even in the case where a focusing-type transmission beam is irradiated and reception scanning lines are disposed outside of the irradiation area of the transmission beam, reception beamforming, which does not generate discontinuity in the vicinity of the transmit focus of the transmission beam, can be executed. 
     Hereinafter, the ultrasound image pickup apparatus of the second embodiment will be explained more concretely. 
     The entire configuration of the ultrasound image pickup apparatus will be explained more detailedly with reference to  FIG. 2 ,  FIGS. 3( a ) and ( b ) . 
     As shown in  FIG. 3( a ) , the ultrasound image pickup apparatus includes an ultrasound probe  106 ; an apparatus body  102 ; an image display unit  103 ; and a console  110 . As shown in  FIG. 3( b ) , the transmission beamformer  104 ; a transmission/reception separation circuit (T/R)  107 ; the reception beamformer  108 ; an image processing unit  109 ; and a control unit  111  that controls the operations of these components are disposed in the apparatus body  102 . 
     As shown in  FIG. 2 , the reception beamformer  108  includes: a delay time memory  123 ; a delaying/adding/phasing unit  204 ; a beam memory  206 ; an inter-transmission synthesis unit  205 ; and a frame memory  207  as well as the abovementioned scanning line setting unit  116 ; the delay time calculation unit  112 ; the delay time generating unit  114  for discontinuity elimination. 
     The transmission beamformer  104  shown in  FIG. 3( b )  generates a transmission beam signal for generating an ultrasound transmission beam. The transmission beam signal is transferred to the ultrasound probe  106  via the transmission/reception separation circuit  107 . The ultrasound probe  106  transfers the transmission beam signal to the respective ultrasound elements  105  of the ultrasound element array  101 . The respective ultrasound elements  105  transmit ultrasound waves to the inside of the body of the test object  100 . Echo signals reflected in the body are received by the ultrasound element array  101  of the ultrasound probe  106 . The received signals pass through the transmission/reception separation circuit  107  again, and phasing/adding calculation processing and the like are executed on the received signals by the reception beamformer  108 . 
     Before the detailed operations of the respective units of the reception beamformer  108  are explained, beamforming executed by means of a typical dispersing-type transmission beam and beamforming executed by means of a typical focusing-type transmission beam will be explained. 
       FIG. 7( a )  is a diagram for explaining beamforming by means of an existing dispersing-type transmission beam. In the case where the divergence angle θ of the dispersing-type transmission beam is small, there is not a large difference between the flight travel of an ultrasound wave transmitted from the outermost side of the transmission beam and the flight travel of an ultrasound wave transmitted in the direction of the transmission sound axis. However, in the case where the divergence angle θ of the transmission beam is large, a difference between the flight travel of an ultrasound wave transmitted from the outermost side of the transmission beam and the flight travel of an ultrasound wave transmitted in the direction of a transmission sound axis becomes large. Therefore, because the divergence angle θ of the dispersing-type transmission beam cannot be set very large, it is difficult to set necessary and sufficient number of scanning lines for high-speed imaging and aperture synthesis. 
     On the other hand,  FIG. 7( b )  is a diagram for explaining beamforming by means of a focusing-type transmission beam. In the irradiation area of the focusing-type transmission beam (an area where ultrasound energy is converged)  32 , delay times are calculated using the virtual source method. The procedure for calculating the time of flight (TOF) of a sound wave using the virtual source method will be explained with reference to  FIG. 7( b ) . The virtual source method is performed under the assumption that a sound wave is reirradiated in a spherical dispersion fashion from the position of a transmit focus that is regarded as a virtual source. For example, in the case of  FIG. 7( b ) , the sound wave travels in the direction of the far side from the virtual source, and travels back in time and returns in the direction of the near side to the ultrasound elements. Here, let&#39;s assume that the origin of time (zero time) is set as the time when a sound wave is transmitted from the center position of the transmission aperture ( 201 ) of the ultrasound element array  101  (the center between elements in the case where the number of the elements in the transmission aperture are even), and the time of flight tof from the time when the sound wave is transmitted to the time when the sound wave reaches a certain ultrasound element  105  after being reflected at an imaging point (a reception phasing point  5 ) is given by the next Expression (1). In this Expression, d 1  is a distance from the center of the transmission aperture to the virtual source (a focal distance in the case of the focusing-type transmission); d 2  is a distance from the virtual source to the reception phasing point  5 ; d 3  is a distance between the reception phasing point  5  and the reception ultrasound element  105 ; and C is the speed of sound in a medium. In Expression (1), the sign “−” of the double sign ± is adopted in the case where the reception phasing point  5  is at the side of the ultrasound element array  101  viewed from the virtual source, and the sign “+” of the double sign ± is adopted in the case where the reception phasing point  5  is at the opposite side of the ultrasound element array  101  viewed from the virtual source. Here, all the distances d in Expression (1) are scalars.
 
[Expression 1]
 
tof=( d   1   +d   2   +d   3 )/ C   (1)
 
     Sign −: in the case where the imaging point is in a transmission irradiation area at the side of the probe. 
     Sign +: in the case where the imaging point is in a transmission irradiation area at the opposite side of the probe. 
     Using the virtual source method makes it possible that reception phasing points  5  are set throughout the entire irradiation area  32  of the transmission beam, and a time of flight for each reception ultrasound element  105  is calculated. Furthermore using the calculated times of flight as delay times makes it possible to execute phasing processing. Therefore, in the focusing-type transmission beam, the divergence angle  9  can be set large, and the width of an area within which the sound wave is propagated can be broadened. 
     However, as shown in  FIG. 4( a ) , if plural reception scanning lines  31  are disposed in the entirety of the irradiation area  32  of the focusing-type transmission beam, an area B which passes through the outer side of the irradiation area  32  is generated. In the present invention, as shown in  FIG. 8 , delay times outside of the irradiation area  32  are calculated under the assumption that spherical waves (diffracted waves) are propagated from ultrasound elements  105   a  and  105   b  at the ends of the transmission aperture  201  of the ultrasound element array  101  that transmits a transmission beam. 
     For example, as for an area on the left side of the irradiation area  32 , it can be considered that a spherical wave (referred to as the diffracted wave hereinafter)  62  irradiated from the ultrasound element  105   a  at the left end is propagated in an area on the shallow side of a transmit focus  33 , and it can be also considered that a spherical wave (referred to as the diffracted wave hereinafter)  63  irradiated from the ultrasound element  105   b  at the right end is propagated in an area on the deep side of the transmit focus  33 . On the other hand, as for an area on the right side of the irradiation area  32 , it can be considered that a diffracted wave  63  irradiated from the ultrasound element  105   b  at the right end is propagated in an area on the shallow side of the transmit focus  33 , and it can be also considered that a diffracted wave  62  irradiated from the ultrasound element  105   a  at the left end is propagated in an area on the deep side of the transmit focus  33 . 
     As shown in  FIG. 8 , the shape of a diffracted wave can be geometrically obtained. For example, in an area that is located on the shallow side of the transmit focus  33  and on the left side of the irradiation area  32 , the shape of the diffracted wave  62  becomes a circular arc whose center is the ultrasound element  105   a  at the left end and whose radius is r 1 . In an area that is located on the deep side of the transmit focus  33  and on the left side of the irradiation area  32 , the shape of the diffracted wave  62  becomes a circular arc whose center is the ultrasound element  105   b  at the right end and whose radius is r r . Therefore, in the area that is located on the left side of the irradiation area  32 , the shape of the diffracted wave is switched from the diffracted wave  62  to the diffracted wave  63  with the vicinity of the transmit focus  33  as a boundary. In the area that is located on the right side of the irradiation area  32 , the shape of the diffracted wave is switched from the diffracted wave  63  to the diffracted wave  62  with the vicinity of the transmit focus  33  as a boundary. 
     Therefore, in the case where a reception scanning line  31  is disposed as shown in  FIG. 4  or  FIG. 5( a ) , delay times calculated using the virtual source method are adapted to areas inside of the irradiation areas  32  of the transmission beam (the inner areas A and C), and the curve of the delay times is shown by a curve  71  in the inner area A on the shallow side of the transmit focus  33  (near to the ultrasound element array  101 ), and shown by a curve  74  in the inner area C on the deep side of the transmit focus  33  as shown in  FIG. 5( b ) . In addition, delay times generated by the diffracted wave  62  is shown by a curve  72  in an area B 1 , which is located on the shallow side of the transmit focus  33 , of the outer area B, and delay times generated by the diffracted wave  63  is shown by a curve  73  in an area B 2 , which is located on the deep side of the transmit focus  33 , of the outer area B. 
     As is clear from  FIG. 5( b ) , the curve  72  of delay times generated by the diffracted wave  62  and the curve  73  of delay times generated by the diffracted wave  63  do not get contact with each other, and therefore if these curves are adopted as they are, there arises a problem in that the delay times become discontinuous at the transmit focus  33  as shown by solid lines  82  in  FIG. 9  (However, the discontinuity between the solid lines  82  is connected by a straight line at the transmit focus  33  as shown in  FIG. 9 ). This discontinuity of the delay times makes phased signals  12  to be generated and the pixel values of an ultrasound image to be generated discontinuous in the vicinity of the depth of the transmit focus  33 , so that an artifact is generated. 
     The delay time generating unit  114  for discontinuity elimination generates delay times that continuously connect these discontinuous delay times. To put it concretely, delay times along a curve that asymptotically approaches the curve  72  or the curve  73  such as a curve  91  or a curve  92  are generated. Herewith, the generated delay times can connect the discontinuity of the delay times in the vicinity of the transmit focus  33  like a curve  81  shown in  FIG. 9 . 
     Here, the offset parts of delay times caused by plane wave propagation are subtracted from delay times shown by the vertical axis in a graph shown in  FIG. 5( b )  or in a graph shown in  FIG. 6 . The offset parts of delay times caused by plane wave propagation are not subtracted from delay times in a graph shown in  FIG. 9 . 
     Hereinafter, the operation of the delay time generating unit  114  for discontinuity elimination will be concretely explained. In this case, the delay time generating unit  114  for discontinuity elimination generates delay times that asymptotically approaches the delay times caused by the forward diffracted wave  62  (shown by the curve  72 ) in the area B 1  and the delay times caused by the backward diffracted wave  63  (shown by the curve  73 ) in the area B 2  in this order as shown by the curve  91  in  FIG. 6 . 
     Here, in  FIG. 4 ,  FIG. 5 , and  FIG. 6 , although the border between the area A and the area B, and the border between the area B and the area C are set as intersection points  34  between the reception scanning lines  31  and the outline of the irradiation area  32  of the transmission beam, it is not always necessary for the borders to correspond to the intersection points  34 . By setting the area of the delay times caused by the diffracted waves in the area B wide, an SN ratio that is homogeneous all over the entirety of an image can be realized. Furthermore, if the area A and the area C are set too wide when the virtual source method is used, the gradient of the variation between delay times in the vicinity of the depth of the transit focus  33  becomes steep, and therefore the discontinuity in an image in the vicinity of the depth of the focus  33  is apt to become obvious. As a result, it is desirable to set the area of the delay times caused by the diffracted waves in the area B wide. 
     In this embodiment, the delay time generating unit  114  for discontinuity elimination generates delay times shown by the curve  91 , which continuously connects the curve  72  of the delay times caused by the forward diffracted wave and the curve  73  of the delay times caused by the backward diffracted wave, from Expression (4) using Expression (3) that uses a sigmoid function defined below by Expression (2). 
     
       
         
           
             
               
                 
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     The sigmoid function defined by Expression (2) is a function that asymptotically behaves at its both ends. Expression (3) is a function to which Expression (2) is applied, and a function of a depth x measured from the ultrasound element array  101  for defining a weighting factor w, where the range of x in Expression (3) is equal to the range of the area B, x f  is a depth from the ultrasound element array  101  to the transmit focus  33 , and α is a coefficient. Using Expression (3), a weighting function w single (x) is calculated whose value changes symmetrically on the shallow side and on the deep side of the transmit focus with the depth of the transmit focus  33  as a symmetric center. The shape of the weighting function w single (x) varies by changing the coefficient α. In this embodiment, a predefined value or a value indicated from the control unit  111  is used as the coefficient α. It is conceivable that the control unit  111  is configured to be able to accept the value of the coefficient α from an operator via the console  110 . 
     The delay time generating unit  114  for discontinuity elimination weights TOF edge_near , which is a delay time (a time of flight) caused by the forward diffracted wave and a value shown by the curve  72 , and TOF edge_far , which is a delay time (a time of flight) caused by the backward diffracted wave and a value shown by the curve  73 , using the weighting function w single (x) obtained from Expression (3), and adds these weighted values as shown in Expression (4). Herewith, the delay time generating unit  114  for discontinuity elimination can generate delay times TOF approx  (values shown by a curve  91 ) that asymptotically approach the curve  72  and the curve  73  respectively at its both ends as shown by the curve  91 . To put it concretely, delay times shown by points o in  FIG. 6  can be generated. Here, because a sigmoid-type function does not become zero at its ends, in order to match the values of TOF approx  with the values shown by the curves  72  and  73  respectively at both ends of the area B, it is necessary to modify the values of TOF approx  a little. 
     Although the sigmoid function is used as a model function that continuously connect the curve  72  and the curve  73  in the above Expressions (2) to (4), a function such as a generalized raised cosine-type function using a coefficient α, which is given by Expression (5), can be used instead of Expression (3). For example, a window function such as a Hanning function (Expression (6)), a Hamming function (Expression (7)), or the like can be used as one of such functions. In addition, the exponent of a cosine function part of a generalized cosine-type function can be 1 or more, for example, 2 or 4. By changing the exponent, the sharpness owing to the change of the coefficient of the function can be changed.
 
[Expression 5]
 
 w ( x )=α−(1−α)cos(2π x )  (5)
 
[Expression 6]
 
 w ( x )=0.5−0.5 cos(2π x )  (6)
 
[Expression 7]
 
 w ( x )=0.54−0.46 cos(2π x )  (7)
 
     Because the weighting functions given by the above Expressions (5) to (7) are cosine-type functions, the values of these functions become 0 or 1 at the ends of a target area, and therefore it is not necessary to modify the values at the ends. By defining the domain of Expressions (5) to (7) as 0≤x≤0.5, functions that continuously vary from 0 to 1 can be realized. For example, by defining w(x) as shown by Expression (8), the curve  91  that continuously connects the both ends x edge1  and X edge2  of the area B can be realized. In Expression (8), X edge1  and x edge2  is the depths of the both ends of the area B on a certain scanning line as shown in  FIG. 5  and  FIG. 6 . 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     8 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     w 
                     ⁡ 
                     
                       ( 
                       x 
                       ) 
                     
                   
                   = 
                   
                     α 
                     - 
                     
                       
                         ( 
                         
                           1 
                           - 
                           α 
                         
                         ) 
                       
                       ⁢ 
                       cos 
                       ⁢ 
                       
                         { 
                         
                           2 
                           ⁢ 
                           π 
                           ⁢ 
                           
                             
                               0.5 
                               ⁢ 
                               
                                 ( 
                                 
                                   x 
                                   - 
                                   
                                     x 
                                     
                                       edge 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       1 
                                     
                                   
                                 
                                 ) 
                               
                             
                             
                               ( 
                               
                                 
                                   x 
                                   
                                     edge 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     2 
                                   
                                 
                                 - 
                                 
                                   x 
                                   
                                     edge 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     1 
                                   
                                 
                               
                               ) 
                             
                           
                         
                         } 
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     Because the sigmoid function, that is, the abovementioned Expression (3), and Expressions (5) to (8) can be controlled by using only one parameter α respectively, only a little amount of calculation is required, and therefore these functions are advantageous in terms of the cost reduction and simplification of the implementation of hardware and software. 
     Furthermore, a weighting function w(x) can be defined not only by one of the abovementioned functions, but also by any of a Blackman window, a Kaiser window, and the like. 
     Hereinafter, the operations of the respective units of the reception beamformer  108  shown in  FIG. 2  will be concretely explained. The delay time calculation unit  112  and the delay time generating unit  114  for discontinuity elimination include processing units such as CPUs and memories. After reading out and executing programs stored in advance in the memories, the delay time calculation unit  112  and the delay time generating unit  114  for discontinuity elimination operate on the basis of software processing. Alternatively, the delay time calculation unit  112  and the delay time generating unit  114  for discontinuity elimination can be comprised of pieces of hardware that execute predefined operations such as ASICs, FPGAs, and registers. Output values for each of transmission conditions, for each of depths, or for each of coefficients α are stored in advance in the registers. After the ASICs or FPGAs read out appropriate values corresponding to a condition from the registers, the delay time calculation unit  112  and the delay time generating unit  114  for discontinuity elimination operate. In addition, it is also possible that parts of the delay time calculation unit  112  and the delay time generating unit  114  for discontinuity elimination are realized by software processing and the other parts are realized by hardware. 
     The scanning line setting unit  116  of the reception beamformer  108  receives a transmission condition, the number and position information of scanning lines  31  from the control unit  111 , and sets the predefined number of reception scanning lines  31  in the area  32  to which the transmission beam is irradiated as shown in  FIG. 4 . 
     In the case where the operation of the delay time calculation unit  112  is realized by software processing, the delay time calculation unit  112  calculates the outline of the irradiation area  32  using the transmission condition received from the control unit  111 , calculates the positions of intersection points  34  between the outline of the irradiation area  32  and the reception scanning lines  31 , and sets inner areas A and C and an outer area B on the reception scanning lines  31  with these intersection points as boundaries. On the other hand, in the case where the delay time calculation unit  112  is configured by hardware, data showing the ranges of the inner areas A and C, and the ranges of the outer area B for respective transmission conditions are calculated in advance with reference to the positional relationship between the shape of the irradiation area  32  and the reception scanning lines  31 , and the data are stored in registers or memories according to the respective transmission conditions. The delay time calculation unit  112  reads out the ranges of the areas A, B, and C corresponding to a transmission condition, which are received from the control unit  111 , and the set reception scanning lines  31  from the registers or memories, and outputs these ranges. 
     Furthermore, the delay time calculation unit  112  includes a register and a memory. Values on the curve  71  of the delay times of the area A, values on the curve  72  of the delay times of the area B 1 , values on the curve  73  of the delay times of the area B 2 , and values on the curve  74  of the delay times of the area C are stored in advance according to the respective transmission conditions, the respective reception scanning lines  31 , or the positions of the respective ultrasound elements  105  in the register and memory. The delay time calculation unit  112  reads out delay times regarding plural points (segment nodes) of the reception scanning lines  31  in the areas A and C according to the respective transmission conditions, the respective reception scanning line  31 , and the positions of the respective ultrasound elements  105  from the register or the memory. 
     The delay time generating unit  114  for discontinuity elimination generates delay times regarding plural points (segment nodes) in the area B along the curve  91  for discontinuity elimination. To put it concretely, if the delay time generating unit  114  for discontinuity elimination is configured to include a processing unit  301  and a memory  302  as shown in  FIG. 10 , and its operation is realized by software processing, delay times for discontinuity elimination are generated by the operation of the delay time generating unit  114  for discontinuity elimination as described below. That is, the delay time generating unit  114  for discontinuity elimination calculates Expression (3) using a coefficient α, a range of the depth x of the area B, and the depth of the transmit focus  33  that are received from the control unit  111  and the scanning line setting unit  116 , and calculates a weighting function w single (x) for each depth x. Next, the delay time generating unit  114  for discontinuity elimination reads out the delay time TOF edge_near  on the curve  72  in the area B 1  and TOF edge_far  on the curve  73  in the area B 2  from the register and the memory of the delay time calculation unit  112 . Expression (4) is calculated using the weighting function w single (x), the delay time TOF edge_near , and TOF edge_far  to obtain a delay time for discontinuity elimination TOF approx  for each depth x. Here, as mentioned above, any of Expressions (5) to (7) can be used instead of Expression (3). In addition, because the gradient of the curve  91  is steep, it is desirable to set distances between segment nodes in the area B shorter than distances between segment nodes in the areas A and C. 
     Furthermore, in the case where the delay time generating unit  114  for discontinuity elimination is configured with hardware as shown in  FIG. 11( c ) , the hardware is configured to include registers  303  (refer to  FIG. 11( a ) ) that store the values of the weighting function w single (x), which are calculated in advance for the combinations of coefficients α and the depths of the area B using Expression (3), according to the depths of the transmit focus  33 . The delay time generating unit  114  for discontinuity elimination reads out a weighting function w single (x) corresponding to a coefficient α, a range of the depth x of the area B, and a depth of the transmit focus  33 , which are received from the control unit  111  and the scanning line setting unit  116 , from the plural registers  303 . Next, the delay time generating unit  114  for discontinuity elimination reads out TOF edge_near  on the curve  72  in the area B 1  and TOF edge_far  on the curve  73  in the area B 2  from the register and the memory of the delay time calculation unit  112 . The delay time generating unit  114  for discontinuity elimination calculates Expression (4) to obtain a delay time for discontinuity elimination TOF approx  using these TOF edge_near , TOF edge_far , and the weighting function W single (x) for each depth x of a predefined segment point in the area B. Here, the values of the weighting function w single (x), which are stored in advance in the registers  303 , can be calculated using any of Expressions (5) to (7) instead of Expression (3). 
     In addition, the configuration of the delay time calculation unit  111  is not limited to the configuration including plural registers  303  in which all the values corresponding to all combinations of conditions are stored as shown in  FIG. 11( c ) , and the configuration including registers whose number is less than the number of all the combinations of conditions, and an interpolation circuit  304  that calculates a weighting function w single (x) corresponding to a required condition using values stored in the registers  303  by means of interpolation calculation can be utilized as shown in  FIG. 11( b ) . For example, the interpolation circuit  304  can be realized using pieces of hardware such as FPGA. Herewith, the circuit size shown in  FIG. 11( c )  can be reduced. 
     The delay time calculation unit  112  and the delay time generating unit  114  for discontinuity elimination transfer delay times calculated regarding plural segment points in the areas A, B, and C to the delay time memory  123 . Delay times are set for each ultrasound element  105  regarding one reception scanning line  31 . 
     The delaying/adding/phasing unit  204  reads out delay times and position information for the respective segment nodes from the delay time memory  123 , and calculates delay times at the positions of reception phasing points between segment nodes on a reception scanning line using interval linear interpolation calculation. After received signals at each ultrasound element  105  are delayed using the calculated delay times, and the delayed received signals are phased, these signals are added together to obtain a phased signal. Because delay times that continuously change are set in the outer area B by the curve  91 , the values of phased signals also become continuous. 
     This is executed regarding all the reception scanning lines  31 . Phased signals calculated regarding reception phasing points of each reception scanning line  31  are stored in the beam memory  206 . The above operation is repeated a predefined times while the irradiation position of the transmission beam is changed. 
     The inter-transmission synthesis unit  205  reads out plural phased signals at the same phasing point from the beam memory  206 , and synthesizes the read-out phased signals to perform aperture synthesis. Next, using the synthesized phased signals, an image in the imaged area is generated. The generated image is stored in the frame memory  207 , and at the same time it is output to the image processing unit  109 . The image processing unit  109  displays the image, on which image processing is executed as required, on the image display unit  103 . 
     The displayed image does not generate a discontinuous artifact even in the vicinity of the transmit focus, and can display a highly accurate image. 
     Third Embodiment 
     In a third embodiment, the delay time generating unit  114  for discontinuity elimination generates the curve  92  (refer to points A in  FIG. 6 ) that continuously connects the curve  72  of delay times caused by a forward diffracted wave and the curve  73  of delay times caused by a backward diffracted wave using below Expressions (9-1), (9-2), (10-1), and (10-2). 
     As is clear from  FIG. 6 , the curve  92  is a two-step curve that has a part (B 1 ) connecting the curve  72  showing the variation between delay times in the shallow area B 1  and a straight line  75  showing the variation between delay times determined on the basis of plane wave propagation, and a part (B 2 ) connecting the straight line  75  based on the plane wave propagation and the curve  73  showing the variation between delay times in the deep area B 2 . 
     Both Expressions (9-1) and (9-2) are expressions used for determining weighting functions using a sigmoid function as is the case of Expression (3). In this embodiment, an area B is divided into the area B 1  and the area B 2  with the transmit focus as a boundary between the areas B 1  and B 2 , and Expression (9-1) and Expression (9-2) are used for generating weighting functions for the area B 1  and area B 2  respectively. Here, in Expression (1), x 1  is the depth of a middle point between the edge of the area B on the side of the area A and the depth of the transmit focus (focus), and x 2  is the depth of a middle point between the depth of the transmit focus (focus) and the edge of the area B on the side of the area C. Here, the range of x used in Expressions (9-1), (9-2), (10-1), and (10-2) is equal to the range of the area B. 
     By calculating Expressions (10-1) and (10-2) using the weighting functions w double (x) obtained from Expressions (9-1) and (9-2), the delay times (times of flight) TOF edge_near  (the values of the curve  72 ) caused by the forward diffracted wave, the delay times TOF PW  (the values of the straight line  75 ) caused by plane wave propagation, and the delay times (time of flight) TOF edge_far  (the values of the curve  73 ) caused by the backward diffracted wave are respectively weighted and added. Herewith, the delay time generating unit  114  for discontinuity elimination can generates the delay times TOF approx  (the curve  92 ), which crosses the straight line  75  based on plane wave propagation after asymptotically approaching the curve  72 , and then asymptotically approaches the curve  73 , as shown by the curve  92 . 
     
       
         
           
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     [ 
                     
                       Expression 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       9 
                     
                     ] 
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       
                         w 
                         double 
                       
                       ⁡ 
                       
                         ( 
                         x 
                         ) 
                       
                     
                     = 
                     
                       { 
                       
                         
                           
                             
                               
                                 1 
                                 
                                   1 
                                   + 
                                   
                                     e 
                                     
                                       - 
                                       
                                         a 
                                         ⁡ 
                                         
                                           ( 
                                           
                                             x 
                                             - 
                                             
                                               x 
                                               1 
                                             
                                           
                                           ) 
                                         
                                       
                                     
                                   
                                 
                               
                               ⁢ 
                               
                                 : 
                               
                             
                           
                           
                             
                               x 
                               ≤ 
                               
                                 x 
                                 f 
                               
                             
                           
                         
                         
                           
                             
                               
                                 1 
                                 
                                   1 
                                   + 
                                   
                                     e 
                                     
                                       - 
                                       
                                         a 
                                         ⁡ 
                                         
                                           ( 
                                           
                                             x 
                                             - 
                                             
                                               x 
                                               2 
                                             
                                           
                                           ) 
                                         
                                       
                                     
                                   
                                 
                               
                               ⁢ 
                               
                                 : 
                               
                             
                           
                           
                             
                               x 
                               &gt; 
                               
                                 
                                   x 
                                   f 
                                 
                                 ( 
                                 
                                   9 
                                   - 
                                   2 
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     9 
                     - 
                     1 
                   
                   ) 
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     [ 
                     
                       Expression 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       10 
                     
                     ] 
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       
                         ( 
                         i 
                         ) 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       x 
                     
                     ≤ 
                     
                       x 
                       f 
                     
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     TOF 
                     
                       approx 
                       . 
                     
                   
                   = 
                   
                     
                       
                         
                           w 
                           double 
                         
                         ⁡ 
                         
                           ( 
                           x 
                           ) 
                         
                       
                       × 
                       
                         TOF 
                         edge_near 
                       
                     
                     + 
                     
                       
                         ( 
                         
                           1 
                           - 
                           
                             
                               w 
                               double 
                             
                             ⁡ 
                             
                               ( 
                               x 
                               ) 
                             
                           
                         
                         ) 
                       
                       × 
                       
                         TOF 
                         PW 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     10 
                     - 
                     1 
                   
                   ) 
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       
                         ( 
                         ii 
                         ) 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       x 
                     
                     &gt; 
                     
                       x 
                       f 
                     
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     TOF 
                     
                       approx 
                       . 
                     
                   
                   = 
                   
                     
                       
                         
                           w 
                           double 
                         
                         ⁡ 
                         
                           ( 
                           x 
                           ) 
                         
                       
                       × 
                       
                         TOF 
                         PW 
                       
                     
                     + 
                     
                       
                         ( 
                         
                           1 
                           - 
                           
                             
                               w 
                               double 
                             
                             ⁡ 
                             
                               ( 
                               x 
                               ) 
                             
                           
                         
                         ) 
                       
                       × 
                       
                         TOF 
                         edge_far 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     10 
                     - 
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
     Furthermore, using cosine-type functions in Expressions (11-1) and (11-2) also in this case makes it possible to generate delay times TOF approx  (the curve  92 ), which crosses the straight line  75  based on plane wave propagation after asymptotically approaching the curve  72 , and then asymptotically approaches the curve  73 , as shown by the curve  92 . In other words, the curve  92  that can continuously connect both ends X edge1  and X edge2  of the area B can be realized. In Expression (11-1) and Expression (11-2), X edge1  and x edge2  are the depths of both ends of the area B on a certain scanning line as shown in  FIG. 5  and  FIG. 6 , and x f  is the depth of the transmit focus. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     11 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     w 
                     ⁡ 
                     
                       ( 
                       x 
                       ) 
                     
                   
                   = 
                   
                     
                       α 
                       - 
                       
                         
                           ( 
                           
                             1 
                             - 
                             α 
                           
                           ) 
                         
                         ⁢ 
                         cos 
                         ⁢ 
                         
                           { 
                           
                             2 
                             ⁢ 
                             π 
                             ⁢ 
                             
                               
                                 0.5 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     x 
                                     - 
                                     
                                       x 
                                       
                                         edge 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                       
                                     
                                   
                                   ) 
                                 
                               
                               
                                 ( 
                                 
                                   
                                     x 
                                     f 
                                   
                                   - 
                                   
                                     x 
                                     
                                       edge 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       1 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           } 
                         
                         ⁢ 
                         
                           : 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         x 
                       
                     
                     ≤ 
                     
                       x 
                       f 
                     
                   
                 
               
               
                 
                   ( 
                   
                     11 
                     - 
                     1 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     w 
                     ⁡ 
                     
                       ( 
                       x 
                       ) 
                     
                   
                   = 
                   
                     
                       α 
                       - 
                       
                         
                           ( 
                           
                             1 
                             - 
                             α 
                           
                           ) 
                         
                         ⁢ 
                         cos 
                         ⁢ 
                         
                           { 
                           
                             2 
                             ⁢ 
                             π 
                             ⁢ 
                             
                               
                                 0.5 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     x 
                                     - 
                                     
                                       x 
                                       f 
                                     
                                   
                                   ) 
                                 
                               
                               
                                 ( 
                                 
                                   
                                     x 
                                     
                                       edge 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       2 
                                     
                                   
                                   - 
                                   
                                     x 
                                     f 
                                   
                                 
                                 ) 
                               
                             
                           
                           } 
                         
                         ⁢ 
                         
                           : 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         x 
                       
                     
                     &gt; 
                     
                       x 
                       f 
                     
                   
                 
               
               
                 
                   ( 
                   
                     11 
                     - 
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
     Other configurations, operations, and advantageous effects of the third embodiment are similar to those of the second embodiment, so descriptions about them will be omitted. However, the operations of the delay time calculation unit  112  and the delay time generating unit  114  for discontinuity elimination can be also realized by means of software. Alternatively, the operations of the delay time calculation unit  112  and the delay time generating unit  114  for discontinuity elimination can be also realized by means of hardware. 
     Fourth Embodiment 
     An ultrasound image pickup apparatus of a fourth embodiment will be explained below. 
     As shown in  FIG. 12 , the ultrasound image pickup apparatus of the fourth embodiment includes a discontinuity extracting unit  113 . The discontinuity extracting unit  113  includes a detection unit  113   a  that detects whether or not discontinuity has been generated in phased signals and the like, and an optimal coefficient setting unit  113   b  that continues changing delay times generated by a delay time generating unit  114  for discontinuity elimination until the degree of discontinuity becomes equal to or less than a predefined value. 
     The detection unit  113   a  of the discontinuity extracting unit  113  receives phased signals in front of and at the rear of a transmit focus  33  from a delaying/adding/phasing unit  204 , and sets two areas  96  and  97  between which the transmit focus  33  is sandwiched as phased signals as shown in  FIG. 13( a ) . It will be assumed that the width of the depth of the area  96  and the width of the depth of the area  97  have predefined values respectively. Next, as an index showing the degree of discontinuity between the phased signal in the area  96  and the phased signal in the area  97 , a correlation coefficient (for example, a pearson&#39;s correlation coefficient or the coefficient of the maximum value of the cross-correlation function calculated by convolution operation at sample points within the range of the depth of the area  96  plus the depth of the area  97 ) between the two phased signals is calculated. If the value of the calculated correlation coefficient is equal to or less than a predefined value (for example, if the value is 0.95 or 0.8 assuming the maximum value of the correlation coefficient is 1.0), the detection unit  113   a  judges that discontinuity is generated, and instructs the optimal coefficient setting unit  113   b  to calculate an optimal coefficient. 
     Furthermore, after setting an area  98 , which includes the transmit focus  33  and has a predefined depth width, as a phased signal as shown in  FIG. 13( b ) , the detection unit  113   a  can also calculate the differential coefficients and statistical quantities of the phased signal in the area  98  instead of the correlation function. If the calculated differential coefficients are equal to or larger than a predefined value (for example, if the calculated differential coefficients are ten times the average value of already-calculated differential coefficients in an area having moderate differential coefficients such as the area A or C), the detection unit  113   a  judges that the degree of discontinuity larger than a predefined value is generated. In addition, the value of the variance of the phased signals or the rank of the correlation matrix calculated from the phased signals can be used as one of the statistical quantities. If the statistical quantity is near to a desired value, it is judged that the degree of discontinuity is small, and if the statistical quantity is displaced from the desired value by a predefined value or more, it is judged that the degree of discontinuity is larger than a predefined value. 
     The operation of the above detection unit  113   a  can be realized by configuring the detection unit  113   a  with a processing unit and a memory, and by executing software processing in which the processing unit reads out programs stored in the memory and executes the programs. 
     If the degree of discontinuity detected by the detection unit  113   a  is larger than the predefined value, the optimal coefficient setting unit  113   b  operates as shown by a flowchart in  FIG. 14  and reassigns an appropriate value to the coefficient α used in the abovementioned Expression (3) and the like. To put it concretely, first the optimal coefficient setting unit  113   b  sets the lower limit value αmin and the upper limit value αmax of the coefficient α (at step  311 ). The upper limit value αmax and the lower limit value αmin can be predefined values or can be values that the control unit  111  receives from an operator via the console  110 . 
     Next, after the lower limit value αmin is assigned to a, the transmission/reception of ultrasound waves between the detection unit  113   a  and a test object  100  or a phantom is performed with a predefined transmission condition and a predefined reception condition respectively under the control by the control unit  11  (at step  312 ). The reception beamformer  108  executes reception beamforming. The detection unit  113   a  detects the degree of discontinuity regarding phased signals obtained by this transmission/reception, and stores the result in the memory (at step  314 ). 
     Next, after α+δα is assigned to α (where δ is a predefined coefficient), transmission/reception is performed (at step  313 ), and the detection unit  113   a  detects the degree of discontinuity regarding the phased signals, so that the result is stored in the memory (at step  314 ). Subsequently, the degree of discontinuity in the case of the previous coefficient α and the degree of discontinuity in the case of a new coefficient α+δα is compared with each other, and if the degree of discontinuity in the case of the new coefficient α+δα is smaller, the new coefficient α+δα is stored in the memory as a tentatively optimal value, and if the degree of discontinuity in the case of the new coefficient α+δα is larger, the previous coefficient α is stored in the memory as a tentatively optimal value (at step  315  to  318 ). Next, the flow goes back to step  313 , and δα is added to the current coefficient α, and the above steps  314  to  318  are repeated. After the above procedure is repeated until α+δα becomes equal to or more than αmax (at step  319 ), the optimal coefficient αopt that makes the degree of discontinuity of the phased signals the smallest can be obtained by assigning the tentatively optimal value α to αopt (at step  320 ). The obtained optimal coefficient αopt is set in the delay time generating unit  114  for discontinuity elimination (step  321 ). 
     Through the above-described steps, because whether discontinuity is generated or not can be examined, and furthermore, an optimal coefficient α that does not cause discontinuity between actual phased signals can be set, an image can be generated by performing transmission/reception under the condition that a discontinuity between phased signals does not generated owing to a discontinuity of delay times. 
     Here, although the abovementioned optimal coefficient setting unit  113   b  obtains an optimal coefficient αopt by actually repeating the transmission/reception, a register  151 , in which optimal coefficients α are stored after the optimal coefficients α are calculated according to respective transmission conditions (such as transmit focuses  33 ) and respective reception conditions (such as the positions of reception scanning lines) in advance, can be used as the optimal coefficient setting unit  113   b  (refer to  FIG. 15( b ) ). In this case, the optimal coefficient setting unit  113   b  reads out an optimal coefficient α according to a transmission condition and a reception condition received from the control unit  111  from the register  151 , and outputs the optimal coefficient α. The output coefficient α is set in the delay time generating unit  114  for discontinuity elimination. In this case, if the delay time generating unit  114  for discontinuity elimination includes registers  303  in each of which the relationship of an α and an x is stored as shown in  FIG. 11( c ) , it is also possible to dispose the register  151  in the delay time generating unit  114  for discontinuity elimination after combining the registers  303  with the register  151  shown in  FIG. 15( b )  (refer to  FIG. 15( a ) ). 
     Fifth Embodiment 
     In an ultrasound image pickup apparatus of a fifth embodiment, a detection unit  113   a  of a discontinuity extracting unit  113  detects the degree of discontinuity in the vicinity of the depth of a transmit focus  33  using image data generated by a reception beamformer  108 . As the image data, image data that is synthesized by aperture synthesis and is stored in the frame memory  207  shown in  FIG. 12  of the fourth embodiment, or image data, on which image processing is executed by the image processing unit  109 , are used. In addition, image data that is generated without being synthesized by aperture synthesis can be also used as the image data. 
     The detection unit  113   a  extracts statistical quantities in the vicinity of the transmit focus  33  regarding image data such as that shown in  FIG. 16 . The entropy of an image is used as one of the statistical quantities. If the entropy of an image in the vicinity of the transmit focus  33  is smaller than a predefined value, it is judged that the degree of discontinuity is equal to or larger than a predefined value. An optimal coefficient setting unit  113   b  sets a coefficient α so that the entropy of the image becomes the maximum. 
     Because other configurations are the same as those described in the fourth embodiment, explanations about those configurations will be omitted. 
     LIST OF REFERENCE SIGNS 
     
         
           100 : Test Object 
           101 : Ultrasound Element Array 
           102 : Ultrasound Image Pickup Apparatus Body 
           103 : Image Display Unit 
           104 : Transmission Beamformer 
           106 : Ultrasound Probe 
           107 : Transmission/Reception Separation Circuit (T/R) 
           108 : Reception Beamformer 
           109 : Image Processing Unit 
           110 : Console 
           111 : Control Unit 
           112 : Delay Time Calculation Unit 
           113 : Discontinuity Extracting Unit 
           114 : Delay Time Generating Unit For Discontinuity Elimination 
           116 : Scanning Line Setting Unit 
           123 : Delay Time Memory