Abstract:
An ultrasonic imaging method capable of increasing density of sampling points in accordance with depths within an object. In this method, a predetermined area contained in the object is divided into at least a first area located most shallowly and a second area located deeper. The method includes the steps of: (a) transmitting and receiving ultrasonic waves focused in one focus direction within the first area to take samples of an ultrasonic image at a plurality of points in the focus direction, and changing the focus direction to scan the first area; (b) sequentially transmitting ultrasonic waves focused in respective focus directions within the second area in a predetermined time period, thereafter receiving ultrasonic waves reflected from the respective focus directions to take samples of the ultrasonic image at a plurality of points in the respective focus directions.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to an ultrasonic imaging method and an ultrasonic imaging apparatus, capable of performing nondestructive tests by employing ultrasonic waves. More specifically, the present invention is directed to such ultrasonic imaging method and apparatus, capable of scanning a radial-shaped area contained in an object to be inspected (sector scanning) by employing ultrasonic waves so as to obtain an image information of this radial-shaped area.  
           [0003]    2. Description of a Related Art  
           [0004]    Normally, in ultrasonic imaging apparatuses utilized as ultrasonic diagnostic apparatuses or industrial-purpose flaw detecting apparatuses, ultrasonic probes are employed each contains a plurality of ultrasonic transducers and has ultrasonic transmission/reception functions. In one typical ultrasonic imaging apparatus equipped with such an ultrasonic probe, image information related to an object to be inspected may be obtained in such a manner that this object to be inspected is ultrasonically scanned by using ultrasonic beams, while the ultrasonic beams are produced by synthesizing ultrasonic waves transmitted from the plurality of ultrasonic transducers. Then, the ultrasonic imaging apparatus may reproduce images of either two-dimensional areas or three-dimensional areas contained in the object to be inspected based upon the obtained image information. As one of scanning methods for scanning an object to be inspected by way of such ultrasonic beams, a so-called “sector scanning operation” is carried out by which a two-dimensional fan-shaped region is ultrasonically scanned along angular directions.  
           [0005]    [0005]FIGS. 7A to  7 C are explanatory diagrams for illustratively explaining one typical example of the conventional sector scanning operation.  
           [0006]    As shown in FIG. 7A, since ultrasonic waves transmitted to an object to be inspected from a plurality of ultrasonic transducers contained in an ultrasonic probe are synthesized with each other, an ultrasonic beam  101  is formed in the object to be inspected, while this ultrasonic beam  101  is extended from a transmission point  100  in a depth direction. Then, a fan-shaped two-dimensional area  103  which is contained in the object to be inspected is sequentially scanned by this ultrasonic beam  101  “N” times in a direction of an angle “O” in an equi-interval. It should be noted that symbol “N” is a natural number.  
           [0007]    Furthermore, as illustrated in FIG. 7B, at a plurality of sampling points  102 , image information is sequentially sampled, while these plural sampling points  102  are distributed in an equi-interval in the depth direction along the ultrasonic beam  101  at the respective angles. As previously described, while the scanning operation by using one ultrasonic beam is carried out, image information related to a plurality of sampling points located on this single ultrasonic beam is sampled every time a predetermined time period has passed.  
           [0008]    [0008]FIG. 7C shows a time chart for explaining such a scanning process operation of the ultrasonic beam. As shown in FIG. 7C, in order to perform a scanning operation of a single ultrasonic beam, a constant repetition time PRT (namely, pulse repetition time period) is consumed. Furthermore, a total value of pulse repetition time period PRT which is consumed to execute scanning operations of a plurality of ultrasonic beams-constitutes imaging time required for scanning an entire portion of a two-dimensional area. With respect to one pulse repetition time period PRT, a plurality of ultrasonic waves are transmitted to an object to be inspected so as to form one ultrasonic beam within a pulse transmission time slot TP. Then, at time instants indicated by white-colored points (see FIG. 7C), ultrasonic echoes are received which are reflected from a plurality of sampling points distributed along one ultrasonic beam, and then, image information related to the respective sampling points is sampled based upon these received ultrasonic echoes.  
           [0009]    However, when such a sampling operation of the image information as shown in FIG. 7B is carried out, a total number of ultrasonic beams  101  (namely, density of ultrasonic beams) employed in a scanning operation of a unit area with respect to a deeper portion  105  within a two-dimensional area  103  becomes smaller than that of a shallower portion  104  thereof. As a result, an image quality of image information related to the deeper portion  105  becomes coarser than that related to the shallower portion  104 .  
           [0010]    As a consequence, as shown in FIG. 8A, the following solution method is conceivable. That is, since a total time of scanning operations for the deeper portion  105  is made larger than that of the shallower portion  104 , density of sampling points  102  within the deeper portion  105  can be increased substantially equal to density of the sampling points  102  within the shallower portion  104 . In FIG. 8A, black-colored points indicate such sampling points which are newly added. In this case, as shown in FIG. 8A, a scanning operation only directed to the deeper portion  105  is carried out between a first scanning operation and a second scanning operation of the conventional sector scanning operations, and then, such a scanning process operation is repeatedly carried out. FIG. 8B is a time chart for explaining such a scanning process operation of the ultrasonic beam. In this time chart, black-colored points represent time instants when ultrasonic echoes reflected from the newly added sampling points are received.  
           [0011]    However, even in such a scanning operation directed only to the deeper portion, a time duration is required for ultrasonic waves transmitted from an ultrasonic probe to reach these sampling points and return to the ultrasonic probe. As a result, even when the scanning operation directed only to the deeper portion is carried out, such a time duration substantially equal to the time duration required for a single scanning operation in the conventional sector scanning operation would be consumed. As a consequence, a total scanning number of the sector scanning operation as shown in FIG. 8A is equal to substantially two times as large as a total scanning number of the sector scanning operation as shown in FIG. 7B. Thus, a frame rate of this sector scanning operation as shown in FIG. 8A, which corresponds to an inverse number of imaging time, would be lowered to a substantially half of a frame rate in the sector scanning operation as shown in FIG. 7B.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention has been made to solve the above-described problems, and therefore, has an object to provide an ultrasonic imaging method and an ultrasonic imaging apparatus, capable of increasing density of sampling points in accordance with a depth degree within an object to be inspected, while a frame rate is not necessarily decreased.  
           [0013]    To achieve the above-described object, an ultrasonic imaging method according to one aspect of the present invention, of obtaining image information in such a manner that a predetermined area contained in an object to be inspected is divided into at least a first area located most shallowly and a second area located deeper than the first area so as to scan the first and second areas by employing ultrasonic waves, comprises the steps of: (a) transmitting and receiving ultrasonic waves focused in one focus direction within the first area by using a plurality of ultrasonic transducers included in an ultrasonic probe to take samples of an ultrasonic image at a plurality of points in the focus direction, and changing the focus direction to scan the first area; (b) sequentially transmitting ultrasonic waves focused in respective focus directions within the second area by using the plurality of ultrasonic transducers in a predetermined time period, thereafter receiving ultrasonic waves reflected from the respective focus directions by using the plurality of ultrasonic transducers to take samples of the ultrasonic image at a plurality of points in the respective focus directions; (c) obtaining image information as to the plural points within the first area on the basis of detection signals obtained from the plurality of ultrasonic transducers at step (a); and (d) obtaining image information as to the plural points within the second area on the basis of detection signals obtained from the plurality of ultrasonic transducers at step (b).  
           [0014]    Also, an ultrasonic imaging apparatus according to one aspect of the present invention, for obtaining image information in such a manner that a predetermined area contained in an object to be inspected is divided into at least a first area located most shallowly and a second area located deeper than the first area so as to scan the first and second areas by employing ultrasonic waves, comprises: drive signal generating means for delaying input signals to supply drive signals having specific phases, respectively; an ultrasonic probe having a plurality of ultrasonic transducers, for transmitting ultrasonic waves in accordance with the drive signals and for receiving ultrasonic waves to output detection signals based upon the received ultrasonic waves; signal processing means for processing the detection signals to obtain image information of an object to be inspected on the basis of the processed detection signals; and control means for controlling the drive signal generating means and the signal processing means to (a) transmit and receive ultrasonic waves focused in one focus direction within the first area by using the plurality of ultrasonic transducers to take samples of an ultrasonic image at a plurality of points in the focus direction, and change the focus direction to scan the first area and (b) sequentially transmit ultrasonic waves focused in respective focus directions within the second area by using the plurality of ultrasonic transducers in a predetermined time period, thereafter receive ultrasonic waves reflected from the respective focus directions by using the plurality of ultrasonic transducers to take samples of the ultrasonic image at a plurality of points in the respective focus directions.  
           [0015]    According to the present invention, a radial-shaped area contained in the object to be inspected is divided into a plurality of areas having different depth degrees, and these plural areas are independently scanned. In particular, as to an area having a deep depth degree within the radial-shaped area, a plurality of ultrasonic waves are transmitted to the object to be inspected so as to sequentially form a plurality of ultrasonic beams extended in the different directions within a time period in which ultrasonic echoes reflected from an area located more shallowly than the deep area are obtained. As a consequence, density of ultrasonic beams in the deep area within the radial-shaped area can be increased, while the scanning time is not much increased. Accordingly, the density of the sampling points can be increased in accordance with the depth degrees of the object to be inspected, while the frame rate is not unnecessarily decreased. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    A more better understanding of the present invention may be made by reading a detailed description in conjunction with the accompanying drawings, in which:  
         [0017]    [0017]FIG. 1 is a schematic block diagram showing a main arrangement of an ultrasonic imaging apparatus according to a first embodiment of the present invention;  
         [0018]    [0018]FIG. 2A is a transparent perspective view illustratively showing an example of a construction of an ultrasonic probe employed in the ultrasonic imaging apparatus as shown in FIG. 1, and FIG. 2B is a perspective view showing a portion of a one-dimensional array in an enlarge manner, which is contained in the ultrasonic probe as shown in FIG. 2A;  
         [0019]    [0019]FIGS. 3A to  3 C are explanatory diagrams for explaining one example of an imaging method with employment of the ultrasonic imaging apparatus as shown in FIG. 1;  
         [0020]    [0020]FIGS. 4A to  4 C are explanatory diagrams for explaining another example of an imaging method with employment of the ultrasonic imaging apparatus as shown in FIG. 1;  
         [0021]    [0021]FIG. 5 is an illustration schematically showing a partial construction of an ultrasonic imaging apparatus according to a second embodiment of the present invention;  
         [0022]    [0022]FIG. 6 is a diagram illustratively showing a scanning area within an object to be inspected and an example of a division pattern of the scanning area which is scanned by employing the ultrasonic probe as shown in FIG. 5;  
         [0023]    [0023]FIGS. 7A to  7 C are explanatory diagrams for explaining an example of the conventional ultrasonic imaging method; and  
         [0024]    [0024]FIGS. 8A and 8B are explanatory diagrams for explaining another example of the conventional ultrasonic imaging method. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    Referring now to drawings, various preferred embodiments of the present invention will be described in detail. It should be understood that the same reference numerals will be employed as those for indicating the same, or similar structural elements, and therefore, descriptions thereof are omitted.  
         [0026]    [0026]FIG. 1 is a system diagram showing a main arrangement of an ultrasonic imaging apparatus according to a first embodiment of the present invention. This ultrasonic imaging apparatus may be employed as, for example, an ultrasonic diagnostic apparatus to be used for diagnosing a human body etc. or a flaw detecting apparatus to be used in an industrial field.  
         [0027]    As shown in FIG. 1, this ultrasonic imaging apparatus includes an ultrasonic probe  10  which is used by abutting against an object to be inspected. The ultrasonic probe  10  includes a plurality of ultrasonic transducers  11  having a transmission/reception function of ultrasonic waves. As the ultrasonic transducer  11 , for example, piezoelectric elements are employed which involve piezoelectric ceramics typically known as PZT (Pb(lead) zirconate titanate) or a polymer piezoelectric element typically known as PVDF (polyvinyl difluoride).  
         [0028]    The plurality of ultrasonic transducers  11  transmit ultrasonic pulses to the object to be inspected in accordance with drive signals which are input from a plurality of pulse generating circuits (pulsers)  12  corresponding to these plural ultrasonic transducers  11 , respectively. Then, these ultrasonic transducers  11  receive ultrasonic pulses reflected from the object to be inspected so as to output detection signals.  
         [0029]    The plurality of pulsers  12  are energized in response to output signals of a plurality of digital delay devices  13  corresponding to these pulsers  12 , respectively, and then output the drive signals. As a pulser  12 , a high-speed pulser which can output a drive signal in a high repetition cycle may be preferably employed. As will be explained later, it is necessary to successively produce ultrasonic beams extended in different directions within a short time period by synthesizing a plurality of ultrasonic pulses with each other in accordance with a portion of the object to be scanned.  
         [0030]    A system control unit  14  for controlling an entire system of this ultrasonic imaging apparatus controls delay time as to the plurality of digital delay devices  13 . Under control of this system control unit  14 , a plurality of ultrasonic pulses having phase differences corresponding to time differences of those drive signals are transmitted from the plurality of ultrasonic transducers  11  to the object to be inspected, and also an ultrasonic beam which is formed by synthesizing these ultrasonic pulses to each other is deflected in a desirable direction.  
         [0031]    On the other hand, the detection signals output from the plural ultrasonic transducers  11  are processed in the analog processing manner by a plurality of pre-amplifiers  15  and a plurality of TGC (time gain compensation) amplifiers  16 , which correspond to these plural ultrasonic transducers  11 . By this analog processing operation, signal levels of those detection signals are matched to input signal levels of a plurality of A/D converters  17 . The analog signals output from the plural TGC amplifiers  16  are converted into digital signals (namely, detection data) by the plural A/D converters  17  under control of the system control unit  14 .  
         [0032]    The detection data output from a plurality of A/D converters  17  are entered into a plurality of digital beam formers  19  in a parallel mode. A plurality of phase control units  21  corresponding to a plurality of ultrasonic transducers  11  are provided in the respective digital beam formers  19 . Each of these phase control units  21  may apply a desirable delay to the corresponding detection data by employing a shift register delay line, a digital very small-sized delay line, or a combination of a CPU and software, otherwise a combination of these delay means. The digital data output from the phase control units  21  are digitally added to each other in a digital adder  22 , so that phase matching operations of the plural detection data obtained by using a series of ultrasonic transducers  11  contained in the ultrasonic probe  10  are carried out. As explained above, since a plurality of digital beam formers  19  are employed, reception focusing operations related to plural directions within the object to be inspected can be achieved at the same time. The digital data output from these digital formers  19  are once stored in a memory  24 , and then, these digital data are read out from this memory  24  so as to be processed in a data processing unit  25 . That is, waveforms of the digital detection data are detected, the detected digital data are converted into image data, and a predetermined image processing operation is carried out as to these digital detection data. Thereafter, the processed detection data are again stored in the memory  24 .  
         [0033]    Furthermore, since a scanning format conversion as to the digital detection data is carried out in a DSC (digital scan converter)  26 , the image data of a scanning space of ultrasonic beams is converted into image data of a physical space. In the case where a three-dimensional image is displayed, a three-dimensional image constructing unit  27  may be interposed between the memory  24  and the DSC  26 . The three-dimensional image constructing unit  27  produces voxel data from plural sets of tomographic data stored in the memory  24 , while this voxel data corresponds to data as to a certain volume. The image data, the scanning format of which has been converted by the DSC  26 , is converted into an analog signal by a D/A converter  28 , and then, this analog signal is displayed on an image display unit  29 .  
         [0034]    [0034]FIG. 2A is a transparent perspective view showing one example of a construction of an ultrasonic probe  30  employed in the ultrasonic imaging apparatus according to this first embodiment, and FIG. 2B is a perspective view showing a portion of one-dimensional array in an enlarge manner, which is contained in the ultrasonic probe  30  as shown in FIG. 2A. As shown in FIGS. 2A and 2B, within a probe main body  31  of the ultrasonic probe  30 , a plurality of arrayed vibration elements  32  which constitute the ultrasonic transducers are assembled in a comb-shaped array, and thus, may constitute a one-dimensional array  33 . A plurality of these arrayed vibration elements  32  are vibrated in response to drive signals entered into these vibration elements  32  so as to transmit ultrasonic pulses toward an object to be inspected. Then, these arrayed vibration elements  32  receive ultrasonic echoes reflected from the object under inspection to be vibrated, so that detection signals (electric signals) are output therefrom. As these arrayed vibration elements, for example, a piezoelectric element containing the above-described PZT and PVDF is employed.  
         [0035]    Next, a description will now be made of one example of a method of imaging an object to be inspected, while using the ultrasonic imaging apparatus according to the first embodiment.  
         [0036]    As illustrated in FIG. 3A, in such a case where a fan-shaped two-dimensional area  40  contained in the object to be inspected is scanned, this two-dimensional area  40  is subdivided into two portions  41  and  42  having different depth degrees. Thereafter, a shallower portion  41  located on the shallower side of this fan-shaped two-dimensional area  40  is firstly scanned. Subsequently, a deeper portion  42  located on the deeper side of this fan-shaped two-dimensional area  40  is scanned. Alternatively, after the deeper portion  42  has been scanned, the shallower portion  41  may be scanned.  
         [0037]    In the case where the shallower portion  41  located in the two-dimensional area  40  as shown in FIG. 3A is scanned, as represented in FIG. 3B, a plurality of ultrasonic pulses are transmitted from the ultrasonic probe  30  to this shallower portion  41  in a pulse transmission time slot TP(S) contained in one pulse repetition time period PRT(S). Since these plural ultrasonic pulses are synthesized with each other, as shown in FIG. 3C, an ultrasonic beam  44  is formed. This ultrasonic beam  44  is extended from a transmission point  43  in the shallower portion  41  in a depth direction (z-direction in FIG. 3C). Then, image information at a plurality of sampling points  45  is sequentially sampled, while these sampling points  45  are distributed along the ultrasonic beam  44  in, for example, an equi-interval. In other words, as shown in FIG. 3B, image information related to the respective sampling points is sampled every time a constant time period has passed. In FIG. 3B, a white-colored point represents such a time point when image information related to a sampling point contained in the shallower portion  41  is sampled.  
         [0038]    As a result, as shown in FIG. 3C, such a scanning process operation is repeatedly carried out “N 1 ” times (symbol “N 1 ” is a natural number), for example, in an equi-angular interval with respect to an angle direction (namely, θ-direction) of the shallower portion  41  to scan an entire area of this shallower portion  41 , so that image information related to all of the sampling points  45  involved in the shallower portion  41  is obtained. As explained above, according to this embodiment, since the firstly-imaged portion in the two-dimensional area is limited to the shallower portion  41 , ultrasonic echoes reflected from this shallower portion  41  are recorded within a short time period. As a consequence, the resultant imaging time with respect to the shallower portion  41  can be shortened, as compared with that required in the conventional sector scanning operation (see FIG. 7).  
         [0039]    When the shallower portion  41  is scanned, strengths of ultrasonic pulses for transmission may be decreased as compared with those of the ultrasonic pulses used in the conventional sector scanning operation. Also, it is preferable to optimize strengths of ultrasonic pulses for transmission when the shallower portion is scanned. If so, then insensible time may be shortened which occurs because high voltages of pulsers are applied to the ultrasonic signal reception system. Then, image information related to an area in the vicinity of a surface of an object to be inspected (for example, an area near a body surface) can be sampled. In addition, it is possible to suppress fogging which is caused by multi-path reflections of ultrasonic echoes reflected from the shallower portion, so that images having a better SN ratio can be reproduced as to the shallower portion. Also, it is preferable that density of ultrasonic beams in the shallower portion is made equal to, or lower than the density of the ultrasonic beams in the conventional sector scanning operation. If so, then imaging time required to image the shallower portion can be furthermore shortened.  
         [0040]    On the other hand, in such a case where the deeper portion  42  within the two-dimensional area  40  as shown in FIG. 3A is scanned, as represented in FIG. 3B, a plurality of ultrasonic pulses are transmitted from the ultrasonic probe to this deeper portion  42  in each of “n” pieces of pulse transmission time slots TP(D) which are contained in one pulse repetition time period PRT (D). Note that symbol “n” indicates a natural number. A plurality of ultrasonic pulses which are transmitted from the ultrasonic probe within each of the pulse transmission time slots TP (D) are synthesized with each other to form a plurality of synthesized beams having different propagation directions from each other as shown in FIG. 4A. That is, the respective ultrasonic pulses are propagated through the object to be inspected with having time differences, a plurality of ultrasonic beams  46  are formed which are extended in different directions. It should also be noted that these ultrasonic beams are preferably formed under such a condition that these ultrasonic beams are spatially separated from each other. If so, then reception ultrasonic echoes which are reflected from the respective different directions can be readily separated from each other by way of the reception focusing operation.  
         [0041]    The transmission of the ultrasonic pulses with respect to the plural directions of the deeper portions  42  is carried out “n” times within a time duration which is required for the ultrasonic pulses to return to the ultrasonic probe after being reflected from the shallower portion  41 . This time duration will be referred to as “shallower portion reciprocation time”. Since the transmission timing of the ultrasonic pulses is set in accordance with the above-described manner, a single scanning time as to the deeper portion can be largely shortened as compared with that of the conventional sector scanning operation as shown in FIG. 8.  
         [0042]    Then, image information is sampled at time instants indicated by black-colored time points (see FIG. 4C) as to a plurality of sampling points  47  which are distributed, for instance, in an equi-interval along “n” lines of ultrasonic beams formed within the deeper portion. At this time, positions of reception focal points are varied by employing detection signals of a plurality of ultrasonic transducers contained in the ultrasonic probe, phases of these detection signals are matched, and then, these detection signals are separated from each other. It should also be understood that most of these received ultrasonic echoes are formed by summing the ultrasonic pulses with each other reflected from a plurality of places within the deeper portion. However, properly-selected weights and also properly-selected time delays have been given to the respective detection signals, and then, the resultant detection signals are added to each other, so that only such a detection signal corresponding to a focal region formed within the deeper portion can be emphasized.  
         [0043]    As shown in FIG. 4C, while the transmission direction is slightly shifted in the angle direction (namely, O-direction) of the deeper portion, such a scanning process operation is repeatedly carried out “m” times, so that an entire portion of this deeper portion can be scanned. It should also be noted that symbol “m” indicates a natural number. Thus, a total number “N 2 ” of ultrasonic beams in the deeper portion becomes N 2 =n×m. In this case, entire imaging time “T F ” related to the fan-shaped two-dimensional area according to this first embodiment is expressed by the following equation:  
           T   F   =N   1   ×PRT ( S )+ m×PRT ( D )  
         [0044]    Assuming now that a total scanning time is expressed as “N” and a duration of repetition time is indicated as “PRT” in such a case where the same fan-shaped two-dimensional area is scanned by way of the conventional sector scanning operation as shown in FIG. 7, it is ideal that the above-explained imaging time “T F ” is set to satisfy the below-mentioned formula:  
         
       T 
       F 
       &lt;N×PRT  
     
         [0045]    However, in an actual case, the entire imaging time “T F ” may be set to satisfy the following formula:  
           T   F &lt;2( N×PRT )  
         [0046]    Next, an ultrasonic imaging apparatus according to a second embodiment of the present invention will now be described. The ultrasonic imaging apparatus according to this second embodiment is featured by that a three-dimensional image is obtained by employing an optical detection type two-dimensional sensor array to receive ultrasonic waves.  
         [0047]    [0047]FIG. 5 illustratively shows a basic idea of an arrangement of the ultrasonic imaging apparatus according to this third embodiment. As illustratively shown in FIG. 5, a two-dimensional sensor array  50  is constituted in such a manner that sectional planes of very fine optical fibers  51  are arranged in a two-dimensional matrix shape. In this case, ultrasonic detecting elements are constructed by either a Fabry-Perot resonator (abbreviated as an “FPR” hereinafter) or a fiber Bragg grating, which are formed on tip portions of a plurality of optical fibers.  
         [0048]    Light produced from a light source  60  passes through a beam splitter  61  and then is entered into the optical fiber array. The light which is entered into the respective optical fibers  51  is reflected by both a half mirror  53  and a total reflection mirror  54 , which are formed on both edges of the FPR. Since a total reflection plane of this total reflection mirror  54  is influenced by geometrical displacement which is caused by ultrasonic waves applied to the ultrasonic detecting elements, the reflection light is modulated by this geometrical displacement, and the modulated reflection light is again entered into the beam splitter  61 . The reflection light entered into the beam splitter  61  is directly focused onto a photodetector  63 , or is focused via an optical fiber and the like onto the photodetector  63 . Alternatively, this reflection light is focused via an image focusing system  62  such as a lens onto this photodetector  63 . It should be noted that in this second embodiment, ultrasonic transmission elements are independently provided with respect to the above-explained ultrasonic detection elements.  
         [0049]    [0049]FIG. 6 is an illustration showing an example of a scanning area and divisions thereof within an object to be inspected, which is scanned by employing the ultrasonic imaging apparatus as shown in FIG. 5. As shown in FIG. 6, in the case where a circular-cone-shaped three-dimensional area  70  contained in the object to be inspected is scanned, this three-dimensional area  70  is divided into a plurality of divisions having different depth degrees, and thereafter, a shallower portion  71  located on the shallower side of the area  70  is firstly scanned, and subsequently, a depth deeper portion  72  on the deeper side of this area  70  is scanned. In this case, the three-dimensional area  70  is scanned in such a manner that density of ultrasonic beams used to scan the deeper portion  72  becomes higher than that of the shallower portion  71 . Alternatively, after the deeper portion  72  has been scanned, the shallower portion  71  may be scanned.  
         [0050]    In the case where the shallower portion  71  within the three-dimensional area  70  is scanned, a plurality of ultrasonic pulse are transmitted to the shallower portion  71  so as to form an ultrasonic beam  44  within such a pulse transmission time slot contained in a single repetition time duration. Next, image information is sequentially sampled at a plurality of sampling points  45  along the ultrasonic beam  44 , and these sampling points  45  are distributed, for example, in an equi-interval. While such an ultrasonic scanning process operation is repeatedly carried out plural times with respect to the shallower portion  71 , an entire portion of the shallower portion  71  is scanned, so that image information related to all of these sampling points  45  contained in the shallower portion  71  can be obtained. In order to realize an ultrasonic imaging method involving such an ultrasonic scanning process operation, for example, the ultrasonic transmission elements may transmit the ultrasonic pulses to the object to be inspected in accordance with the time chart as shown in FIG. 3B.  
         [0051]    On the other hand, in such a case where the deeper portion  72  contained in the three-dimensional area  70  is scanned, a plurality of ultrasonic pulses are transmitted to the deeper portion  72  and thus, a plurality of ultrasonic beams  46  extended in different directions are sequentially formed within each of a plurality of pulse transmission time slots which are involved in a single repetition time duration. Next, image information is sequentially sampled at a plurality of sampling points  47  along the ultrasonic beams  46 , and these sampling points  46  are distributed, for instance, in an equi-interval in the deeper portion  72 . While such an ultrasonic scanning process operation is repeatedly carried out with respect to the deeper portion  72  by slightly shifting the transmission direction, an entire portion of the deeper portion  72  is scanned, so that image information related to all of these sampling points  47  contained in the deeper portion  72  can be obtained. In order to realize an ultrasonic imaging method involving such an ultrasonic scanning process operation, the ultrasonic transmission elements may transmit the ultrasonic pulses to the object to be inspected in accordance with, for example, the time charts as shown in FIGS. 3B and 4B.  
         [0052]    According to the present invention, either the fan-shaped two-dimensional area or the circular-cone-shaped three-dimensional area contained in the object to be inspected is divided into at least the shallower portion and the deeper portion, and these portions are independently scanned one by one. In particular, as to this deeper portion, a plurality of ultrasonic waves are transmitted to the object to be inspected so as to sequentially form a plurality of ultrasonic beams extended in the different directions within a time period in which the ultrasonic echoes reflected from the shallower portion are obtained. As a consequence, the ultrasonic beam density of the deeper portion can be made higher than that of the shallower portion, while the scanning time is not much increased. As a result, the density of the sampling points can be increased in response to the depths of the object to be inspected, while the frame rate is not unnecessarily decreased. As a result, the ultrasonic resolution of the depth degree within the object can be improved up to at least approximately the ultrasonic resolution of the shallower portion, so that the ultrasonic images as to the object can be obtained in high precision.  
         [0053]    As apparent from the foregoing descriptions, the present invention is not limited to the above-described embodiments. For example, while either the fan-shaped two-dimensional area or the circular-cone-shaped three-dimensional area contained in the object under inspection may be divided into three, or more portions having different depth degrees, a total scanning number of times may be increased in response to depths of the respective portions. In this alternative case, the ultrasonic beam scanning operations may be sequentially carried out from the shallowest portion to the deepest portion, or from the deepest portion to the shallowest portion. Alternatively, one of these portions is sequentially selected at random, and the selected portion may be scanned. If the object to be inspected is scanned in accordance with the above-described alternative scanning manner, ultrasonic images of the object would be reproduced in higher precision.  
         [0054]    As previously described in detail, according to the present invention, the density of the sampling points can be increased in accordance with the depths within the object to be inspected, while the frame rate is not unnecessarily decreased.