Patent Publication Number: US-2010125201-A1

Title: Ultrasound imaging apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ultrasound imaging apparatus that generates an image by transmitting and receiving ultrasound. More particularly, the present invention relates to an ultrasound imaging apparatus that supports identifying multiple fetuses present in the womb. 
     2. Description of the Related Art 
     An ultrasound imaging apparatus is an apparatus that generates an image by transmitting and receiving ultrasound to and from the inside of a subject. An examination using this ultrasound imaging apparatus is less invasive for the subject. Therefore, this examination can be repeatedly performed on the subject. Also, with an ultrasound imaging apparatus, it is easy to implement a real time display. For these reasons, an ultrasound imaging apparatus is often utilized in the field of obstetrics when tracking a fetus present in the womb of a pregnant woman over time (e.g., see Japanese published unexamined application No. 2001-120544). Recently, a three-dimensional technology for an ultrasound imaging apparatus has been developed. 
     Physicians and others may confirm the condition of the fetus in three dimensions based on this three-dimensional technology in order to comprehend the development and shape of the fetus. 
     In the tracking of the fetus, physicians and others determine whether the fetus is growing well or not by representing its development using a growth curve. When multiple fetuses are present in the womb, these growth curves are used to represent each fetus. 
     Therefore, in order to confirm the degree of growth for each of the multiple fetuses, physicians and others must specify the same fetuses in both the previous and current examinations by distinguishing the fetuses in the womb based on their individual experiences. 
     However, in ultrasonic images, it is rare that multiple fetuses are clearly distinguished and represented. Thus, it has not been easy to specify each of the multiple fetuses. Therefore, physicians and others might make a mistake in specifying each of multiple fetuses. 
     SUMMARY OF THE INVENTION 
     The present invention is considered in the light of the aforementioned background. The object of the present invention is to provide an ultrasound imaging apparatus that supports physicians and others in specifying the same fetuses when multiple fetuses are present in the womb. 
     The first aspect of this invention is an ultrasound imaging apparatus having a probe configured to transmit ultrasound and receive the reflected waves, a generator configured to generate an image based on the reflected waves, and a display that displays the image, wherein the ultrasound imaging apparatus comprises: a first analyzer configured to specify respectively each umbilical cord present in said image; a second analyzer configured to respectively specify a structure continuing into one end of said umbilical cord; and a display controller configure to cause each said structure to be displayed on said display. 
     According to the first embodiment of this invention, even when multiple fetuses are present in the womb, it will be easy to confirm the condition of each fetus by physicians and others, and possible to improve the accuracy of measuring the growth condition of the fetuses, etc. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration diagram showing the appearance of an ultrasound imaging apparatus according to the present embodiment. 
         FIG. 2  is a configuration diagram showing an ultrasound probe. 
         FIG. 3  is a configuration diagram showing an internal configuration of an apparatus body. 
         FIG. 4  is a diagram showing a detailed configuration of an analyzer according to the first embodiment. 
         FIG. 5  is a pattern diagram showing a Color Doppler image of an umbilical cord. 
         FIG. 6  is a pattern diagram showing a B-mode image of a region including a fetus. 
         FIG. 7  is a flowchart showing a process of specifying an umbilical cord. 
         FIG. 8  is a flowchart showing a process of specifying a structure. 
         FIG. 9  is a flowchart showing a process of identifying and displaying a structure. 
         FIG. 10  is a pattern diagram showing a screen from the ultrasound imaging apparatus. 
         FIG. 11  is a pattern diagram showing a second aspect of a screen from the ultrasound imaging apparatus. 
         FIG. 12  is a pattern diagram showing a third aspect of a screen from the ultrasound imaging apparatus. 
         FIG. 13  is a configuration diagram of an analyzer according to the second embodiment. 
         FIG. 14  is a data configuration diagram for data including placenta positional information. 
         FIG. 15  is a pattern diagram showing a display of a probe movement route. 
         FIG. 16  is a flowchart showing an operation for storing the placenta position, etc. 
         FIG. 17  is a flowchart showing an operation for reproducing an aspect of identifying the fetus, even for a new examination. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of an ultrasound imaging apparatus according to the present invention is specifically described as follows, with reference to the drawings. 
       FIG. 1  is a configuration diagram showing the appearance of the ultrasound imaging apparatus according to the present embodiment. 
     The ultrasound imaging apparatus  1  transmits and receives ultrasound to and from the inside of a subject such as a pregnant woman. Moreover, the ultrasound imaging apparatus  1  generates and displays an image of the inside of the subject based on reflected waves that have been received. More particularly, if tracking of fetuses is an item that is part of the examination, this ultrasound imaging apparatus  1  identifies and displays multiple fetuses in the image. 
     This ultrasound imaging apparatus  1  is configured to include an ultrasound probe  2  and an apparatus body  3 . The ultrasound probe  2  and the apparatus body  3  are connected via a cable  4 . 
     The ultrasound probe  2  is controlled by the apparatus body  3  and transmits and receives ultrasound to and from the inside of the subject. The apparatus body  3  generates ultrasonic images such as B-mode (Brightness Mode) and Color Doppler mode images by processing signals of the reflected waves that have been received. A B-mode image represents the intensity of the received ultrasound using brightness, and represents each position of those tissues in the subject that reflect the ultrasound sent from ultrasound probe  2 , arranged two-dimensionally or three-dimensionally. A Color Doppler mode image is an image of blood flow information, the image representing frequency changes in the ultrasound, converted into the flow rate of blood flow using colors. In general, in a Color Doppler mode image, something that is close to the ultrasound probe  2  is processed using red, and something that is distant using blue. 
     When physicians and others implement three-dimensional scanning with the ultrasound imaging apparatus  1 , the physicians and others move the ultrasound probe  2  held by them in a direction perpendicular to the scan plane. Three-dimensional scanning is scanning in which a scan frame (scan plane) is manually or mechanically moved in a three-dimensional region of the subject to collect a signal received from each point within that three-dimensional region. 
       FIG. 2  is a configuration diagram showing the ultrasound probe  2 . 
     The ultrasound probe  2  comprises multiple aligned piezoelectric elements  21 , an ultrasound-transmitting circuit  22 , an ultrasound-receiving circuit  23 , and a position detector  24 . 
     The piezoelectric element  21  is an acoustically/electrically reversible conversion element consisting of a piezoceramic such as lead titanate. When a signal voltage is applied to the piezoelectric element  21 , the signal voltage produces a piezoelectric effect, and ultrasound is sent from the piezoelectric element  21 . Also, when the piezoelectric element  21  receives ultrasound reflected from the subject, a piezoelectric effect is produced by the ultrasound. Ultrasound is converted by the piezoelectric effect into an echo signal having a voltage value according to the intensity of that ultrasound. 
     The ultrasound-transmitting circuit  22  applies a voltage pulse to each piezoelectric element  21 . This ultrasound-transmitting circuit  22  has an oscillator  221 , a frequency divider  222 , a transmission delay circuit  223 , and a pulse generator  224 . 
     The oscillator  221  controls the transmission timing of voltage pulses. The oscillator  221  generates a timing signal for each predefined time period, and transmits the timing signal to the transmission delay circuit  223  via the frequency divider  222 . The frequency divider  222  reduces the timing signal to a rate pulse of, e.g., about 5 KHz. 
     The transmission delay circuit  223  determines the scanning direction of the ultrasound beam. The transmission delay circuit  223  delays the timing signal per piezoelectric element  21 , setting a time lag. 
     This transmission delay circuit  223  receives a scanning direction signal that selects the scanning direction from the apparatus body  3 . 
     Moreover, the transmission delay circuit  223  sets a delay in applying voltage pulse to each piezoelectric element  21  so that the ultrasound beam is focused in the scanning direction based on the received signal. 
     The pulse generator  224  applies a voltage pulse to each piezoelectric element  21  in tune with the timing signal received from the transmission delay circuit  223 . 
     The ultrasound-receiving circuit  23  receives an echo signal output by each piezoelectric element  21 . This ultrasound-receiving circuit  23  has a preamplifier  231 , a partial reception delay circuit  233 , and a partial adder  234 . 
     The preamplifier  231  amplifies echo signals. The partial reception delay circuit  233  provides a delay time that is required to determine the receiving directionality for the amplified echo signals. 
     In order to decrease the number of signal lines in the cable  4 , once the partial adder  234  partially phases and adds signals from each piezoelectric element  21 , it decreases the number of signal lines to, e.g.,  16 . The partial adder  234  sends the partially phased and added echo signals respectively via the cable  4  to the apparatus body  3 . 
     The position detector  24  detects positional information indicating the position of the ultrasound probe  2 . The positional information is information indicating the reference position PO (x0, y0, z0), angle θa (rotational angle around the X-axis), and angle θB (rotational angle around the Y-axis), angle θY (rotational angle around the Z-axis) of the ultrasound probe  2 . 
       FIG. 3  is a configuration diagram showing the internal configuration of the apparatus body  3 . As shown in  FIG. 3 , the apparatus body  3  has a receiving circuit  31 , an amplitude detector  32 , a data memory  33 , a blood flow information detection circuit  34 , a DSC (digital scan converter)  35 , an analyzer  36 , a display controller  37 , a display  38 , and an operating part  39 . 
     Echo signals are input into the receiving circuit  31  from the ultrasound probe  2 . This receiving circuit  31  has an A/D (analog to digital) converter  311 , a main reception delay circuit  312 , and an adder  313 . The A/D converter  311  converts signals that are input from the ultrasound probe  2  into digital signals. The main reception delay circuit  312  provides a delay time that is required to determine the receiving directionality for the echo signals. The adder  313  generates a signal echo signal by phasing and adding multiple echo signals that have been input. 
     Echo signals are input into the amplitude detector  32  from the receiving circuit  31 . The amplitude detector  32  detects envelopes. The amplitude detector  32  detects the intensity of the received ultrasound by implementing compression with logarithmic conversion on the detected data. The data of that intensity value becomes an element of a B-mode image due to its correspondence with the reflected position of the ultrasound. 
     The blood flow information detection circuit  34  has a perpendicular detection circuit  342 , a MTI (moving target indicator) filter  343 , an autocorrelator  344 , a mean flow rate calculator  345 , a dispersion calculator  346 , and a power calculator  347 . 
     The perpendicular detection circuit  342  extracts Doppler signals that have shift frequency components from echo signals, using perpendicular detection processing. The MTI filter  343  implements high-pass filter processing that separates tissue signals and blood flow signals from the Doppler signals. The autocorrelator  344  implements frequency analysis of the Doppler signals to obtain the shift frequency of the blood flow. The mean flow rate calculator  345  calculates the mean velocity of the blood flow from the shift frequency. The dispersion calculator  346  calculates velocity distribution of the blood flow from the shift frequency. The power calculator  347  calculates power that reflects the blood flow volume from the shift frequency. 
     Data representing the mean average, velocity distribution, and power of the blood flow constitute a Color Doppler mode image, and is sent to the data memory  33 . 
     The data memory  33  stores the data of each pixel value constituting a B-mode image corresponding to the positions where the ultrasound was reflected. In addition, the data memory  33  stores the data representing the mean average, velocity distribution, and power of the blood flow, which constitute a Color Doppler mode image corresponding to the positions where the ultrasound was reflected. 
     The DSC  35  constructs volume data according to the actual space from signals of B-mode image and Color Doppler mode image stored in the data memory  33 . The DSC  35  distributes B-mode image and Color Doppler mode image of each scan frame stored in the data memory  33  in a three-dimensional memory space based on positional information detected by the position detector  24  and the oscillation angle of the scanning line. 
     The analyzer  36  specifies a fetus based on a three-dimensional image generated by the DSC  35 . If there are multiple fetuses present in the three-dimensional image, each fetus is separately specified. 
       FIG. 4  is a diagram showing the detailed configuration of the analyzer  36  according to the first embodiment. As shown in  FIG. 4 , the analyzer  36  has an umbilical cord-analyzer  361  that specifies an umbilical cord, and a structure-analyzer  362  that specifies a structure continuing from the umbilical cord. The analyzer  36  first specifies the umbilical cord from a three-dimensional image. Next, the analyzer  36  specifies multiple fetuses respectively by specifying the structures continuing from the umbilical cord. The structure continuing from the umbilical cord is a fetus. 
     Specifying the umbilical cord is described based on  FIG. 5 .  FIG. 5  is a pattern diagram showing a Color Doppler image of the umbilical cord. 
     The umbilical cord-analyzer  361  specifies a region occupied by the umbilical cord P 3  using the Color Doppler mode image. The umbilical cord P 3  has a characteristic pattern in blood vessels running therein. For the blood vessels in the umbilical cord P 3 , an artery P 1  and a vein P 2  run in a spiral. So, the umbilical cord-analyzer  361  specifies the region occupied by the umbilical cord P 3  by detecting this spiral running pattern of blood vessels. 
     Specifying the region of the umbilical cord P 3  with the umbilical cord-analyzer  361  is specifically as follows. The umbilical cord-analyzer  361  has prestored a predicted running pattern of blood vessels  361   a.  This predicted running pattern of blood vessels  361   a  is image data with crossing lines that represent blood vessels. The umbilical cord-analyzer  361  compares the data of each image, for the entire region of the Color Doppler mode image, to this predicted running pattern of blood vessels  361   a.  The umbilical cord-analyzer  361  makes the comparison while zooming in and out the predicted running pattern of blood vessels  361   a  according to a predefined rule and varying the crossing angle of the lines. Then, when the image data array pattern of the comparison subject and the predicted running pattern of blood vessels  361   a  match, a region comprising that image data is specified as a portion of the region occupied by the umbilical cord P 3 . 
     The umbilical cord-analyzer  361  scans the entire region of the Color Doppler mode image for all the umbilical cords P 3  respectively. 
     The umbilical cord-analyzer  361  regards as one umbilical cord P 3 , any region where the region matching the predicted running pattern of blood vessels  361   a  continues on. To continue on means that matched regions are connected with each other by the same artery P 1  and vein P 2 . 
     In other words, the umbilical cord-analyzer  361  determines whether any blood vessel is present between neighboring regions matched to the predicted running pattern of blood vessels  361   a.  If a blood vessel is present, the umbilical cord-analyzer  361  tracks that blood vessel to determine whether the regions to be determined are connected with each other by this blood vessel. Moreover, if the umbilical cord-analyzer  361  determines that the regions are connected with each other, it determines that those regions and the blood vessel region therebetween constitute the umbilical cord P 3 . Then, the umbilical cord-analyzer  361  puts a series of regions connected by the same blood vessels into one group. On the other hand, other matching regions that have been determined by the umbilical cord-analyzer  361  not to be in a relationship with the series of regions for this group are regarded as constituting another umbilical cord P 3 . Moreover, the umbilical cord-analyzer  361  further puts a group of regions that have a continuing region relationship with those matching regions into the other group. Grouping by the umbilical cord-analyzer  361  is done by associating with the same fetus ID a collection of positional information that indicates the position of each region that belongs to one group. 
     In addition, the umbilical cord-analyzer  361  may detect a corded structure using a B-mode image to specify the umbilical cord P 3 . In this case, the umbilical cord-analyzer  361  detects the corded structure by detecting a collection of portions having diameters below the predefined value in the image. 
     Next, specifying the fetus is described based on  FIG. 6 .  FIG. 6  is a pattern diagram showing a B-mode image of a region including a fetus. 
     The structure-analyzer  362  specifies a fetus using the B-mode image. This structure-analyzer  362  specifies a collection of voxels continuing into the region of the umbilical cord P 3  specified by the umbilical cord-analyzer  361  and having a pixel value above a predefined threshold. The predefined threshold is a value that characterizes brightness exhibited by a structure P 4  present in the womb and that is also an empirical value. 
     Specifying a fetus using the structure-analyzer  362  is specifically performed as follows. The structure-analyzer  362  tracks the region occupied by the specified umbilical cord P 3 , with a reference point P 5  as its end point. This reference point P 5  is a connection point between the umbilical cord P 3  and the structure P 4 . 
     Then, the structure-analyzer  362  expands the voxels of the comparison subject from this reference point P 5  and compares the pixel values of the voxels of the comparison subject with the predefined threshold. As a result of comparison by the structure-analyzer  362 , if a pixel value is above the predefined threshold, the structure-analyzer  362  regards that voxel of the comparison subject as one component of the structure P 4 . On the other hand, if the pixel value is below the predefined threshold, the structure-analyzer  362  does not regard that voxel as one component of the structure P 4 . In addition, the structure-analyzer  362  does not expand the voxels around the comparison subject having pixel values below the predefined threshold. 
     If there are multiple umbilical cords P 3  specified by the umbilical cord-analyzer  361 , the structure-analyzer  362  specifies a structure P 4  continuing into the umbilical cord for each umbilical cord P 3 . When the structure P 4  is specified by the structure-analyzer  362 , the umbilical cord P 3  continuing into that structure P 4  is regarded as being in the same group as the structure P 4 . In other words, it further associates a collection of positional information for voxels continuing from one umbilical cord P 3  and having a pixel value above the predefined threshold, with the fetus ID associated with that one umbilical cord P 3 . 
     In addition, the analyzer  36  may model a collection of associated positional information using models such as rectangular solids and spheres and further associate the type and arranged position of the models and the coupling relationship between the models with the fetus ID. 
     The display controller  37  receives voxels of all the structures P 4  output from the DSC  35 , and implements volume rendering processing and color processing on them to constitute one screen. The display controller  37  outputs it to the display  38 . The display  38  is a liquid crystal display, an organic EL display, CRT, etc. 
     In volume rendering processing, the display controller  37  takes a sample of the pixel value of each voxel in all the structures P 4  from the direction of the viewpoint, and calculates light transmission according to the opacity and reflection with respect to the viewpoint, and applies shading, thereby generating a projected image. The viewpoint in volume rendering processing is input by an operator via an operating part  39 . The operating part  39  is a keyboard, a mouse, a trackball, etc. When the direction of the viewpoint, angle around the axis of the viewpoint, viewpoint, and distance to the three-dimensional image are input by physicians and others via the operating part  39 , the display controller  37  sets the viewpoint corresponding to that operation, i.e., the direction of the viewpoint, rotational angle of the image, and zoom-in volume, so as to implement rendering processing on the three-dimensional image. 
     In color processing, a hue, such as one in the RGB format, is assigned to each voxel of the structures P 4 . This hue that is assigned is different for each structure P 4 . 
     This color processing is specifically performed as follows. The display controller  37  obtains a collection of positional information associated with one fetus ID from the analyzer  36 . The display controller  37  also assigns the same hue to each voxel specified based on that obtained positional information. Similarly, the display controller  37  obtains a collection of positional information for other fetus IDs and assigns the same hue for each collection of the positional information based on the obtained positional information. The display controller  37  assigns a different hue to a group with a different fetus ID. For example, the ultrasound imaging apparatus  1  has prestored a list of corresponding fetus IDs and hues. The analyzer  36  assigns a fetus ID to a group from the list. Then, the display controller  37  refers to the list and assigns a hue corresponding to the fetus ID. 
       FIGS. 7 through 9  are flowcharts showing operation of the ultrasound imaging apparatus  1  to identify and display the fetus. 
     First,  FIG. 7  is a flowchart showing the process of specifying the umbilical cord P 3 . The umbilical cord-analyzer  361  obtains image data of the next region in a Color Doppler mode image (S 01 ). When the image data is obtained, the umbilical cord-analyzer  361  compares the predicted running pattern of blood vessels  361   a  to the array pattern in that image data (S 02 ). The umbilical cord-analyzer  361  implements S 01  and S 02  until all the regions are scanned (S 03 , No). 
     When scanning using the predicted running pattern of blood vessels  361   a  is finished (S 03 , Yes), among matching regions matched by comparison at S 02 , the umbilical cord-analyzer  361  defines as a reference region any matching region that is located in the outermost outline on the Color Doppler mode image and has not yet become a reference region (S 04 ). Moreover, the umbilical cord-analyzer  361  issues a new fetus ID to associate the fetus ID with the positional information of that reference region (S 05 ). 
     When the reference region is determined, the umbilical cord-analyzer  361  searches for each matching region present within a predefined search range from this reference region, which is defined in advance. As a result of the search, the umbilical cord-analyzer  361  defines regions other than the reference regions as continuous regions to be determined (S 06 ). 
     Then, the umbilical cord-analyzer  361  determines whether any blood vessel is present between any of respective regions to be determined and the reference regions (S 07 ). When there is a blood vessel (S 07 , Yes), the umbilical cord-analyzer  361  tracks that blood vessel to determine whether it leads to a reference region and a region to be determined (S 08 ). If it does (S 08 , Yes), the umbilical cord-analyzer  361  associates the positional information indicating that region to be determined with the same fetus ID as the reference region (S 09 ). 
     Moreover, the umbilical cord-analyzer  361  defines the region to be determined associated with the same fetus ID as a new reference region (S 10 ) and repeats S 06 -S 09 . 
     As a result of the determination by the umbilical cord-analyzer  361 , there may be no regions to be determined where blood vessels are present between those regions and a reference region (S 07 , No) and also there may be no regions to be determined that are connected with a reference region via a blood vessel (S 08 , No), and moreover, there may be the case where all the matching regions have not been set as reference regions (S 11 , No). In these cases, the umbilical cord-analyzer  361  repeats processing from S 04  to S 10 . In other words, the umbilical cord-analyzer  361  issues a new fetus ID for a matching region not associated with a fetus ID and searches for a new umbilical cord P 3  that includes that region. 
     If the umbilical cord-analyzer  361  has set all the matching regions as reference regions (S 11 , Yes), the process of specifying the umbilical cord P 3  is finished. 
     Next,  FIG. 8  is a flowchart showing the process of specifying the structure P 4 . When the process of specifying the umbilical cord P 3  using the umbilical cord-analyzer  361  is finished, the structure-analyzer  362  reads a B-mode image (S 12 ). Then, the structure-analyzer  362  specifies a fetus ID that has not yet been identified for the structure P 4  (S 13 ). Moreover, the structure-analyzer  362  reads the collection of positional information associated with that fetus ID (S 14 ). 
     When the structure-analyzer  362  reads the B-mode image and collection of positional information, the structure-analyzer  362  further searches for a reference point P 5  located at the farthest point of that positional information (S 15 ). Then, the structure-analyzer  362 , using as comparison subjects, from among the voxels around that reference point P 5 , voxels that have not yet been determined to be one component of the structure P 4 , compares the predefined threshold to the pixel value of those voxels. (S 16 ). 
     As a result of the comparison, when there is any voxel that has a pixel value above the predefined threshold (S 16 , Yes), the structure-analyzer  362 , treating that voxel as one component of the structure P 4 , associates positional information of that voxel with the specified fetus ID (S 17 ). 
     Moreover, the structure-analyzer  362  handles each voxel that has been made one component of the structure P 4  as a reference point P 5  (S 18 ), and repeats S 16  and S 17 . On the other hand, as a result of the comparison, if there is no voxel that has a pixel value above the predefined threshold (S 16 , No) and if specifying all the fetus IDs is not finished (S 19 , No), structure-analyzer  362  repeats S 13 -S 19 . In this way, it implements the process of specifying the structure P 4  for a new fetus ID. 
     If specifying the structures P 4  for all the fetus IDs is finished (S 19 , Yes), the structure-analyzer  362  finishes the process of specifying the structure P 4  continuing into the umbilical cord for each fetus ID, in other words, for each specified umbilical cord P 3 . 
     Next,  FIG. 9  is a flowchart showing the process of identifying the structure P 4 . When the process of specifying the structure P 4  is finished, from the viewpoint set by the operator via the operating part  39 , the display controller  37  implements the process of volume rendering on the collection of voxels indicated by the positional information associated with all the fetus IDs (S 20 ). 
     Moreover, the display controller  37  specifies one pixel representing a projected image generated by the volume rendering process (S 21 ). The display controller  37  obtains positional information of the voxel corresponding to the specified pixel (S 22 ). When the display controller  37  obtains positional information, it searches for a fetus ID associated with the obtained positional information (S 23 ). The display controller  37  refers to the list and reads hue information corresponding to the fetus ID (S 24 ). 
     When the display controller  37  reads hue information, it adds a pixel value representing the hue corresponding to that hue information to the pixel specified at S 20  (S 25 ). 
     The display controller  37  repeats processing from S 21  to S 25  for all the pixels representing the projected image (S 26 , No). When the display controller  37  finishes processing all the pixels (S 26 , Yes), the display controller  37  outputs the projected image that has been generated to the display  38  (S 27 ). 
       FIG. 10  is a pattern diagram showing a screen from the ultrasound imaging apparatus  1 . As shown in  FIG. 10 , the structures P 4 , i.e., the fetuses, are displayed on the screen, with each individual being distinguished by color. Therefore, even when there are multiple fetuses in the womb, it is easy for physicians and others to confirm the condition of each fetus, thus improving the accuracy of measuring the growth condition of the fetuses, etc. 
       FIG. 11  is a pattern diagram showing a second aspect of a screen from the ultrasound imaging apparatus  1 . Although the case where the fetuses are displayed such that they are distinguished by color has been described above, alternatively, as shown in  FIG. 11 , the screen may be divided so as to display a separate window for each fetus. 
     In this case, from the viewpoint set by the operator via the operating part  39 , the display controller  37  implements a volume rendering process for each collection of voxels indicated by positional information associated with each fetus ID. Then, the display controller  37  causes the respective projected images to be displayed in separate windows. 
       FIG. 12  is a pattern diagram showing a third aspect of a screen from the ultrasound imaging apparatus  1 . Although the examples of identification and display by means of distinguishing fetuses by color and dividing the screen for each fetus have been described above, alternatively, as shown in  FIG. 12 , the display controller  37  may display only one fetus in response to a switching operation via the operating part  39 . 
     In this case, in the volume rendering process at S 20 , from the viewpoint set by the operator via the operating part  39 , the display controller  37  implements a volume rendering process per collection of voxels indicated by positional information associated with each fetus ID. Then, the display controller  37  switches each P 4  that has undergone volume rendering to cause it to be displayed in series, according to the number of times the switching operation has taken place. 
     For example, suppose that there is a collection of positional information associated with fetus IDs A, B, and C respectively, i.e. there are three fetuses. In the first display, the display controller  37  implements a volume rendering process on the collection of voxels indicated by positional information associated with the fetus ID A. The display controller  37  causes only the fetus obtained by the process to be displayed. 
     When the first switching operation is performed, the display controller  37  implements a volume rendering process on the collection of voxels indicated by positional information associated with the fetus ID B. The display controller  37  causes only the fetus obtained by the process to be displayed. 
     When the second switching operation is performed, the display controller  37  implements a volume rendering process on the collection of voxels indicated by positional information associated with the fetus ID C. The display controller  37  causes only the fetus obtained by the process to be displayed. 
     When the third switching operation is performed, returning to the first display, the display controller  37  implements a volume rendering process on the collection of voxels indicated by positional information associated with the fetus ID A The display controller  37  causes only the fetus obtained by the process to be displayed. 
     With these modes of identification and display indicated by  FIGS. 11 and 12  also, it becomes easy for physicians and others to identify multiple fetuses, improving the accuracy of measuring the growth condition of the fetuses, etc. 
     Next, the second embodiment for identifying and displaying fetuses is described. The ultrasound imaging apparatus  1  according to the second embodiment specifies the same fetuses both in the past and in new examinations. More particularly, specification using this ultrasound imaging apparatus  1  is as follows. The ultrasound imaging apparatus  1  stores the position of the placenta connected to a fetus. 
     The ultrasound imaging apparatus  1  specifies the same fetus in a new examination based on the stored placenta position. Because the fetus moves about freely in the womb and the umbilical cord also moves following it, it is difficult for physicians and others to specify the same fetus based on the position where the fetus is present, etc. 
     However, because the placenta position is specific to each fetus and that position has been determined, by linking the placenta position with the fetus connected to it, the same fetus can be identified. 
       FIG. 13  is a configuration diagram of the analyzer  36  according to the second embodiment. As shown in  FIG. 13 , in addition to the umbilical cord-analyzer  361  and the structure-analyzer  362 , this analyzer  36  has a storage  363  and a placenta-analyzer  364 . 
     The storage  363  is nonvolatile memory. As shown in  FIG. 14 , for each patient ID, the storage  363  stores combinations of fetus IDs and placenta positional information, and display mode information. The placenta positional information is information indicating the position at the farthest point opposite the structure P 4  of the umbilical cord P 3 , which is represented using coordinates (X, Y, Z). The display mode information is indicated by the position of the viewpoint, rotational angle and distance to the three-dimensional image, which are set by the operator via the operating part  39 . The patient ID is the ID of a subject, which is preinput by the operator via the operating part  39 . In other words, the storage  363  stores placenta positional information for each structure P 4  that continues into the same umbilical cord as that placenta; that is, for each fetus ID. 
     The placenta-analyzer  364 , in each examination, specifies the position of the placenta that continues into each umbilical cord P 3  and generates placenta positional information based on the specified position. More particularly, it tracks the collection of positional information obtained by the umbilical cord-analyzer  361  and defines the positional information as the farthest position of the placenta. The placenta-analyzer  364 , from both ends of the umbilical cord P 3 , defines as the placenta positional information an end point that is opposite to the end point defined as reference point P 5  by the structure-analyzer  362 . 
     In addition, the placenta-analyzer  364  causes the placenta positional information to be stored in the storage  363 . More particularly, the placenta-analyzer  364  obtains a fetus ID associated with the collection of positional information that indicates the tracked umbilical cord P 3 . The placenta-analyzer  364  associates this obtained fetus ID with the placenta positional information to cause it to be stored under the patient ID. 
     If the combination of fetus ID and placenta positional information has been already stored for the patient ID, in order to incorporate changes in the placenta position depending on differences in gestational weeks so as to improve the accuracy, the placenta-analyzer  364  may replace the combination with the new combination, thereby causing it to be stored. It is desirable that the replacing and storing is implemented after the placenta-analyzer  364  reproduces the identifying mode of the fetus used in the previous examination, e.g., after the examination is finished. Alternatively, first-time only storage of the placenta positional information may be implemented. 
     In a new examination, if the patient ID input using the operating part  39  has been stored in the storage  363 , the display controller  37  adds a hue that corresponds to the fetus ID in the list, combined with the placenta positional information generated at the placenta-analyzer  364 , to the structure P 4  that continues from the placenta position indicated by the placenta positional information generated by this placenta-analyzer  364 , and causes it to be displayed. 
     In comparing the placenta positional information obtained in a new examination to the placenta positional information stored in the storage  363 , it is necessary that these sets of placenta positional information be represented with the same coordinate system. Therefore, before starting the examination, the display controller  37  causes a probe movement route R that guides the movement route of the ultrasound probe  2  to be displayed on the display  38 , as shown in  FIG. 15 . The probe movement route R is the start position of scanning with the ultrasound probe  2 ; in other words, an arrow indicating the origin position at which the ultrasound probe  2  is initially placed, and the direction in which the ultrasound probe  2  is to be moved. 
     The display controller  37  prestores a subject model M, origin position, and directional information for drawing this probe movement route R. The display controller  37  draws the probe movement route R pointing in the direction indicated by the directional information from the origin point, and causes it to be displayed on the display  38 . 
     In addition, if the patient ID input using the operating part  39  has been stored in the storage  363 , the display controller  37  implements a volume rendering process at the viewpoint indicated by the display mode information stored in association with that patient ID. This display mode information is stored in association with the patient ID by the display controller  37 . 
       FIG. 16  is a flowchart showing an operation to store the placenta position in the storage  363 . First, the patient ID is input in advance by the operator via the operating part  39  (S 31 ). If the input patient ID has not been stored in the storage  363 , the placenta-analyzer  364  causes that patient ID to be stored (S 32 ). 
     Subsequently, after the umbilical cord-analyzer  361  specifies the umbilical cord P 3 , the structure-analyzer  362  specifies the structure P 4 . 
     Either after the structure P 4  is specified or after an image is output, the placenta-analyzer  364  first specifies a fetus ID that has not yet been specified for the placenta position (S 33 ). The placenta-analyzer  364  reads an umbilical cord P 3  part based on the collection of positional information associated with the specified fetus ID (S 34 ). When the umbilical cord P 3  is read, the placenta-analyzer  364 , from among the positional information of the umbilical cord P 3 , obtains positional information of the location at the opposite end from the structure P 4  (S 35 ). 
     When the positional information is obtained, the placenta-analyzer  364  links the association between the placenta positional information represented by this positional information and the fetus ID specified at S 31  with the input patient ID, and causes it to be stored in the storage  363  (S 36 ). 
     Next, triggered by end of the examination, etc., the display controller  37  links the viewpoint used for the volume rendering process, as the display mode information, with the input patient ID, and causes it to be stored in the storage  363  (S 37 ). 
     In this way, for the past examinations of the subject represented with the patient ID, the fact that structure P 4  specified with the placenta positional information has been displayed in the identifying mode is stored, represented by the fetus ID, in addition to which the viewpoint at that time is stored. 
       FIG. 17  is a flowchart showing an operation to reproduce the mode of identifying the fetus, even in a new examination. 
     First, among fetus IDs issued at S 05  in the current examination, the placenta-analyzer  364  specifies a fetus ID that has not yet been identified for the placenta position (S 41 ). The placenta-analyzer  364  reads an umbilical cord P 3  part from the collection of positional information associated with that fetus ID (S 42 ). When the umbilical cord P 3  is read, the placenta-analyzer  364 , from among the positional information of the umbilical cord P 3 , obtains positional information of a location at the opposite end from the structure P 4 , as placenta positional information (S 43 ). 
     When the placenta positional information is obtained, the display controller  37  compares the obtained placenta positional information to each placenta positional information associated with the preinput patient ID (S 44 ). When the comparison finds matching placenta positional information (S 44 , Yes), the display controller  37  replaces the fetus ID specified at S 41  by the fetus ID associated with that placenta positional information (S 45 ). 
     In addition, in the comparison of placenta positional information, if a coordinate (x, y, z) indicated by the obtained placenta positional information is within a sphere of a predefined diameter around the coordinate (X, Y, Z) indicated by the placenta positional information stored in the storage  363 , both sets of information are considered to be a match. 
     In the current examination, if processing is not finished for all fetus IDs issued at S 05  (S 46 , No), S 41 -S 46  are repeated for the next fetus ID. Then, if processing is finished for all fetus IDs (S 46 , Yes), the display controller  37  implements the processes of identifying and displaying, starting from S 20 . 
     In addition, the display controller  37  may store the origin position of the probe movement route R for each gestational week, and cause the probe movement route R, extending from the origin point according to the gestational weeks input via the operating part  39 , to be displayed. 
     Also, the ultrasound imaging apparatus  1  may store the route along which the ultrasound probe  2  was actually moved, and convert the coordinates of placenta positional information obtained in a new examination from differences in the movement route of the new examination. 
     Specifically, the display controller  37  causes the subject model M to be displayed on the display  38  after the examination. Then, when the actual route along which the ultrasound probe  2  was moved in the examination is input by the operator via the operating part  39 , the display controller  37  causes that route to be drawn on the subject model M and also associates the origin position and directional information indicated by that route with the patient ID, and causes it to be stored in the storage  363 . 
     Upon a new examination, the display controller  37  generates a determinant that converts the coordinate system of the route input using the operating part  39  into the coordinate system indicated by the route stored in the storage  363 . The display controller  37  converts the coordinates indicated by placenta positional information obtained in the new examination using this determinant. 
     Then, the display controller  37  compares this placenta positional information after conversion to each placenta positional information stored in the storage  363  and colors the structure P 4  with a hue corresponding to the fetus ID associated with matching placenta positional information. 
     In this way, because the ultrasound imaging apparatus  1  according to the second embodiment specifies the same fetus in both past and new examinations, it is possible to reproduce, even in a new examination, the mode of identifying each fetus identified and displayed in past examinations. Therefore, it is possible for physicians and others to eliminate the task of confirming whom an identified and displayed fetus is, which provides good operational efficiency.