A three-dimensional ultrasonic diagnostic apparatus includes a two-dimensional array type of ultrasonic probe. A three-dimensional scan operation of scanning a three-dimensional region within a human body under examination with ultrasound and a two-dimensional scan operation of scanning a two-dimensional plane within the three-dimensional region with ultrasound are selectively performed by a beam former unit. Under the control of a controller, the three-dimensional scan operation is repeated intermittently and the two-dimensional scan operation is repeated during the interval between each three-dimensional scan. Three-dimensional ultrasonic image data concerning the three-dimensional region is produced on the basis of received echo signals obtained by the three-dimensional scan operation. Two-dimensional ultrasonic image data concerning the two-dimensional plane section is produced on the basis of received echo signals obtained by the two-dimensional scan operation.

BACKGROUND OF THE INVENTION
 The present invention relates to a 3D (three-dimensional) ultrasonic
 diagnostic apparatus which visualizes a 3D region within a human body
 under examination and more specifically to a technique for improving the
 real-time imaging.
 This application is based on Japanese Patent Application No. 10-311367
 filed on Oct. 30, 1998 and Japanese Patent Application No. 10-316584 filed
 on Nov. 6, 1998, the entire content of which is incorporated herein by
 reference.
 Recent 2D types of ultrasonic probes can scan a 3D region within a human
 body under examination with ultrasound to produce a 3D image. This type of
 scanning is referred to as 3D scanning or volume scanning. Whereas
 conventional 2D scanning is required only to move ultrasound along a plane
 section of a human body, the 3D scanning needs to move ultrasound in all
 directions within a 3D region of the human body. In order to reproduce
 natural movements of internal organs in real time, it is required to
 reduce the time required to scan the 3D region thoroughly for the purpose
 of improving temporal resolution (volume rate). That is, it is required to
 set the number of times that the 3D region is scanned every second to
 about 30 times per second as in the 2D scanning.
 As is well known, the velocity of propagation of ultrasound through human
 body is nearly constant; therefore, the number of times per unit time that
 ultrasound is transmitted and received is limited. That is, since the time
 required for transmission and reception of an ultrasound beam is
 absolutely determined by the depth of field and the ultrasound propagation
 velocity, the transmission/reception rate is almost fixed.
 In order to satisfy the real-time requirements of the 3D scanning,
 therefore, it is required to reduce the spatial resolution (the density of
 ultrasound scanning lines). In order to increase the number of ultrasound
 scanning lines per second, the adoption of a simultaneous reception scheme
 known as digital beam forming has been considered. However, even with the
 digital beam forming, echoes are only received from some directions at
 most for each transmission, resulting in a failure to gain sufficient
 spatial resolution. It might be expected to increase the spatial
 resolution by increasing the number of directions from which echoes are
 received simultaneously. However, this approach would require applied
 energy to be considerably high and therefore might cause damage to the
 array probe and fail to meet safety standards.
 The ultrasonic 3D imaging method, as its typical operation, extracts
 concerned parts from 2D image data obtained, and superimposes the
 extracted concerned parts one on another to create a 3D image. In this
 method, therefore, part of 2D image data drops off.
 Further, it is very useful in diagnosis to display a tissue image (B-mode
 image) and a blood-flow image (color Doppler image) in combination.
 However, the 3D representation capability is still being improved.
 With the ultrasonic imaging, although its imaging range is narrower than
 the imaging range of X-ray computerized tomography apparatus and
 magnetic-resonance imaging apparatus, . . . This causes a problem in that
 it is difficult for an observer to understand the orientation and position
 of a 3D image in a human body under examination.
 BRIEF SUMMARY OF THE INVENTION
 It is an object of the present invention to provide a 3D ultrasonic
 diagnostic apparatus which permits the compatibility of the attainment of
 relatively high spatial and temporal resolution with the provision of 3D
 image information.
 The three-dimensional ultrasonic diagnostic apparatus of the present
 invention has an arrangement required to repeat a three-dimensional scan
 operation of scanning a three-dimensional region within a human body under
 examination intermittently with ultrasound and to repeat a two-dimensional
 scan operation of scanning a two-dimensional plane section within the
 three-dimensional region with ultrasound during the interval between each
 three-dimensional scan.
 The three-dimensional ultrasonic diagnostic apparatus of the present
 invention has an arrangement required to scan two two-dimensional plane
 sections within a three-dimensionally scannable three-dimensional region
 with ultrasound and to display two images concerning the two plane
 sections in combination according to their positional relationship so that
 internal three-dimensional structure can be estimated.
 Additional objects and advantages of the invention will be set forth in the
 description which follows, and in part will be obvious from the
 description, or may be learned by practice of the invention. The objects
 and advantages of the invention may be realized and obtained by means of
 the instrumentalities and combination particularly pointed out
 hereinafter.

DETAILED DESCRIPTION OF THE INVENTION
 Hereinafter, the preferred embodiments of three-dimensional ultrasonic
 diagnostic apparatus of the present invention will be described in detail
 with reference to the accompanying drawings. In the description below,
 "three-dimensional" and "two-dimensional" are abbreviated to 3D and 2D,
 respectively.
 [First Embodiment]
 Referring now to FIG. 1, there is illustrated in block diagram form a 3D
 ultrasonic diagnostic apparatus according to a first embodiment of the
 present invention. In FIG. 1, a 2D array ultrasonic probe 1 has a number
 of piezoelectric elements each formed on top and bottom with electrodes.
 The piezoelectric elements are arranged in a 2D array.
 To the ultrasonic probe 1 is connected an image gathering and processing
 unit 2, which has a 3D beam former 3 that performs on a
 transmitter/receiver section 5 delay control required to scan a 3D region
 within a human body under examination with ultrasound through the probe 1
 and a 2D beam former 4 that performs on the transmitter/receiver section 5
 delay control required to scan a plane section within the 3D region with
 ultrasound through the probe 1. The 2D beam former and the 3D beam former
 are separated functionally for the sake of description but in practice
 they are implemented in the same hardware. The image gathering and
 processing unit 2 has an image processing section (1) 6 that produces 2D
 image data, such as B-mode image data, on the basis of received echo
 signals from the transmitter/receiver section 5 based on the 2D scan and
 produces 3D image data on the basis of the outputs of the
 transmitter/receiver section 5 for the 3D scan.
 The position of a plane section to be subjected to a 2D scan is initially
 set at the center of the 3D region.
 A scan converter 9 is provided to combine the 2D image data and the 3D
 image data produced by the image gathering and processing unit 2 into one
 frame of image data and convert the resulting composite image data into a
 video signal format. The composite image data is displayed on a display
 unit 10 such as a cathode ray tube (CRT) or liquid crystal display (LCD).
 The scan converter 9 has a function of converting non-isotropic 3D image
 data which is obtained by the sector scanning method, which will be later
 described, into isotropic 3D image data. Therefore, in the case of the
 sector scanning method in particular, the apparatus is, in some cases,
 equipped with an image processing unit (2) 11 for generating 3D image data
 by processing 3D image data from the scan converter 9. A controller 7 has
 control of the entire ultrasonic diagnostic apparatus. A pointing device
 8, such as a mouse, a trackball, or a keyboard, is connected to the
 controller 7 in order to allow an operator to enter various commands.
 The operation of the 3D ultrasonic diagnostic apparatus thus arranged will
 be described below. In this embodiment, sector scan, linear scan or any
 other scan can be used. In FIG. 2, there is illustrated a 3D region and a
 2D plane section (indicated hatched) each subjected to sector scan. In
 FIG. 3, there is illustrated a 3D region and a 2D plane section (indicated
 hatched) each subjected to linear scan. In FIG. 4, there is illustrated
 mixed scan of 3D scan of the 3D region shown in FIGS. 2 or 3 with
 ultrasound and 2D scan of the 2D plane section within the 3D region with
 ultrasound. The 3D scan is made repeatedly but intermittently. During the
 interval between each 3D scan, the 2D scan is repeated. As a typical case,
 a 2D scanning frame rate (the number of times of scanning cross sections
 per 1 second) is sufficiently higher (more frequent) than a 3D scanning
 volume rate (the number of times of scanning 3D regions per 1 second). In
 the 3-dimensional scanning operation, a so-called parallel simultaneous
 signal receiving mode which generates a plurality of ultrasonic echo
 signals in the signal receiving direction per one ultrasonic signal
 reception, is employed. It should be noted here that generally, the
 parallel simultaneous signal receiving mode is not employed in the
 2-dimensional scanning operation. However, the parallel simultaneous
 signal receiving mode may be employed in the 2-dimensional scanning
 operation. In this case, the number of signals received in the parallel
 simultaneous mode for the 2-dimensional scanning operation (the number of
 ultrasonic echo signals generated per one ultrasonic signal reception) is
 set lower than the number of signals received in the parallel simultaneous
 mode for the 3-dimensional scanning operation.
 The period C of the 3D scan operation is set equal to or longer than the
 length of time required by 3D processing and display processing, i.e., the
 time .DELTA.t between the moment that a 3D scan is terminated and the
 moment that 3D image data is produced. Thereby, a delay of 3D image
 display with respect to the progress of the 3D scan operation is
 compensated for and 3D images can be displayed one after another at the
 period C of the 3D scan operation.
 The image processing section (1) 6 or (2) 11 employs general 3D image
 processing such as volume rendering or MIP to produce 3D image data one
 after another at the period C from received signals gathered by each 3D
 scan. The resulting 3D image data are displayed in succession after
 another at the display period equal to the 3D scan period C.
 The image processing section in the image gathering and processing unit 2
 produces 2D image data one after another at time intervals of .DELTA.T
 required for each 2D scan from received signals gathered by each 2D scan
 as shown in FIG. 4. The resulting 2D image data are displayed one after
 another at the display period equal to the 2D scan period .DELTA.T.
 While a 3D scan is being made, no 2D scan is carried out and hence no 2D
 image data is produced. Therefore, the 2D image display is interrupted
 temporarily, failing to display the motion of an organism smoothly.
 However, this embodiment compensates for the effects of the temporal
 interruption of 2D image display by producing and displaying 2D image data
 concerning the plane section which is an object of the 2D scan operation
 on the basis of a portion of received signals gathered by a 3D scan.
 The 3D image data are displayed on the same screen as the 2D image data
 while being switched at the same period C as the 3D scan operation. The 3D
 image data and the 2D image data may be displayed on separate portions of
 the same screen. Alternatively, the 2D image data may be displayed
 superimposed upon the 3D image data according to its position in the 3D
 region.
 Here, since the time .DELTA.t required by 3D processing and display
 processing is very longer than the time .DELTA.T for 2D processing and
 display processing, the timing of displaying 3D image data is delayed
 considerably with respect to the display of 2D image data. To compensate
 for that delay, as shown in FIG. 5, the reading of 2D image data from the
 scan converter 9 may be delayed by the time C which elapses from the
 termination of a 3D scan to the time when 3D image data is produced.
 The aforementioned 3D scan operation can be modified as follows. FIG. 6
 shows three subregions that make up a 3D region. As shown in FIG. 7, a 3D
 subscan operation of scanning the first subregion, a 3D subscan operation
 of scanning the second subregion and a 3D subscan operation of scanning
 the third subregion are carried out in this sequence at regular intervals.
 In other words, the 3D subscan operation is repeated intermittently and a
 separate 3D subregion is scanned for each 3D subscan operation. According
 to such a 3D scan approach, 3D image data concerning a 3D region is
 produced by three successive 3D subscan operations. Thus, the period at
 which 3D image data is produced can be shortened to improve the temporal
 resolution of a 3D image. Alternatively, the spatial resolution of 3D
 image data can be improved with the period at which 3D image data is
 produced maintained.
 As stated previously, the location of a plane section subjected to 2D scan
 is initially set at the center of a 3D region. The operator can operate
 the pointing device 8 to parallel shift the plane section any distance
 within the 3D region as shown in FIGS. 8A and 8B or to slant the plane
 section at an angle with respect to the 3D region as shown in FIG. 8C. The
 operator can perform this operation prior to scan or during scan while
 viewing the image.
 Thus, if the 2D plane section is slanted with the direction of view fixed,
 then the operator will have to view the 2D image at an angle. This may
 sometimes make it difficult to view the image. For this reason, as shown
 in FIGS. 9A and 9B, the image processing section 6 is arranged to shift
 the point of view for 3D processing onto the center line of the slanted
 plane section and then change the direction of view to conform to that
 center line. As a result, the 2D image will always be displayed as a front
 image and the 3D image will look as if it has been rotated in the
 direction opposite to the direction in which the plane section was
 slanted.
 As obvious from the above description, since this embodiment repeats a 3D
 scan operation and repeats a 2D scan operation during the interval between
 each 3D scan, the 3D structure of an organism of a human body under
 examination can be observed from a 3D image and internal movement can be
 observed through a 2D image with high temporal resolution. In addition,
 since the driving energy per unit time to the probe 1 can be made lower
 than in the case where the 3D scan operation alone is repeated in
 succession, the possibility of damage to the probe can be reduced and the
 safety of the probe can therefore be improved.
 [Second Embodiment]
 FIG. 10 schematically shows an ultrasonic diagnostic apparatus according to
 a second embodiment of the present invention. This apparatus is composed
 of an ultrasonic probe 101, an apparatus body 112, a display unit 107, and
 a scan panel 111. The ultrasonic probe 101 used is of a 2D array type in
 which a number of piezoelectric elements adapted for inter-conversion
 between electric and acoustic signals are arranged in a matrix.
 The apparatus body 112 is constructed from a transmitter/receiver unit 102,
 a digital beam former switch unit 103, an image processing unit 113, a
 host CPU 110, and a display unit 109. The transmitter/receiver unit 102
 comprises a transmission/reception switch 123 for switching between
 transmission and reception, a transmitter 121, and a preamplifier 122. The
 switch 123, at the time of transmitting ultrasound, connects the
 transmitter 121 to the ultrasonic probe 101 and, at the time of receiving
 echoes, connects the preamplifier 122 to the ultrasonic probe.
 The transmitter 121, which is comprised, through not shown, of a clock
 generator, a frequency divider, a transmission delay circuit, and a
 pulser, converts clock pulses from the clock generator to rate pulses of,
 say, 6 KHz through the frequency divider, then applies the rate pulses
 through the delay control circuit to the pulser to produce high-frequency
 voltage pulses and drives the piezoelectric elements with the voltage
 pulses. Namely, the piezoelectric elements are subjected to mechanical
 vibrations to produce ultrasound. The ultrasound thus produced is
 reflected by acoustic impedance boundaries within the human body back to
 the ultrasonic probe 101, thus subjecting the piezoelectric element to
 mechanical vibrations. An electrical echo signal is thus produced
 individually in each piezoelectric element. The resulting electrical echo
 signals are amplified in the preamplifier 122 and then sent to a beam
 former unit 103 where they are added in phase. In this way, signals having
 directivity (received echo signals) are produced.
 The diameter of a beam of ultrasound is increased intentionally by delay
 control of voltage pulses to the piezoelectric elements. This is intended
 to reduce the time required to scan through a 3D region within a human
 body under examination with ultrasound, i.e., the time required by a 3D
 scan (also referred to as a volume scan), and thereby improve the temporal
 resolution, i.e., the number of times per second that the 3D region is
 scanned, or the volume rate and increase the real-time operability. In
 order to produce a plurality of (n in this example) received signals
 differing in directivity each time a large-diameter beam of ultrasound is
 transmitted, that is, in order to implement multidirectional simultaneous
 reception, the digital beam former unit 103 is equipped with a plurality
 of (n in this example) digital beam formers 131-1 to 131-n each of which
 is arranged to sum received echo signals in phase at a different time.
 The image processing unit 113 is equipped with four processors 104, 105,
 106, and 108 which are connected with a bus. The application processor 106
 has a processing facility mainly required for display and measurement. An
 echo processor 104 is responsive to received signals from the digital beam
 former unit 103 to produce B-mode image data that provides morphological
 information of tissues (information about the structure and form of
 tissues). The echo processor 104 extracts from the received signals
 harmonic components having frequencies that are integral multiples of the
 fundamental frequency and then produces tissue harmonic image data that is
 capable of providing the morphological information of tissues more
 clearly.
 A unit that implement the so-called color flow mapping (CFM), the Doppler
 processor 105 is arranged to subject the received echo signals from the
 digital beam former unit 103 to quadrature detection to derive a
 frequency-shifted Doppler signal, cause specific frequency components in
 the resulting Doppler signal to pass through an MTI (moving target
 indicator) filter, determine the frequencies that passed through the
 filter by the use of an autocorrelator, and compute average velocity,
 variance, and power. By adjusting the passband of the MTI filter, general
 CFM image data that mainly visualize tissues and blood-flow velocity and
 tissue Doppler image data that mainly visualize the tissue shapes of
 cardiac muscles and the like can be produced selectively. In addition, the
 Doppler processor is capable of producing power Doppler image data from
 the power that visualize the shape of tissues and blood flow.
 The 3D processor 108 is arranged to produce 3D-like image data as will be
 described later from any of the B-mode image data, the tissue harmonic
 image data, the CFM image data, the tissue Doppler image data, and the
 power Doppler image data.
 In addition, the 3D processor 108 can form a simple graphic, such as a
 wire-frame model, that represents the contour of a 3D scan region within a
 human body under examination. The 3D processor can execute processing
 required to implement various display modes such as of superimposing
 B-mode image data upon that wire-frame model. Image data produced by the
 3D processor is displayed through the display unit 109 upon the display
 screen 107.
 [Basic Scanning and 3D-like Display Screen Configuration]
 In the present embodiment, the key to the operation of displaying
 tomography data, such as B-mode image data, in a 3D-like fashion is to
 display schematically a 3D region that can be scanned with ultrasound
 using a wire-frame model with its vertex at the ultrasound emitting
 surface of the ultrasonic probe 101 as shown in FIG. 11A. Two plane
 sections A and B within the 3D region that meet at a point on the center
 axis of the ultrasonic probe 101 are alternately subjected to a 2D scan.
 The resulting tomography data for those two plane sections are
 superimposed on the wire frame model with registration as shown in FIG.
 12. In this registration, the two 2D-scan cross-sectional images are
 fitted accurately in their respective positions on the wire frame model.
 At the lower right of the display screen each of the two 2D-scan
 cross-sectional images is separately displayed in full as a B-mode image.
 To facilitate the understanding of the positional relationship between two
 plane sections to be 2D scanned and set easily such two plane sections, a
 3D region guide figure as viewed from the probe side is displayed at the
 upper right of the display screen. By operating on the guide figure with
 the pointing device on the scan panel 111, two plane sections can be
 changed or shifted. When the plane sections are changed, the conditions
 under which the probe 101 is driven by the transmitter/receiver unit 102
 are changed accordingly, whereby the 2D scan plane sections are changed
 automatically.
 The 2D plane sections may be curved as shown in FIG. 13. This scan can be
 made by changing delay times regularly not only for the azimuth direction
 in which ultrasound is directed but also the direction normal to the
 azimuth direction each time ultrasound is transmitted.
 According to the present embodiment, as described above, sine only two
 plane sections within a 3D region are scanned with ultrasound, under the
 same temporal resolution the density of ultrasound scan lines can be
 improved significantly as compared with the case where the 3D region is
 scanned in its entirety, whereby high spatial resolution, that is, high
 image quality is attained. In addition, since the two tomography images
 for the two plane sections that meet are combined in such a way as to
 conform to their actual position and then registered with and fitted in
 the wire-frame model that schematically represents the 3D region,
 morphological 3D information can be recognized by intuition in comparison
 with the case where the two tomography images are simply displayed in a
 line. Furthermore, by shifting the two plane sections manually, the
 structure and form of tissues and the blood flow can be understood well in
 a 3D-like fashion.
 Although, in the above description, only two plane sections are subjected
 to 2D scan, this is not restrictive. Three or more plane sections may be
 subjected to 2D scan within some temporal and spatial resolution
 tolerance.
 [Automatic Shifting of Plane Section]
 Automatic shifting of at least one of the two plane sections allows the
 further promotion of the understanding of the structure and form of
 tissues and the blood flow in a 3D-like fashion. FIGS. 14A, 14B, 15A and
 15B illustrate display examples. At the lower right of the display screen
 is displayed a menu for setting up automatic shifting conditions. On the
 menu, the plane to be shifted automatically can be specified to be either
 A plane or B plane. The scan scheme can be specified to be either type a
 or type b. The range over which the plane is to be shifted can be
 specified in terms of an angle. The pitch at which the plane is shifted
 can be specified. The speed at which the plane is shifted can be
 specified. The transmitter/receiver unit 102 automatically shifts the
 selected plane to be subjected to 2D scan in accordance with the automatic
 shifting conditions thus set and the display unit 110 displays the
 resulting tomography images one after another in the wire frame model.
 FIG. 14A illustrates the manner in which the selected plane is shifted in
 accordance with the scan type a. In the scan type a, the plane is rotated
 automatically with the center axis of the 3D region as the center of
 rotation. In FIG. 15A, there is illustrated the manner in which the
 selected plane is shifted in accordance with the scan type b, in which
 case the plane is shifted along the direction perpendicular to it.
 With such automatic shifting of a plane section, after the ultrasonic probe
 101 is applied to the human body surface, two plane sections that pass
 through the center axis of a 3D region are displayed in real time (a live
 image or a moving image), and, upon depression of the automatic shift
 button, one of the plane sections is shifted through the 3D region and the
 resulting tomography images are displayed one after another within a wire
 frame model indicating the 3D region. The selected plane section can be
 selectively shifted in one direction only or in two directions.
 The plane sections to be shifted are not limited to the two plane sections
 A and B. An additional plane section .alpha. to be scanned may be set as
 shown in FIGS. 14A or 15B. For example, if the plane section .alpha. is
 set at the boundary between the plane sections A and B, then the area to
 be observed will become easy to recognize as a whole. When the plane
 section .alpha. is being shifted and displayed, both the plane sections A
 and B may be displayed in the form of either live images or still images,
 which are switched as required by means of switches or the like. In the
 case where the plane sections A and B are displayed in the form of still
 image, only the plane section .alpha. is scanned, that allowing the
 temporal and spatial resolution to be prevented from lowering.
 Next, the specific operation of the ultrasonic diagnostic apparatus of this
 embodiment will be described. The apparatus can be selectively operated in
 three scan modes. The aforementioned display of FIG. 12 forms the basis
 for any of the three scan modes. The three scan modes will be explained
 below in sequence.
 [First Scan Mode]
 FIG. 16 shows a display image in the first scan mode. The first scan mode
 is suitable for the purpose of providing B-mode images or CFM images of
 two orthogonal plane sections corresponding to orthogonal arrangements of
 piezoelectric elements in the 2D array probe 101. This scan is similar to
 conventional scanning of two orthogonal plane sections with a bi-plane
 probe and is an application of bi-plane probe-based scan to the 2D array
 probe.
 The first scan mode subjects only two plane sections within a 3D region to
 2D scan and is not therefore means for directly obtaining a 3D image in
 its original sense. The scan range is N.times.2 scan lines, smaller than
 the 3D scan range of N.times.M scan lines. Therefore, images can be
 provided which have such a high temporal resolution that the real-time
 operability is not much damaged and a high spatial resolution nevertheless
 only two plane sections are scanned.
 It is therefore desirable to use the first scan mode as premeans for
 obtaining a 3D-like image, i.e., as a guide to the operator in positioning
 the probe on the human body. The outline of the procedure for executing
 the first scan mode will be explained below.
 The operator first selects the first scan mode on the operating panel 111.
 A selection signal from the operating panel 111 is entered over the bus
 into the host CPU 110, which in turn outputs a control signal for the
 first scan mode to the transmitter/receiver unit 102 and the beam former
 unit 103. Thereby, a scan is made in the first scan mode. The host CPU 110
 sends a control signal for signal processing corresponding to the first
 scan mode to the echo processor 104, the Doppler processor 105, the 3D
 processor 108, and the display unit 109 over the bus.
 The echo processor 104 then produces a tissue-based ultrasonic image signal
 (a basic signal for B-mode image) from received echo signals. The Doppler
 processor 105 produces a blood-flow-based ultrasonic signal (a basic
 signal for CFM image) from received echo signals obtained in a sequential
 order of time from the same position. The 3D processor 108 performs
 operations of projecting 3D coordinate data referenced to the position of
 the probe 101 onto 2D images on the following display elements and
 performs transformation for image display.
 The display elements include a wire frame model representing the contour of
 a 3D region, a tomography image of the plane section A (B-mode image or
 the like), a tomography image of the plane section B, and a probe graphic
 (including scan marks).
 As for the probe graphic and the 3D-region wire frame model, transformation
 to projected image is performed by setting a given estimated position for
 the probe 1. As for the tomography images of the plane sections A and B
 obtained from the B-mode image basic signal and the CMF image basic signal
 as well, the same estimated position is used to perform transformation to
 projected image. The positions of the plane sections A and B are set on
 the basis of initial-state values in the host CPU 110.
 In shifting a plane section, the operator changes the initial-state values
 through the operating panel 111. The display unit 109 superimposes each
 display element on top of the other to produce ultrasonic image data. A
 ultrasonic image is then displayed on the display monitor 107.
 FIG. 16 shows a typical display example in the first scan mode when the
 left ventricle is viewed from the tip of the heart. The operator, while
 observing such a display as shown in FIG. 16, can move the probe 101 or
 shift the plane A or B to confirm that a region he or she wants to watch
 is covered.
 [Second Scan Mode]
 The second scan mode is suitable for the purpose of providing a 3D
 ultrasonic image of a local region (3D-ROI) forming a portion of a 3D
 region, more specifically, a 3D local region surrounded by the two
 orthogonal planes corresponding to the orthogonal arrangements of
 piezoelectric elements in the 2D array probe 1 set in the first scan mode.
 In the second scan mode in which a 3D scan (N'.times.M' scan lines) is
 made, the real-time operability tends to become lower than in the first
 scan mode (N.times.2 scan lines) in which only two plane sections are
 extracted from 3D space within a human body. However, by restricting the
 3D scan region to a local region of proper size (N'&lt;N, 2&lt;M'&lt;M), the
 lowering of the real-time operability can be minimized. Above all, 3D
 information can be provided which is never available in the first scan
 mode.
 Therefore, it is preferable to use the second scan mode in order to obtain
 a 3D image after image display in the first scan mode serving as
 positioning guide.
 The outline of the procedure of executing the second scan mode will be
 described below.
 The operator first switches the scan mode to this mode through the
 operating panel 111. The following procedure remains basically unchanged
 from that in the first scan mode. In this scan mode, the 3D processor 108
 performs operations of projecting 3D coordinate data that is referenced to
 the position of the probe 101 onto 2D images on the following display
 elements and performs basic coordinate and size transformation processing
 for image display and, particularly for a 3D image, generally known 3D
 image reconstruction processing including: (1) setting of transparency for
 perspective representation, (2) maximum value projection processing called
 MIP, and (3) contour extraction preprocessing and volume rendering
 postprocessing.
 The display elements include a wire frame model representing the contour of
 a 3D region, a 3D tomography image, and a probe graphic (including scan
 marks).
 The probe graphic and the wire frame model for a 3D region may or may not
 be displayed. The operation when they are displayed remains unchanged from
 that in the first scan mode. The graphic of a local region subjected to a
 3D scan is transformed to a projected image by setting a given estimated
 position for the probe.
 A 3D ultrasonic image obtained from B-mode image basic signals and CFM
 image basic signals for a local region subjected to a 3D scan is also
 transformed to a projected image using the same estimated position. The
 setting of the estimated position is performed on the basis of the initial
 values in the host CPU 10. To shift the estimated position, the operator
 simply updates the initial values through the operating panel 111. To
 shift a region of interest, the operator simply performs a similar
 operation through the operating panel.
 FIGS. 17A and 17B show a typical display example in the second scan mode
 for observation of the bicuspid when the left ventricle is viewed from the
 tip of the heart. A 3D image shown in FIG. 17B may be displayed within the
 3D-ROI or may be displayed separately outside the 3D-ROI as shown in FIG.
 17A. In the latter case, the 3D-ROI graphic itself serves the function of
 a guide for understanding the orientation and facilitates the
 understanding of the position of the 3D image cut out. Even if the cut 3D
 image has hidden portions difficult to view under the default estimated
 position, the estimated position can be rotated or the image itself can be
 enlarged by the 3D processor, thus making such portions easy to view.
 The operator is allowed, while viewing such a display as shown in FIG. 17
 in real time, to record moving images on video tape or to photograph a
 still image after scanning is frozen at an appropriate time. Since a 3D
 image can be provided in real time, the time and accuracy of stress
 echo-based diagnosis useful in examining (???) heart diseases can be
 improved. The reason is as follows: The conventional stress echo method,
 which evaluates the behavior of cardiac muscles through tomography images
 only, needs to record a plurality of tomography images one after another
 in a short time. In contrast, this inventive method allows easy
 positioning in the first scan mode and moreover allows an originally
 desired 3D-like image to be recorded in real time in the second scan mode.
 [Third Scan Mode]
 The third scan mode is suitable for the purpose of providing B-mode images
 or CFM images for two orthogonal planes corresponding to orthogonal
 arrangements of piezoelectric elements in the 2D array probe 101 set in
 the first scan mode and providing a 3D ultrasonic image for a 3D local
 region included in space surrounded by the two orthogonal planes. This
 scan mode is a hybrid of the first and second scan modes and consequently
 the display is also a combination of the display in the second scan mode
 and the display of two tomography images. For the third scan mode, there
 are roughly two modes of use according to the purpose.
 The first mode of use mainly aims to make full use of the merits of high
 image quality and a large number of frames in the first scan mode. In this
 mode it is preferable that, after the first scan mode is carried out as a
 guide for positioning, the third scan mode be used to obtain a 3D-like
 image. For example, the first mode of use is suitable for the first
 medical examination when the operator has no previously obtained
 information concerning positioning.
 In the second mode of use, the third scan mode is selectively used from the
 beginning with no operation of switching the scan modes. For example, the
 second mode is suitable for the observation of progress of the state of a
 human body in the case where the operator has previously obtained
 information concerning the human body.
 The outline of the procedure of executing of the operation in the third
 scan mode will be explained below.
 The operator first makes a transition to the third scan mode through the
 operating panel 111. The following operation remains basically unchanged
 from that in the second scan mode and description thereof is therefore
 omitted. In this scan mode, the 3D processor 108 performs given processing
 on the following display elements.
 The display elements include a wire frame model representing the contour of
 a local region (3D-ROI), a 3D ultrasonic image, a probe graphic (including
 scan marks), a wire frame model for a 3D region, a tomography image of the
 plane section A (B-mode image or the like), and a tomography image of the
 plane section B.
 In FIGS. 18A and 18B there is illustrated a typical display example in the
 third scan mode for observation of the bicuspid when the left ventricle is
 viewed from the tip of the heart. A 3D image may be displayed within the
 3D-ROI or may be displayed separately outside the 3D-ROI as shown. In the
 latter case, the 3D-ROI itself serves the function of a guide for
 understanding the orientation and facilitates the understanding of the
 position of the 3D image cut out. Even if the cut 3D image has hidden
 portions difficult to view under the default estimated position, the
 estimated position can be rotated or the image itself can be enlarged by
 the 3D processor, thus improving visibility.
 When the 3D image is displayed inside the 3D-ROI, it is displayed
 overlapped with the two tomography images A and B in the background. To
 permit the tomography images and the 3D image to be observed separately,
 it is preferable that they be displayed with different transparency. As an
 example, those portions of the tomography images which are not overlapped
 with the 3D image are displayed usually. Those portions of the tomography
 images which are overlapped with the 3D image are displayed with a
 transparency of .alpha. and the 3D image is displayed with a transparency
 of 1-.alpha.. By so doing, even in the overlapping portions, the
 tomography images in the background will be displayed through the
 semi-transparent 3D image, making comparison in orientation between the
 tomography images and the 3D image easy to understand. Of course, in
 addition to the transparency setting, tomography image color map setting
 and 3D image color map setting can be made different. In that case, the
 tomography images and the 3D image can be observed separately with
 different shades of color. The color map setting can be implemented in the
 3D processor 108 or the display unit 109.
 The image display in the present invention is not limited to the examples
 described so far. Various modifications are possible.
 For example, in FIG. 18, the tomography images are tissue-based B-mode
 images and the 3D image is a tissue-based 3D image subjected to MIP or
 volume rendering. The tomography images may be blood-flow-based CFM
 images, CFM images containing tissue motion called tissue Doppler, or
 B-mode images containing information dependent on the non-linearity of
 tissue propagation called tissue harmonic. The same is true for the 3D
 image.
 It does not matter if a contrast medium is given to a patient and
 information based on the contrast medium is contained in received echo
 signals. This is because the present invention aims at new display
 procedures, display methods, and ultrasound scanning methods. As another
 display example in the third scan mode, a typical display example for
 examination of blood flow within a lever tumor is illustrated in FIGS. 19A
 and 19B.
 In the third scan mode, it may follow from various constraints and
 examination purposes that there are suitable combinations and unsuitable
 combinations of the modes of tomography and 3D images. Particularly in the
 case where the heart is examined as shown in FIG. 18, the ultrasonic
 diagnostic apparatus is required to have real-time operability. In such a
 case, a combination in which the frame rates is maximized is desirable. As
 indicated in the accompanying sheet 1, in order to provide an appropriate
 frame rates for a blood flow-based 3D image, it is required to limit the
 size of a local region actually subjected to 3D scan or decrease the
 density of scan lines. A blood flow-based CFM image, even if it is a
 tomography image, requires the frame rates to be decreased excessively.
 Thus, the combination of a blood flow-based 3D image and a
 blood-flow-based CFM tomography image is not suitable.
 In the third scan mode, where the real-time operability is taken as
 important, it is more desirable to consider tomography images to only
 function as a guide for giving the orientation of a 3D image and to
 display normal B-mode images.
 Moreover, in the scanning sequence for tomography and 3D images, the scan
 ratio between the 2D scan in the first scan mode and the 3D scan in the
 second scan mode can be changed. For example, as shown in FIG. 21A, it is
 recommended to allocate more time to a scan for which temporal resolution
 has great weight, in addition to a general sequence in which a 2D scan for
 tomography image and a 3D scan for 3D image are alternated on a
 time-division basis. For example, if, as shown in FIGS. 21B and 21C, most
 of the scan time is allocated to 3D scan and 2D scan is made occasionally
 (or when switch control is performed), then the lowering of the real-time
 operability for 3D image will be minimized. Of course, the real-time
 operability for tomography images is lowered, but the tomography images
 can serve well as the function of a positioning guide. Conversely, when
 importance is attached to tomography images rather than a 3D image, it is
 only required that most of the scan time be allocated to 2D scan and 3D
 scan be made occasionally (or when switch control is performed) as shown
 in FIGS. 21D and 21E.
 FIGS. 20A through 20E are diagrams for use in explanation of a method of
 setting a local region (3D-ROI) subjected to a 3D scan in the second and
 third scan modes. Here, the method will be described in terms of the case
 of a valve disease. In order to observe the structure and form of a valve
 on a 3D basis, it is required that the valve be included in a 3D-ROI. To
 determine the 3D-ROI more readily, a C-mode image in the halfway position
 in the direction of depth within the 3D-ROI guide wire is displayed
 separately. The C-mode plane can be set arbitrarily at any depth within
 the 3D-ROI guide wire according to the purpose. As is well known, the
 C-mode image is a tomography image in a plane (C-mode plane) substantially
 orthogonal to the direction of a beam of ultrasound.
 When captured in the guiding C-mode image, the valve, an object of
 observation, will have been included in the 3D-ROI guide wire. Thus, if a
 C-mode plane is set in a 3D-ROI guide wire and the guide wire is moved in
 real time to a scan region using a C-mode image displayed in a separate
 area as a guide, the 3D-ROI including the valve captured in the C-mode
 image can be positioned efficiently and surely. This facilitates the
 setting of an ROI, allowing diagnosis to proceed smoothly.
 Alternatively, it is also possible to place the 3D-ROI in a region where a
 valve will probably be present and shift automatically or manually the
 C-mode plane up or down. In this case, at the time when the valve is
 captured well in the C-mode image, the stop position is specified by
 switch control and then the 3D-ROI guide wire is shifted to conform to the
 C-mode plane.
 After the 3D-ROI has been set, the C-mode plane is newly shifted up and
 down to mark upper and lower limits on the 3D display region. This is
 useful in extracting and displaying only a certain part, a valve in this
 example, in a region determined by the 3D-ROI guide wire. This is more
 efficient than simply adjusting repeatedly the height of the 3D-ROI guide
 wire on the basis of the 3D displayed image so that the valve is
 successfully captured in the displayed region.
 Alternatively, the 3D-ROI may be determined in accordance with the
 following procedure. First, the whole of a scan region is displayed in the
 form of a C-mode image. A valve which is an object of observation is
 captured, the stop position is designated by switch control, and a 3D-ROI
 guide wire of appropriate size then appears to conform to a C-mode plane.
 The optimum position of the 3D-ROI guide wire is determined by parallel
 shifting it in the C-mode plane and then its height and width are
 determined so that a desired 3D image display is obtained.
 In addition to being merely used as the 3D-ROI setting guide, the C-mode
 image may be displayed simultaneously with a 3D transmissive display (MIP
 or integral display) of the 3D-ROI. In this case, it becomes possible to
 observe the motion of the entire valve on the 3D display and, at the same
 time, observe the motion of the valve in a specific plane through the
 C-mode image.
 The C-mode image may be displayed not only in black and white but in
 colors. A C-mode color image can be used for blood flow rate measurement.
 Additional advantages and modifications will readily occur to those skilled
 in the art. Therefore, the invention in its broader aspects is not limited
 to the specific details and representative embodiments shown and described
 herein. Accordingly, various modifications may be made without departing
 from the spirit or scope of the general inventive concept as defined by
 the appended claims and their equivalents.