1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus in which a plurality of piezoelectric transducers are arranged in a predetermined direction so as to transmit and receive ultrasonic waves to obtain a tomographic image on the inside of the subject, and more particularly to an ultrasonic diagnostic apparatus adopting such a scheme that a sector scan is electronically performed.
2. Description of the Related Art
Hitherto, there has been used an ultrasonic diagnostic apparatus in which ultrasonic waves are transmitted toward the subject, specially a living body and ultrasonic waves reflecting from a tissue within the living body are received by piezoelectric transducers to generate received signals, and an image of the living body is displayed on the basis of the received signals, thereby facilitating a diagnostic of an intestinal disease or the like in the living body.
FIG. 23 is a schematic diagram showing a functional structure of an ultrasonic diagnostic apparatus. The ultrasonic diagnostic apparatus 100 is provided with, for example, 64 piezoelectric transducers (hereinafter, it may happen that these are each referred to as "element") 12.sub.-- 1, 12.sub.-- 2, . . . , 12.sub.-- 64, which are arranged as a strip. Those elements 12.sub.-- 1, 12.sub.-- 2, . . . , 12.sub.-- 64 are applied to a body surface of the subject (not illustrated), and then a transmitting circuit 102 sends out pulse signals to the piezoelectric transducers in their associated timings, respectively. The pulse signals are converted into high voltage pulses by the associated transmitting driver 103.sub.-- 1, 103.sub.-- 2, . . . , 103.sub.-- 64, respectively. The converted high voltage pulses are applied to the elements 12.sub.-- 1, 12.sub.-- 2, . . . , 12.sub.-- 64, respectively, so that ultrasound beams emanate from the elements 12.sub.-- 1, 12.sub.-- 2, . . . , 12.sub.-- 64 toward the inside of the subject.
The ultrasonic waves reflecting from the inside of the subject again return to the elements 12.sub.-- 1, 12.sub.-- 2, . . . , 12.sub.-- 64 and are received thereat. Signals, which are generated through receiving by the elements 12.sub.-- 1, 12.sub.-- 2, . . . , 12.sub.-- 64, are amplified suitably by receiving amplifiers 104.sub.-- 1, 104.sub.-- 2, . . . , 104.sub.-- 64, respectively, and then supplied to a beamformer circuit 105. The beamformer circuit 105 is arranged to delay the respective entered received signals and then to add the respective delayed received signals, so that the received signals can be generated along the ultrasound beams extending into the subject. The added received signals, which are outputted from the beamformer circuit 105, are applied to a signal transforming circuit 106 so as to be transformed into a displaying signal. The displaying signal outputted from the signal transforming circuit 106 is applied to a CRT display 107, so that a tomographic image 110 on the inside of the subject is displayed on a screen of the display 107.
Incidentally, when the piezoelectric transducers (elements) 12.sub.-- 1, 12.sub.-- 2, . . . , 12.sub.-- 64 are generally named, they are denoted as the piezoelectric transducers (elements) 12, hereinafter. This is the similar as to the matter of the transmitting driver 103.sub.-- 1, 103.sub.-- 2, . . . , 103.sub.-- 64, and the receiving amplifiers 104.sub.-- 1, 104.sub.-- 2, . . . , 104.sub.-- 64.
FIG. 24 is a typical illustration of an example showing a relationship between an arrangement of the piezoelectric transducers and reflecting points of ultrasonic waves within the subject. In this figure, the axis of abscissas X denotes an arrangement direction of 64 pieces of piezoelectric element 12 applied to a body surface, and the axis of ordinates Z and the clinoaxis Z' denote the directions (each of them is referred to as a scan line) of traveling of ultrasound beams within the subject. Here, it is assumed that an acoustic velocity within the subject is uniform independently of a place.
In case of the formation of ultrasound beams having a focus at a point P1 within the subject, the transmitting circuit 102 (Cf. FIG. 23) sends out transmission pulse signals to the piezoelectric elements 12.sub.-- 1, 12.sub.-- 2, . . . , 12.sub.-- 64 in their associated timings, respectively, such that the transmission pulse signals are delayed in accordance with a delay pattern corresponding to an arc R1 described with the point P1 in the center so that the ultrasonic waves emitted from the piezoelectric elements 12.sub.-- 1, 12.sub.-- 2, . . . , 12.sub.-- 64 arrive simultaneously on the arc R1, in such a manner that taking account of an acoustic velocity within the subject, for example, the piezoelectric elements 12.sub.-- 31 and 12.sub.-- 32 at the center radiate the ultrasonic waves at the time point when the ultrasonic waves emitted from the piezoelectric elements 12.sub.-- 1 and 12.sub.-- 64 at the both ends arrive on the arc R1.
In a similar fashion to that of the formation of ultrasound beams having a focus at a point P1, ultrasound pulse beams having focuses at points P2 and P3, respectively, which travel toward a scan line Z direction, are formed by means of generating by the transmitting circuit 102 transmission pulse signals delayed in accordance with delay patterns corresponding to arcs R2 and R3, respectively.
Further, it is possible to form not only a scan line extending to a direction (Z direction) perpendicular to the arrangement direction X of the piezoelectric elements 12.sub.-- 1, 12.sub.-- 2, . . . , 12.sub.-- 64 but also a scan line extending to a direction (Z' direction) oblique with respect to the arrangement direction X. Ultrasound pulse beams having focus at a point P4, which travel toward a scan line Z' direction, are formed through an adjustment of delay patterns for transmission pulse signals so that the ultrasonic waves emitted from the piezoelectric elements 12.sub.-- 1, 12.sub.-- 2, . . . , 12.sub.-- 64 arrive simultaneously on an arc R4 described with the point P4 in the center.
This is the similar as to the matter of receiving. For example, the ultrasonic waves reflecting from the point P1 travels toward the piezoelectric elements 12.sub.-- 1, 12.sub.-- 2, . . . , 12.sub.-- 64 with dispersion and arrive simultaneously on the arc R1. Here, the received signals involved in the ultrasonic waves reflecting from the point P1, which are derived from, for example, the piezoelectric elements 12.sub.-- 31 and 12.sub.-- 32 of the center, are delayed until the ultrasonic waves reflecting from the point P1 are received by the piezoelectric elements 12.sub.-- 1 and 12.sub.-- 64 of the both ends. In this manner, the respective received signals are delayed through a delay pattern corresponding to the arc R1 and the delayed received signals are added, thereby forming at the receiving end the equivalent ultrasound beams having a focus at a point P1 and extending to the scan line Z direction.
In a similar fashion to that of the formation of ultrasound beams having a focus at a point P1 at the receiving end, ultrasound beams having focuses at points P2 and P3, respectively, which extend to the scan line Z direction, are formed at the receiving end by means of delaying the respective received signals in accordance with delay patterns corresponding to arcs R2 and R3, respectively.
Further, ultrasound beams having focus at a point P4, which extend to the scan line Z' direction, are formed at the receiving end by means of delaying the respective received signals in accordance with a delay pattern corresponding to an arc R4.
Here, the ultrasonic waves transmitted from the piezoelectric elements 12.sub.-- 1, 12.sub.-- 2, . . . , 12.sub.-- 64 toward the scan line Z direction first arrive at a shallow point P3 within the subject, then at a point P2 and finally at a point P1. Consequently, the ultrasonic waves reflecting from the point P3 reach the elements 12 earlier than the ultrasonic waves reflecting from the point P2. Likewise, the ultrasonic waves reflecting from the point P2 reach the elements 12 earlier than the ultrasonic waves reflecting from the point P1.
Hence, this aspect is utilized for a control of delay patterns in such a way that a delay pattern for the respective received signals derived through the piezoelectric elements 12.sub.-- 1, 12.sub.-- 2, . . . , 12.sub.-- 64 is adjusted, in timing of receipt of the ultrasonic waves reflecting from the point P3, to provide a delay pattern corresponding to the arc R1; in timing of receipt of the ultrasonic waves reflecting from the point P2, to provide a delay pattern corresponding to the arc R2; and in timing of receipt of the ultrasonic waves reflecting from the point P1, to provide a delay pattern corresponding to the arc R3. In this manner, it is possible to implement a so-called receiving dynamic focus in which a focal point at the receiving end is sequentially shifted, as P1.fwdarw.P2.fwdarw.P3, extending to the scan line Z direction.
FIG. 25 is an illustration showing a pattern of weighting (amplification factor of each of the receiving amplifiers 104.sub.-- 1, 104.sub.-- 2, . . . , 104.sub.-- 64) for the respective received signals derived through the elements 12.sub.-- 1, 12.sub.-- 2, . . . , 12.sub.-- 64.
It is assumed that the center of a group of the elements (receiving aperture) for use in receiving is given by X=0. As a function representative of a pattern of weighting, generally, Gaussian function, which is expressed by formula (1), is adopted. EQU g(x)=exp{-.alpha..sup.z (X/XO).sup.2 } (1)
where
.alpha.: weighting factor, and PA1 XO: coordinates of end of receiving aperture. PA1 transmitting and receiving means, having a plurality of piezoelectric transducers are arranged in a predetermined arrangement direction, for sequentially transmitting ultrasound beams along a plurality of scan lines from the piezoelectric transducers into a subject and for sequentially receiving ultrasonic waves along a plurality of scan lines with the piezoelectric transducers; and PA1 display means for displaying a tomographic image of the subject on the basis of received signals generated from said transmitting and receiving means, PA1 wherein said transmitting and receiving means are arranged to transmit and receive ultrasonic waves along a plurality of scan lines which are sequentially deflected as a sector in the arrangement direction and pass through a first predetermined point within the subject apart from said piezoelectric transducers, and performs transmitting and/or receiving of ultrasonic waves using a larger number of said piezoelectric transducers for transmitting and receiving of ultrasonic waves along the scan lines nearer a central part of the sector configuration. PA1 transmitting and receiving means, having a plurality of piezoelectric transducers are arranged in a predetermined arrangement direction, for sequentially transmitting ultrasound beams along a plurality of scan lines from the piezoelectric transducers into a subject and for sequentially receiving ultrasonic waves along a plurality of scan lines with the piezoelectric transducers; and PA1 display means for displaying a tomographic image of the subject on the basis of received signals generated from said transmitting and receiving means, PA1 wherein said transmitting and receiving means are arranged to transmit ultrasonic waves along a plurality of scan lines which are sequentially deflected as a sector in the arrangement direction and pass through a second predetermined point within the subject apart from said piezoelectric transducers, and receive ultrasonic waves along a plurality of scan lines which are sequentially deflected as a sector in the arrangement direction and pass through a third predetermined point within the subject, the third predetermined point being set up to a place deeper than the second predetermined point. PA1 transmitting and receiving means, having a plurality of piezoelectric transducers are arranged in a predetermined arrangement direction, for sequentially transmitting ultrasound beams along a plurality of scan lines from the piezoelectric transducers into a subject and for sequentially receiving ultrasonic waves along a plurality of scan lines with the piezoelectric transducers; and PA1 display means for displaying a tomographic image of the subject on the basis of received signals generated from said transmitting and receiving means, PA1 wherein said transmitting and receiving means are arranged to transmit ultrasonic waves along a plurality of scan lines which are sequentially deflected as a sector in the arrangement direction and pass through a fourth predetermined point on said piezoelectric transducers, and receive ultrasonic waves along a plurality of scan lines which are sequentially deflected as a sector in the arrangement direction and pass through a fifth predetermined point within the subject apart from said piezoelectric transducers.
The weighting factor.alpha. serves to determine a ratio of gain of the received signal derived through an element located away from the center (X=0) of the aperture.
It is known that the above-mentioned weighting of the received signals may reduce a side-lobe-level of the received ultrasound beams, thereby enhancing resolution.
Incidentally, while Gaussian function is shown in formula (1) as the weighting function, it is noted that the weighting function is not always Gaussian function. It is known that the use of, for example, a trapezium-like shaped weighting function, which approximates to Gaussian function, may also bring the substantially same result.
FIG. 26 is an illustration showing an example in which a size D (the number of elements used for receiving) of the receiving aperture is varied in a state that weighting is fixed.
It is also known that a receiving is performed, as shown in FIG. 26, temporarily using a part of the arranged elements for the purpose of, for example, a control of the intensity and resolution of the received signals at a shallow point and a deep point within the subject, but not using all of the arranged 64 pieces of elements.
This is similar to the matter of a transmission. It is known that a transmission is performed, temporarily using a part of the arranged elements for the purpose of, for example, a control of the intensity of the ultrasonic waves at a shallow point and a deep point within the subject, and a beam width of the transmitting ultrasound beam. There is also known such a technique of weighting that in transmission, in a similar fashion to that of the weighting shown in FIG. 26, the number of pulses of the transmission pulse signal, a pulse voltage and the like are controlled to transmit the ultrasonic waves, which are mutually different in an intensity, from the respective elements in the transmission aperture (a group of elements for use in transmission).
Next, taking account of the various techniques as to the ultrasonic diagnostic system as mentioned above, there will be described the conventional electronic sector scan type of ultrasonic diagnostic apparatus, which is used, for example, for observation of the heart, and the problems involved in such an apparatus.
The sector scan implies such a scanning scheme that as explained referring to FIG. 24, a scan line extending to a direction oblique with respect to the arrangement direction of the elements is formed and sequentially varied in the direction of the scan line so as to spread in a sector configuration in its entirety. The use of such a sector scan serves to form a sector shaped tomographic image 110 as illustrated in the screen of the CRT display 107 in FIG. 23.
FIG. 27 is a typical illustration showing the state of transmitting and receiving of the ultrasonic waves through adjacent ribs toward the heart using the conventional electronic sector scheme of ultrasonic diagnostic apparatus.
According to the conventional typical electronic sector scan type of ultrasonic diagnostic apparatus, a sector shaped tomographic image is formed in such a manner that as seen from FIG. 27, an amount of delay of each of the elements 12 applied to a body surface 11 at the time of transmitting and receiving is controlled so that the ultrasound beams are deflected right and left with the center 1 of the elements 12 in the center. In this manner, in order to form the tomographic image of the heart, a scan is performed, placing a group of elements between rib 10-to-rib 10 each being of 10 mm in an adult. Consequently, as to the aperture with respect to the scan direction (an arrangement direction of the elements, or the right and left direction in FIG. 27), there are two conflicting requirements, one of which is involved in a requirement in which the aperture is formed in size as smaller as possible in view of the fact that the scan is performed through adjacent ribs, another concerns a requirement in which the aperture is formed in size as large as possible to obtain a penetration. As common grounds, usually, the aperture with respect to the scan direction is set up with a size of the order of 10 mm-20 mm. Hence, the scan lines (areas 20) of the edge portions of the sector configuration, where the ultrasonic waves are larger in the deflection angle, involves such problems that the ultrasonic waves are obstructed by the ribs 10 and as a result an observer cannot see portions deeper than the ribs 10, and further multiple reflection echoes due to reflection from the ribs appear and as a result overall image is deteriorated.
FIG. 28 is a typical illustration showing a technique of removing or reducing a bad influence of the ribs, which technique is proposed in, for example, Japanese Utility Model Laid Open Gazette No. 114019/1990.
According to the proposal as noted above, arranging the piezoelectric transducers 12 as a concave sets up the center of curvature (an intersection of scan line-to-line) 2 of the transmitting and receiving wave surface within the human body, and a so-called linear scan is carried out through a ultrasonic propagation medium 14 within a water sack 15, thereby implementing the sector scan in which the center of curvature 2 is placed in the center independent of the ribs 10. However, according to this scheme, the ultrasonic waves are reflected on a boundary between a body surface 11 and the ultrasonic propagation medium 14 and as a result multiple reflection echoes emanate, and thus it is difficult to obtain a good image.
Japanese Patent Publication No. 12971/1992 proposes a method of improving the problem as to the above-mentioned multiple reflection.
FIGS. 29 and 30 are each a typical illustration useful for understanding the proposal disclosed in Japanese Patent Publication No. 12971/1992.
According to the system of this proposal, the scan is performed in such a manner that the ultrasound beams pass through substantially a fixed point (center 2) within the human body, using fixed or semi-fixed delay elements 17 each provided for the associated element, so as to obtain the equivalence to such a situation that as in the related art shown in FIG. 28, the elements 12 are arranged on the arc of a radius R from the center 2 of the sector scan within the human body. In this case, the focal position of the ultrasound beams is equal to the position of the center 2 within the human body. On the other hand, it is necessary for observation of the heart to provide a focus position of the order of 80-100 mm, and thus variable delay elements 19 for use in alteration of the focal position are used in combination. With respect to the aperture width, as shown in FIG. 30, there is provided the same effective aperture (L1'=L1.multidot.COS (.theta.)=L0) on each scan line. The number of elements forming an aperture on each scan line is about 7-9 pieces.
However, this system is poor in the number of transmitting and receiving elements and thus poor in resolution and penetration. In case of the general electronic sector type, the aperture is of about 20 mm, and is comprised of 60 pieces of element. According to such general electronic sector type, 50 pieces of element are used for transmission, and about overall elements are used for receiving. Consequently, according to the conventional systems as proposed in FIGS. 29 and 30, the aperture area (the number of transmitting and receiving elements) is too little to form the converged ultrasound beam, and thus it is apparent that resolution is reduced and penetration is not attained.