Abstract:
An ultrasonic imaging apparatus and method of forming an image of an acoustic image of an object employing the aperture synthesis technique, in which at least one partial aperture for accommodating a predetermined number of successive transducers is established and applied to a transducer array to select the predetermined number of successive transducers accommodated in the partial aperture so that the selected transducers serve to transmit ultrasonic acoustic signals to an object and receive signals reflected from the object, and the at least one partial aperture is specifically moved along the arrayed transducers for similar signal transmission to and/or signal reception from the object, whereby the apparatus and the method do not suffer substantial noise caused during changeover between transmitting and receiving operation modes and/or enjoy and enlarged aperture for signal transmission or enlarged aperture for signal reception with a suppressed decrease of the frame rate.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an ultrasonic imaging apparatus and a method of forming an ultrasonic image of an object, the apparatus and method being based on the aperture synthesis technique and suitable for a high resolution imaging. 
     The conventional aperture synthesis technique is intended to achieve an enlarged receiving aperture frame, in which a similar ultrasonic beam is transmitted several times and the received signals are combined by switching receiving partial apertures so as to obtain a total-aperture reception signal, as disclosed in JP-A No. 58-132677 (corresponding to U.S. patent application Ser. No. 769,805 filed on Aug. 27, 1985 which is a continuation application of U.S. patent application Ser. No. 463,652 filed on Feb. 3, 1983 now abandoned). The above technique, however, does not deal at all with switching noise caused due to the fact that different transducers are selected for ultrasonic beam transmission and reception. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide an ultrasonic imaging apparatus and a method of forming an acoustic image of an object, based on the aperture synthesis technique, which is virtually unsusceptible to noise arising at the switching of transmission and reception modes. 
     Another object of this invention is to provide an ultrasonic imaging apparatus and a method of forming an acoustic image of an object, based on the aperture synthesis technique, which is intended to have an enlarged transmitting aperture. 
     A further object of this invention is to provide an ultrasonic imaging apparatus and a method of forming an acoustic image of an object, based on the aperture synthesis technique, which is intended to have an enlarged receiving aperture, suppressing a decrease of the frame rate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram illustrating the operation of aperture synthesis by an ultrasonic imaging apparatus embodying the present invention. 
     FIG. 2 is a block diagram showing an example of the structure of the embodiment illustrated in FIG. 1. 
     FIGS. 3a and 3b are diagrams explaining examples of the structure of the beam former used in the apparatus shown in FIG. 2. 
     FIGS. 4a and 4b are timing charts used to explain the transmitting and receiving operations of the apparatus shown in FIG. 2. 
     FIGS. 5a, 5b and 5c are graphs comparing directivity of embodiments of this invention. 
     FIG. 6 is a diagram showing the operation of aperture synthesis by the ultrasonic imaging apparatus according to another embodiment of this invention. 
     FIG. 7 is a block diagram showing an example of the structure of the embodiment of FIG. 6. 
     FIG. 8 is a diagram showing aperture synthesis and display range of ultrasonic images. 
     FIG. 9 is a timing chart used to explain the transmitting and receiving operations of the apparatus shown in FIG. 7. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 showing an important aspect of the operation of an ultrasonic imaging apparatus according to an embodiment of this invention, symbol N denotes an array of transducers, T 0  and T 1  denote transmitting partial apertures accommodating a predetermined number m of transducers, R 0  and R 1  denote receiving partial apertures m accommodating transducers, T&#39; denotes a synthesized transmitting aperture, R&#39; denote a synthesized receiving aperture, n denotes an overlap between the partial apertures, and F denotes a convergent point of an ultrasonic beam in an object. 
     In an aperture synthesizing operation, the transmitting and receiving partial apertures T 0  and R 0  are applied to the whole array of transducers to select a predetermined number of (m) transducers, and an ultrasonic beam is emitted from the selected transducers to an object (not shown) and converged to a convergent point F for transmission and reception, as shown in the figure. Subsequently, transducers accommodated in the transmitting and receiving partial apertures T 1  and R 1  (T 1  =T 0 , R 1  =R 0 ), which are the results of movement of the above-mentioned partial apertures T 0  and R 0  by a distance identical with (m-n) transducers (m&gt;n≧0) are selected. The ultrasonic beam is transmitted and received in the same way as above, electric signals received for the receiving partial apertures R 0  and R 1  are phased (phase adjustment for matching the phases of electric signals received from the transducers) and added to each other with the phase information in the received signals retained, and a reception signal for the synthesized receiving aperture R&#39; is produced. The produced synthesized reception signal is equivalent to an output signal obtained by phasing and adding of electric reception signals resulting from conversion of acoustic signals received with aperture R&#39;=R 0  +R 1  -n, the received acoustic signals being produced responsive to acoustic signals transmitted with aperture T&#39;=T 0  +T 1  -n. Or otherwise, the produced synthesized reception signal is at least equivalent to an output signal obtained by phasing and adding electric reception signals resulting from conversion of acoustic signals received with aperture R 0 , the received acoustic signal being responsive to acoustic signals transmitted with aperture T 0 . 
     FIG. 2 shows in block diagram an embodiment of this invention, in which an ultrasonic imaging apparatus may include an array of transducers 1, a transducer selecting circuit 2, transmitting/receiving operation mode switch 3, a beam former 4 for phasing and addition of signals, a compression circuit 5, an envelope detection circuit 6, a display device 7, a driver 8 for transmission, and a control circuit 9. 
     In the above arrangement, m transducers accommodated in the transmitting partial aperture T 0  or T 1  and partial receiving aperture R 0  or R 1 , as shown in FIG. 1, are selected among the transducers in the array 1 by the selecting circuit 2 under the control of the control circuit 9. For signal transmission, the driver 8 supplies a drive pulse signals including pulses, which are controlled in the amplitude and phase by the control circuit 9, to the transducers accommodated in the established transmitting partial aperture T 0  and selected among the transducer array 1 via the switch 3 and selecting circuit 2. For signal reception, the beam former 4 is controlled by the control circuit 9 so that the received signals are fed through the selecting circuit 2 and switch 3, and phased for the receiving partial aperture R , and the result is memorized. Similar operations are repeated for the number of times necessary for a signal synthesis, and an electric output signal for a synthesized aperture produced by phasing and addition of 2m electric signals from (2m-n) transducers accommodated in apertures R 0  and R 1  by the beam former 4, is processed by the compression circuit 5 and detection circuit 6 and delivered to the display device 7. The above operations are repeated, while moving the synthesized transmitting and receiving aperture T&#39;, R&#39;, by a distance identical with one transducer, each time signal transmission and reception has been completed for one synthesized aperture, as shown in FIG. 1, in the azimuthal direction, that is, along the transducer array 1, and an ultrasonic image is displayed on the display device 7. 
     The beam former 4 in FIG. 2 will be described in more detail with reference to FIGS. 3a and 3b, in which reference number 11 denotes a partial beam former for a partial aperture, 12 denotes an adder, 13 denotes a line memory, and 14 denotes an envelope detector. 
     By the arrangement shown in FIG. 3a, the first reception signals Si of m channels (m is a positive integer) for the transmitting and receiving partial apertures T 0  and R 0 , as shown in FIG. 1, are phased and added by the partial beam former 11 for a partial addition, processed by the adder 12, and stored in the line memory 13. Subsequently, next reception signals S i+1  of m channels obtained by switching the transmitting and receiving partial apertures to T 1  and R 1  are phased and added by the partial beam former 11 for a partial addition, thereafter, subjected to coherent-addition by the adder 12 to the reception signals Si which have been stored previously in the line memory 13 and outputted via the line memory 13. 
     The signal phasing and addition as described above enables simplification of the structures of the beam former and the selecting circuit necessary accomplishing a enlarged aperture. 
     In case an object to be imaged tends to move suddenly, the received signals may have a diminished amplitude due to phasic cancellation among the signals. In such a case, the outputs of the partial beam former 11 are first fe to the envelope detector 14 and the outputs of the detector 14 are added for coherent addition, thereby solving the above-mentioned problem. 
     FIGS. 4a and 4b show in timing chart effects in one aspect of this invention, in which t T  and t R  represent time spent in selecting the partial apertures for transmission and reception, respectively. In FIG. 4a, the transmitting and receiving partial apertures accommodate the same transducers in the same range actuatable at one time, so that the transmitting partial aperture T 0  and the receiving partial aperture R are selected simultaneously (t T=t   R ) according to one aspect of this invention. Accordingly, when signal transmission takes place at t x , signal reception is possible immediately following the transmission. However, if the transmitting and receiving partial apertures accommodate one or more transducers in different ranges actuatable at one time, e.g., T for signal transmission and R for signal reception, selection of transducers accommodated in the receiving partial aperture will commence after signal transmission, and therefore signal reception will take place with a delay of t R  following the time point t x , as shown in FIG. 4b. On this account, the depth r o  on the display screen at which reception of can commence becomes unfavorably r o  =t R  ·v/2, where v denotes the sound velocity. As an advantage of this invention, as will be appreciated from FIG. 4a, the receiving partial apertures can be synthesized without the influence of noise which is created in the aperture movement at the switching of the transmission/reception operation modes. In contrast, the case of FIG. 4b obviously results in the creation of noise due to the aperture movement at transmission, which adversely affects reception. 
     FIGS. 5a-5c show the results of measurement of directivity of the ultrasonic beams, two of which are formed by the use of the foregoing aperture synthesis techniques. The conditions of measurement are as follows (also see FIG. 1 for the definition of apertures). The transducer width, i.e., the transducer-to-transducer distance is set for 1 mm; the transmitting partial apertures T 0  and T 1  are equal to the receiving partial apertures R 0  and R 1 , and are set for 20 mm; the convergent point of the ultrasonic beam is set for 70 mm measured on the center line of the overlap section; the ultrasonic beam frequency is set for 3.5 MHz. 
     FIG. 5a shows a beam directivity resulting from transmission and reception with 1.5 T 0  and 1.5 R 0  without synthesis of aperture, in which the beam widths at -20 dB and at -10 dB are 1.7 mm and 12 mm, respectively. FIG. 5b shows a beam directivity achieved by aperture synthesis with a 50% overlap (n), in which the beam widths at -20 dB and at -10 dB are 1.6 mm and 1.2 mm, respectively, these values being comparable with the case of FIG. 5a. FIG. 5c shows a beam directivity achieved by aperture synthesis with a 40% overlap (n), in which the beam widths at -20 dB and at -10 dB are 1.9 mm and 1.3 mm, respectively, these values being also comparable with or slightly inferior in the directivity to the case without aperture synthesis shown in FIG. 5a. Thus, aperture synthesis with less than 40% overlap may result in an extremely inferior directivity, and an overlap of 40% or more is desired. 
     Although the above description has been mainly devoted to the cases of synthesizing the total-aperture signal by moving successively two transmitting and receiving partial apertures (e.g., T 0  and R 0 , and T 1  and R 1 ), such that the two apertures (T 0  and T 1  or R 0  and R 1 ) in both sets accommodate the same number of transducers and have an overlap therebetween, the present invention is not limited to this, but is of course applicable to the case of synthesizing the total-aperture signal by moving successively more than two transmitting and receiving partial apertures such that these different apertures have an overlap between adjacent apertures. 
     The present invention is also effective for the case where the transmitting and receiving synthesized apertures have different values, and is of course applicable to the cases where a plurality of ultrasonic beam convergent points are formed for transmission, and the receiving synthesized aperture is variable. 
     The embodiments described above is particularly effective for the enlargement of aperture in imaging a portion at a relatively small depth in the direction of ultrasonic beam radiation. The following embodiments with reference to FIGS. 6 and 7 are particularly effective for the enlargement of aperture in imaging a portion at a relatively large depth in the direction of ultrasonic beam radiation. 
     In FIG. 6, an aperture is synthesized from five partial apertures with the central partial aperture serving as both transmitting and receiving partial apertures T 2  and R 2 . Symbol N&#39; denotes an array of transducers, T 2  denotes a transmitting aperture, R 2 , R 3 , R 4 , R 5  and R 6  denote receiving partial apertures, and F&#39; denotes a convergent point of the ultrasonic beam. 
     FIG. 7 shows an ultrasonic imaging apparatus embodying the present invention, in which the apparatus may include an array of transducers 31, first and second transducer selecting circuits 32 and 33, an adder 34 for adding those signals which receive same delay processing, a transmission/reception operation mode switch 35, a beam former 36 for phasing and addition of signals, a compression circuit 37, an envelop detection circuit 38, a display device 39, a drive circuit 40 for signal transmission, and a transmission/reception control circuit 41. 
     The following describes the formation of an ultrasonic beam by synthesizing apertures through the selection of transducers by movement of partial apertures as shown in FIG. 6 in the apparatus shown in FIG. 7. For signal transmission, the transmission/reception control circuit 41 sets the mode switch 35 for the transmission mode, and all switches in the adder 34 are turned off. Under the control of the control circuit 41, the second transducer selecting circuit 33 selects a predetermined number of transducers accommodated in the transmitting partial aperture T 2  among the whole transducer array 31. The selected transducers are activated by the operations of the second transducer selecting circuit 33, adder 34, transmission/reception mode switch 35 and transmission drive circuit 40. For the initial signal reception, the control circuit 41 switches the transmission/reception mode switch 35 to the reception mode, the receiving partial aperture R 2  which accommodates the same transducers as the transmitting partial aperture T 2  is applied to the transducer array 31, and the received signals are fed to the beam former 36 through the route opposite to transmission. The beam former 36 has the same structure as that of FIGS. 3a or 3b, and the phased signals are stored in the line memory 13. 
     Subsequently, transmission takes place in the same way as above. For the second reception, the first and second transducer selecting circuits 32 and 33 select transducers accommodated in the receiving partial apertures R 3  and R 4  same in number as those accommodated in T 2  and R 2 . At this time, the receiving partial apertures R 3  and R 4  are located at symmetrical positions with respect to the beam convergent point F, and the signal delay operations for both apertures are the same. The control circuit 41 turns on the switches in the adder 34 so as to deal with two signals with a single processing means by making use of the symmetry of the two signals, whereby the signals received for the two receiving partial apertures R 3  and R 4  can be processed at one time with the single processing means. The reception signals are phased by the beam former 36 and added to the signals which have been received for the receiving partial aperture R 2  and stored in the memory with phase information being retained, and the resulting signal is stored in the line memory 13. These operations are repeated for the partial apertures R 5  and R 6  that are the result of movement from R 3  and R 4  by a distance identical with the predetermined number of transducers and a reception signal for the total receiving aperture that is the sum of R 2 , R 3 , R 4 , R 5  and R 6  is synthesized. The signal from the beam former 36 is processed by the compression circuit 37 and detection circuit 38, and delivered to the display device 39. By repeating the above operations, while moving the transmitting and receiving apertures successively, an ultrasonic image is displayed on the display device 9. 
     By the above-described phasing and addition of received signals, even when signal synthesis is effected n times (n&gt;0), the receiving aperture can be enlarged to (2n+1) times the receiving partial aperture without a significant sacrifice of the frame rate. Thus, the above-described embodiment is particularly effective for forming an ultrasonic beam for a portion at a relatively large depth which generally requires a large aperture. By the arrangement shown in FIG. 7, aperture synthesis shown in FIG. 1 is also applicable. 
     Accordingly, for formation of an ultrasonic image of an object to be displayed on the display device, ultrasonic images of those parts of an object which are to be displayed in a range of a relatively small depth L 1  should be formed by the embodiments described with reference to FIG. 1, while those parts of the object which are to be displayed in a range of a relatively large depth L 2  should be formed by the embodiments described with reference to FIGS. 6 and 7, thereby to complete the total image. 
     FIG. 9 shows in timing chart the operation of FIG. 8. On the chart, symbols T 0 , T 1  and R 0  -R 6  represents ultrasonic imaging according to the illustration in the partial apertures shown in FIGS. 1 and 6, Trig indicates the timing of signal transmission, represents a time length required to form a raster of an ultrasonic image as in FIG. 8, t 1  represents a time length corresponding to a range of depth L 1 , and t 2  represents a time length corresponding to a range of depth L 2 . 
     The number of partial apertures described in the above embodiments is solely for the explanatory purpose, and the aperture may of course be divided into more number of partial apertures depending on the use, frame rate required and circuit complexity. 
     Although the above embodiments relate to a fixed convergent point system, the present invention is not limited to this, but the invention is of course applicable to the cases where transmission takes place with more than one beam convergent point and the total receiving aperture is variable. The present invention is also effective for the case where the composed receiving aperture is varied depending on the depth of the convergent point, with the intention of enhanced resolution of the ultrasonic image.