Ultrasonic Interrogating System and Exciter-Detector Circuit Therefor

This invention relates to real-time ultrasonic interrogating systems, and, more particularly, to circuitry for exciting a transducer and for receiving echos in an ultrasonic interrogating system which is for example useful in the biological investigation arts.

A system environment in which the circuitry of the present invention has found significant utility is described in my copending patent application Ser. No. 389,958, for "Ultrasonic Cross-Sectional Imaging System", filed Aug. 20, 1973, and assigned to a common assignee herewith, now Pat. No. 3,881,466. The ultrasonic imaging system disclosed therein is characterized by the use of an array of relatively narrow transducers which are excited in groups according to a predetermined sequence in order to achieve improved resolution by an overlapping ultrasonic scanning pattern.

The most advanced known prior art real-time ultrasonic imaging system for carrying out biological investigations is disclosed in an article by Bom et al, published in the November, 1971, issue of "Bio-Medical Engineering", pages 500-503, 508, and entitled "Ultrasonic Viewer for Cross-Sectional Analyses of Moving Cardiac Structures". The Bom et al apparatus utilizes a linear array of twenty relatively large transducers excited sequentially on an individual basis. Twenty power transistors and twenty preamplifiers are necessary to achieve transducer excitation and echo detection, and the electronic circuits and transducers are coupled by twenty separate coaxial cables. Each transducer is connected in parallel with a tuning inductor, which must be provided to damp unwanted low frequency cross-mode vibrations.

There are two basic disadvantages to the prior art apparatus as exemplified by the Bom et al system. First, the inordinate inherent clumsiness and unreliability are a serious drawback. Second, the necessity for providing twenty complete discrete electronic channels exhibiting high performance characteristics inevitably results in undesirably high costs. The present invention is directed to overcoming both these disadvantages while further affording superior performance.

It is, therefore, a broad object of my invention to provide improved circuitry for use in a real-time ultrasonic imaging system.

It is another broad object of my invention to provide such improved circuitry in a system especially adapted for use in the medical arts.

It is a more specific object of my invention to provide simplified circuitry which nonetheless exhibits superior performance characteristics in an ultrasonic interrogating system employing a transducer array.

It is also an object of my invention to provide such circuitry in which the necessity for employing a plurality of inductive components and a plurality of echo preamplifiers is obviated thereby affording much lower cost and significantly reduced physical size.

Briefly, these and other objects are achieved as follows. According to one aspect of the invention, a single preamplifier transmits to a display echoes returning to a plurality of ultrasonic transducers. The transducers are selectively excited electrically in a predetermined sequence to produce ultrasonic emission. The respective excited transducers are selectively coupled to the input of the preamplifier in the predetermined sequence in synchronism with the display to present the echoes of the ultrasonic emission on the display. Preferably, the transducers comprise a fixed segmented transducer array having a plurality of discrete adjacent transducer elements, and the preamplifier is packaged with the transducer array.

Another aspect of the invention is a fixed segmented transducer array having a plurality of discrete adjacent transducer elements and a plurality of switches at least equal in number to the transducer elements packaged with the transducer array. The transducer elements are selectively excited electrically in a predetermined sequence to produce ultrasonic emission. Each transducer element is coupled through a respective switch to one end of a common transmission line. The other end of the common transmission line is connected to a display. The switches are selectively closed in the predetermined sequence to transmit to the display via the transmission line echoes of the ultrasonic emission, and the display is synchronized to the switch closures to present the echoes on the display.

Still another aspect of the invention is a plurality of ultrasonic transducers selectively excited to ultrasonic emission in a predetermined sequence, and a single inductive reactance connected effectively in parallel with the respective excited transducers as to echoes of the ultrasonic emission. A plurality of switching devices are each connected in series with a unique one of the transducers to form a plurality of switching device/transducer sets. The switching device/transducer sets are connected in parallel with one another. The single inductive reactance in parallel with the switching device/transducer sets is coupled to the input of a receiver, and the switching devices are selectively closed in the predetermined sequence to transmit to the receiver echoes of the ultrasonic emission. Preferably, the transducers comprise a fixed segmented transducer array having a plurality of transducer elements, and the inductor is packaged with the transducer array.

According to another aspect of the invention applicable either to an ultrasonic tranducer array or to a single ultrasonic transducer, the internal capacitance of the transducer is slowly charged to a first potential and then rapidly discharged to a second different potential to excite the transducer to ultrasonic emission. A tuning inductor and the transducer are effectively connected in series as to direct current during the slow charging interval and effectively connected in parallel as to alternating current during the rapid discharging interval. The transducer and tuning inductor in parallel are coupled across the input of a receiver. Preferably, the receiver cyclically displays ultrasonic echoes returned to the transducer during a first period of each cycle without displaying such signals during a second period of each cycle; the first period corresponds to the discharging interval and the second period corresponds to the charging interval.

The subject matter of the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, may best be understood by reference to the following description taken in connection with the accompanying drawings, in which:

FIG. 1 is a simplified partially schematic and partially block diagram illustrating a circuit configuration exemplary of the prior art;

FIG. 2 is a waveform diagram showing a signal observed at a reference point in the circuit of FIG. 1;

FIG. 3 is a partially schematic and partially block diagram of a circuit configuration comprising one preferred embodiment of the invention;

FIGS. 4a and 4b are waveform diagrams showing time related signals observed at two reference points in the circuit of FIG. 3;

FIG. 5 shows a typical transducer array with which the embodiments of the invention may be used;

FIG. 6 is a partially schematic and partially block diagram of a circuit configuration comprising another preferred embodiment of the invention; and

FIG. 7 is a waveform diagram showing time related signals observed at a reference point in the circuit of FIG. 6.

Referring now to FIG. 1, it will be observed that a twenty-transducer system is depicted by showing only channels 1, 2 and 20, with channels 3-19 omitted to avoid cluttering the diagram. Each transducer 1 is energized by applying an "on" pulse from timing and control logic 2 to the base electrode of an appropriate one of the power transistor switches 3. Any echo received applies a mechanical exertion on the transducer 1 which causes a low level voltage signal to be developed across the transducer and parallel tuning inductor 4. This echo signal is amplified to a useful level by the corresponding preamplifier 5.

An electronic switch 6, under control of the timing and control logic 2, passes the selected echo signals amplified by the preamplifier to a raster scanned display 7 which also receives coordinating signals from the timing and control logic 2. Thus, as the transducers 1 are pulsed in a step-by-step sequence, the echo signals may be observed on the display 7 in a correspondingly time-dependent display to provide a two-dimensional dynamic representation of an organ or area under analysis.

It should be noted that conductors 8 are coaxial cables which are used to couple the electronics to the transducer array which might typically be laid over the chest of a subject under examination or treatment. The aforementioned clumsiness and unreliability in such an environment is particularly undesirable.

FIG. 2 illustrates an exemplary waveform observed at the junction A of a transducer 1, tuning inductor 4, and collector electrode of a transistor switch 3. When an "on" pulse is applied to the base electrode of a given transistor switch 3, V is applied as a pulse 9 across the transducer 1 and inductor 4 coupled to the transistor switch. The consequent mechanical deformation of the transducer introduces a shock wave into the biological subject. As the shock wave propagates through the subject, certain targets reflect echo pulses which deform the transducer 1 to generate echo signals 10, 11 and 12. The amplified and processed echo pulses are applied to a Z-axis input of raster scanned display 7 to modulate the intensity along a scan line corresponding to the specific transducer currently active.

Referring now to FIG. 3, one preferred embodiment of the subject invention is depicted incorporated into a system in accordance with the referenced U.S. Pat. No. 3,881,466.

A significant feature in the referenced system is the sequence and combinations by which the transducers 20 are excited. A typical transducer array comprises a fixed, segmented transducer array of N discrete adjacent transducer elements. A counter-selector means simultaneously transmits a pulsed electrical signal to A selected adjacent transducer elements of the array and sequentially pulses selected different groups of A contiguous elements, each of which are longitudinally displaced along the elongate array. Each such group is displaced B transducer elements from the location of the immediately preceding group. In the presently preferred system embodiment, N=64, A=4 and B=1. Thus, transducers X.sub.1, X.sub.2, X.sub.3 and X.sub.4 are pulsed as a group in unison; then transducers X.sub.2, X.sub.3, X.sub.4 and X.sub.5 are pulsed as a group; X.sub.3, X.sub.4, X.sub.5 and X.sub.6 are pulsed as a group, etc. The result is an overlapping ultrasonic scanning pattern. With this sequence and combination, the mechanical power, and consequently the electrical power, required of each transducer is much less than required of the individually pulsed prior art system transducers. Consequently, the necessity for using physically large power switching transistors is eliminated. The elimination of the power transistors and the further elimination, by the present invention, of multiple inductors and preamplifiers results in a compact and relatively simply system which exhibits improved performance characteristics. The timing and control logic 22 for effecting the sequential energization of the transducer groups may be of a number of different types which are well known in the art, such as ring counters, shift registers, or more complex binary digital counters. The particular type of circuit used is primarily a design consideration. For example, one type of circuit which could be used to effect the pulsing of these transducers is a shift register pulsing circuit of the type disclosed in the patent to Wendt et al U.S. Pat. No. 3,373,602 issued Mar. 19, 1968.

Still referring to FIG. 3 and also to FIG. 4a, all transducers 20 will initially capacitively charge to V.sub.cc (which is opposite in polarity from V.sub.ee) through charging resistors 21. Transducers 20 are each connected in series with a respective switching transistor 23 to form a series set. When timing and control logic 22 issues "on" pulses to the base electrodes of the switching transistor 23 in a group of four channels, the switching transistors become conductive and discharge the charge stored on the transducers 20 to V.sub.ee, as shown in FIG. 4a, electrical point B. The leading edge of the step-like voltage change transiently appearing across the transducers produces a sharp unison mechanical deformation which introduces a highly damped short ultrasonic pulse into the biological subject being analyzed. When the "on" signal is removed from the switching transistors 23, the capacitive charging phase is reestablished in preparation for the next subsequent operating cycle. The capacitance of each of the transducers 20 is considerably smaller than that of the transducers 1 of the prior art apparatus, FIG. 1, by virtue of their smaller physical size. Thus, by appropriately selecting the value of charging resistors 21, the transducers 20 will have adequate time to recharge to V.sub.cc before a succeeding "on" pulse again causes discharge.

During the recharging phase of an operating cycle, the mechanical deformation of the transducers will be smoothly relaxed such that all ultrasonic shock waves introduced into the biological subject result only from the discharging phase.

To summarize as depicted in FIG. 4a, the internal capacitance of the respective transducers 20 is cyclically charged and discharged--the respective transducers 20 are first slowly charged through the respective charging resistors 21, to the potential difference between V.sub.cc and ground, and then, as the respective switching transistors 23 are closed by an "on" pulse, such internal capacitance is rapidly discharged to the potential between V.sub.ee and ground. The rapid discharge of the internal capacitance caused by switch closure excites the discharged transducers to produce ultrasonic emission. After the rapid discharge of the transducer internal capacitance, the corresponding switches remain closed during the interval in which echoes of the ultrasonic emission returned to the transducers are to be sensed. Thus, in this, the preferred embodiment of the invention, the transducers are excited to produce ultrasonic emission by the rapid discharge of their internal capacitance upon switch closure. The terms "charge" and "discharge" as used herein are interchangeable, depending upon what is arbitrarily taken as the starting point of the cycle. Starting from V.sub.ee, the internal capacitance of the transducer is slowly charged to V.sub.cc and rapidly discharged to V.sub.ee. Starting from V.sub.cc, the internal capacitance of the transducer is rapidly charged to V.sub.ee and slowly discharged to V.sub.cc.

It will be noted that as long as an "on" pulse holds a switching transistor 23 in the conducting state, the terminal of the transducer 20 connected to the collector electrode thereof (electrical point B) is effectively clamped to a-c ground. Thus, as shown in FIG. 4b, returning echoes which cause further mechanical strain in the transducer 20 may readily be sensed at the other transducer terminal (electrical point C) to which one end of an inductive reactance, shown as a single tuning inductor 24, is connected. This electrical point is common to all channels such that only a single preamplifier 25 is needed. The other end of the tuning inductor 24 is connected to ground such that, insofar as echoes are concerned, it is disposed in parallel with the group of those transducers 20 which have switching transistors in the conducting state. Thus tuning inductor 26 is shared by the different groups of transducers as they become operative, i.e., as the switching transistors for such transducers close. Further, the switching transistors for the transducers in the preferred embodiment perform three functions--namely, selecting the transducers to be excited to ultrasonic emission, initiating excitation of the selected transducers to ultrasonic emission, and coupling to the preamplifiers only the echoes returned to the selected transducers.

It should be noted that although inductor 24 is connected effectively in parallel with the group of selected transducers (i.e., the transducers whose switching transistors are in the conducting state) as to alternating current (i.e., tht echoes returned to the transducers), inductor 24 is connected effectively in series with the non-selected transducers 20 (i.e., those having switching transistors in the non-conducting state) as to direct current (i.e., the charging and discharging current furnished by V.sub.ee and V.sub.cc). Consequently, inductor 24 does not short-circuit transducers 20, which would inhibit their charging and discharging in the described manner.

Echoes amplified by the preamplifier during conduction of transistors 23 are coupled by a transmission line designated 27 in FIG. 3, to the raster scanned display 26. These echoes are presented on the display 26 which receives control signals for coordinating scanning from timing and control logic 22.

Referring now to FIG. 5, there is shown a typical transducer array which can be used in conjunction with the circuit of FIG. 3 and the circuit of FIG. 6 to form the various transducers X1 through X64. This array is made in accordance with the array disclosed in U.S. Pat. No. 3,881,466 and comprises a central layer formed of a single piece of suitable transducer material, such as piezoelectric crystal 31. A common conductive electode 32 covers the lower surface of the transducer 31 and the upper surface is covered with adjacent individual conductive strips 35. Each of the strips 35 are separated or are insulated from one another, so that they are electrically independent of one another. This creates a a plurality of electrically independent transducer elements in the array of FIG. 5 which can be separately excited by applying a direct current potential to selected ones of the strips 35 through appropriate input leads 36.

In FIG. 6, there is shown another embodiment of the invention in which the same reference numbers used in FIG. 3 are used to identify the same or similar components. Essentially, the circuit of FIG. 6 operates in the same manner as described previously in conjunction with the operation of the circuit of FIG. 3. In the embodiment shown in FIG. 6, however, the lower or common electrode 32 of the transducers 20 is connected directly to ground instead of being connected through the inductor 24 to ground as shown in the embodiment of FIG. 3. This is the side of the transducer array which is placed against the patient's body over the organ or area under analysis.

The circuit of FIG. 6 then further differs from the circuit of FIG. 3 in that the tuning inductor 24 is connected in series between the V.sub.ee positive voltage input terminal and a common point D connected to the emitters of all of the PNP switching transistors 23. In the embodiment shown in FIG. 3, the positive voltage supply terminal V.sub.ee is connected directly to the emitters of these transistors. The final difference between the circuit of FIG. 6 and that of FIG. 3 is that the input to the preamplifier 25 in FIG. 6 is taken from point D, which is the junction of the lower end of the inductor 24 with the emitters of the switching transistors 23.

The timing and control logic circuit 22 sequentially renders different groups of the switching transistors 23 conductive in the same manner described previously in conjunction with the description of the circuit of FIG. 3. The advantages of the circuit of FIG. 3, namely, the elimination of the power transistors and the further elimination of multiple inductors and preamplifiers, also are present in the circuit of FIG. 6.

The circuit of FIG. 6, however, also has the additional advantage of being substantially immune from radio frequency interference (RFI), being coupled to the load from the transducers 20 which are "off" or connected to non-conductive switching transistors 23. In the circuit shown in FIG. 3, all of the transducers 20 have one terminal connected in common to the output terminal of the transducer array supplied to the preamplifier 25. As a consequence, if radio frequency interference is capacitively coupled to an "off" transducer, it is possible for that transducer to capacitively couple the interference to the inductor 24. This spurious signal in turn would be coupled from terminal C to the preamplifier 25 in the same manner as the desired echo pulses from the transducers which are "on" or being driven by pulses from the switching transistors 23. While the circuit of FIG. 3 is a much improved real-time ultrasonic imaging system over systems previously available in the prior art, this disadvantage of the circuit could become a problem if the array were operated in a region subject to substantial radio frequency interference. The circuit array of FIG. 6 is not subject to such interference.

In the circuit of FIG. 6, the common point of the transducers, terminal 32, is connected to ground potential. The other terminal, terminal 35, of each of the transducers is coupled through its corresponding switching transistor 23 to the common terminal D with the inductor 24. Thus, when the corresponding transistor 23 is non-conductive, essentially an open circuit is presented to the transducer 20 with respect to the inductor 24. Thus, no pulses from an "off" transducer 20 can be coupled to the inductor 24 to cause erroneous data to be reproduced by the display 26. The inductor 24 still is connected in a series circuit with the conducting switching transistors 23 and their corresponding transducers 20, as it is in the embodiment shown in FIG. 3. The relocation, however, of the inductor 24 to the positive direct current voltage side of the circuit, rather than the grounded side, i.e., from the side of transistors 23 with transducers 20 to the other side of transistors 23, results in the improved performance noted.

The input for the preamplifier 25 in the circuit of FIG. 6 then is obtained from the common terminal D, which is similar to the terminal C of FIG. 3, except that it is on the positive voltage input side of the system rather than on the grounded side. As a consequence, the pulses applied to the preamplifier 25 from terminal D are reversed in polarity from those obtained from terminal C in the circuit of FIG. 3. These reversed polarity input and echo pulses are shown in FIG. 7. A comparison of FIG. 7 with FIG. 4b shows substantially similar waveforms which are 180.degree. out of phase with one another.

With the exception of the differences noted, the circuit of FIG. 6 operates in the same manner as the circuit of FIG. 3, and both circuits exhibit substantial improvements in ultrasonic imaging systems.

The elimination of power transistors, plural inductors, and plural preamplifiers permits packaging the electronics, i.e., switching transistors 23, tuning inductors 24, and preamplifier 25, directly with the transducer array. Consequently, a very much simplified and more facile physical arrangement is realized, while the above-mentioned objects of the invention are admirably met.

It will be appreciated that other means may be employed to excite the transducers, the same switches still selecting the transducers to be excited and coupling only the echoes returned to the selected transducers to the display, and the single inductor and/or preamplifier still being shared by all the transducers. Further, although the invention has been described as interrogating biological subjects, it is applicable to interrogation of other targets such as objects subject to non-destructive testing. Moreover, plural switches could be provided for each transducer array element if phase controlled focusing is utilized.

Having fully described a presently preferred embodiment of my invention in such terms as to enable those skilled in the art to which it pertains to understand and practice it,