DELAY LINE CIRCUIT ARRANGEMENT AND ULTRASONIC IMAGING APPARATUS UTILIZING THE SAME

Abstract: An analogue delay line circuit arrangement (100) is constructed by a plurality of delay line elements (10A: 10B: 10J) and a plurality of compensation amplifiers (20A: 20B: 20I). These delay line elements (10A: 10B: 10J) and compen­ sation amplifiers (20A: 20B: 20I) are series-connected, one by one, so as to construct a cascade connection. The input echo signals are delayed in the delay line elements (10A: 10B: 10J) and simultaneously the amplitudes thereof abrupt­ ly decrease at the high frequency range. The cascade-­ connected amplifiers (10A: 10B; 10J) can compensate for the abrupt decrease of the signal amplitudes. The delay line elements (10A; 10B: 10J) are constructed by passive ele­ ments such as the capacitors (C1; C2) and inductors (L1; L2). The compensation amplifiers (20A: 20B; 20I) are constructed by active elements such as the emitter follower transistor (TR1).

Description: The present invention is directed to a delay line circuit arrangement and an ultrasonic imaging apparatus utilizing the same. In general, a conventional ultrasonic imaging apparatus employs a circuit arrangement of an analogue delay line DL that is mainly constructed by inductors and capacitors so as to steer and also focus echo signals derived from a transducer during the signal reception. For ultrasonic pulse transmission, a digital delay line is normally utilized, which is technically different from the analogue delay line. A typical analogue delay line circuit arrangement is shown in Fig. 1. The delay line DL has a plurality of taps for various delay times, e.g., 0.8 µS. An analogue switch ASW is connected to these selection taps so as to select the desirable delay time. Between an input terminal IN and an output terminal OUT, both the delay line DL and the analogue switch ASW are connected, and matching resistors Rm are also connected. The circuit arrangement of the delay line DL shown in Fig. 1 is arranged in such a manner that a plurality of delay line elements are connected in a cascade connection, each of which has its own delay time of approximately, e.g., 10 ns (0.01 µS). Accordingly, the desirable delay time can be easily obtained by assembling the given number of cascade-connected delay line elements. Fig. 2 shows a circuit diagram of the typical delay line element. The delay line element is constructed by inductors L₁, L₂ and capacitors C₁, C₂. These circuit elements practically function as a filter or the like for the received echo signals. When the analogue delay line circuit having the delay time of 5 µS is designed, a hundred pieces of the delay line elements having, for example, 0.05 µs delay time are connected in a cascade arrangement. Consequently, the total insertion loss and the entire amplitude decrease in the high frequency range are adversely influenced by multiplying those of a single delay line element by 100. It is therefore very difficult to realize such an analogue delay line circuit having a delay time in the order of several microseconds for higher frequencies, e.g., 10 MHZ. For instance, if the maximum delay time is selected to be 5 µs, the effective maximum frequency is limited to approximately 7 MHZ (-3 dB below). Recently, there has been a tendency to introduce an ultrasonic imaging apparatus for utilizing high frequency transducers. Accordingly, the above-described conventional transducer is not sufficient for these current imaging apparatus. The major cause for the RF frequency problems of the delay line circuit, i.e., an abrupt decrease in the high frequency range of the delayed echo signals, is as follows. A magnetic core that can be driven in the higher frequency of, say, 10 MHZ is not available. Precisely speaking, since there exists a size-limitation on the analogue delay line circuit, the effective sizes of the inductors L1 and L2 are restricted to given small values. Practically, a toroidal core having a diameter of 10 mm is employed. It is, of course, possible to realize a desired RF frequency characteristic if the toroidal core having a larger diameter than the above core could be utilized. The other cause for poor performance of the delay line circuit is an insertion loss of the echo signals. As previously described, the longer the desirable delay time is set, the greater the total insertion loss of the echo signals. That is, if the tap Xn of the analogue switch ASW is selected, the insertion loss is maximum (see Fig. 1). For instance, if the delay time is selected to be about 5 µs, the insertion loss amounts to 1 to 2 dB. Accordingly, the amplitude of the delayed echo signal processed in the delay line circuit attenuates in the higher frequency range (see frequency characteristic curve of Fig. 3) due to both the insertion loss and the high frequency response of the delay line circuit. As easily seen from the characteristic data of Fig. 3, there exist the insertion loss "ℓ" and the amplitude difference "a" due to the degradation of the frequency characteristic, when the tap for the shortest delay time Xl and that for the longest delay time Xn are selected. In other words, the magnitudes of the amplitudes of the output echo signals differ from each other in accordance with the selected delay time, resulting in the poor image quality of the tomographic image obtained by the ultrasonic imaging apparatus employing such a conventional delay line circuit arrangement that is operated under a higher frequency. Another drawback of the conventional delay line circuit arrangement is the inherent reflection phenomenon thereof. That is to say, although the echo signals input in the delay line circuit via the selected input tap of the analogue switch ASW flow through the given delay line elements and are derived from the output terminal, a part of the input echo signals flows through the remaining delay line elements that were not selected to the output terminal. As a result, a partial echo signal causes the reflection therein. Then, these reflected echo signals are added to the normally-delayed echo signals having time delays. It should be noted that since degrees of the delay time for both delayed echo signals are different from each other, the reflected echo signals are not superimposed with the normally-delayed echo signals. Accordingly, the artifact appears in the resultant tomographic image. Prior art document US-A-3 781 722 discloses a system for providing a digitally variable delay time for electrical signals which utilizes a connection of multiple delay line sections each of which contributes to the overall system delay time. Each delay line section comprises first and second selectable transmission lines having different electrical lengths and a different magnitude of attenuation per unit length. Finally, prior art document US-A-4 116 229 describes an apparatus for forming real time images of internal portions of the human body from reflections by internal tissue of transmitted short pulses of ultrasonic waves. This apparatus comprises an array of receiving transducers, a master delay line having a plurality of taps thereon and fixed delays between successive taps, and means for selectively providing predetermined small amounts of incremental delay between each of said transducers and any one of said taps on said master delay line. It is an object of the present invention to provide an ultrasonic imaging apparatus wherein the specific delay line circuit arrangement is employed so as to compensate for the abrupt decrease in the delayed echo signals and the insertion loss thereof, and further to prevent the echo signal reflections in the inoperative delay line elements, and to provide a delay line circuit arrangement therefor constructed by not only such passive elements as coils and capacitors, but also by such active elements as transistors and integrated circuits, these active elements being interposed between the succeeding delay line elements. To solve this object the present invention provides an ultrasonic imaging apparatus as stated in claim 1. For the better understanding of these and other objects of the present invention, reference is made to the following detailed description of the invention to be read in conjunction with the following drawings, in which; Fig. 1 shows a circuit arrangement of the conventional analogue delay line; Fig. 2 is a circuit diagram of the delay line element of the delay line shown in Fig. 1; Fig. 3 is a graphic representation for indicating the frequency characteristic and the amplitude of the output signal; Fig. 4 schematically shows a circuit diagram of an analogue delay line circuit arrangement according to one preferred embodiment; Fig. 5 is a circuit diagram of a basic circuit of an amplifier employed in the delay line circuit arrangement shown in Fig. 4; Fig. 6 illustratively shows amplitude/frequency characteristic curves for explaining operations of the amplifier shown in Fig. 5; Fig. 7 is a circuit diagram of an analogue delay line circuit arrangement according to a second preferred embodiment; Fig. 8 is a block diagram of an analogue delay line circuit arrangement according to a third preferred embodiment; Fig. 9 shows a circuit diagram of a practical circuit of the amplifier according to the invention; Fig. 10 is a graphic representation for showing output signal/frequency characteristic curves of the amplifier shown in Fig. 9; Fig. 11 is a schematic diagram of an ultrasonic imaging apparatus according to the invention; and Fig. 12 is a schematic diagram of a delay line unit employed in the apparatus shown in Fig. 11. Reference first is made to Fig. 4 in which there is shown a block diagram of an analogue delay line circuit arrangement 100 according to one preferred embodiment (the analogue switch has been omitted). As easily understood from the circuit, a plurality of delay line elements 10 are cascade-connected, one by one, in series to a plurality of amplifiers 20. The numbers of these delay line elements 10 are determined by calculating the desired delay time, e.g., 8 µs. It is, for instance, assumed that the delay time Td of the single delay line element 10 is approximately 0.8 µs and the maximum delay time Tdm of the delay line circuit arrangement 100 is selected to be about 8 µs. Accordingly, ten elements of the delay line element 10A to 10J and nine circuits of the amplifiers 20A to 20I are alternately connected one by one to constitute a cascade-connected circuit arrangement. A basic circuit of the amplifier 20 is shown in Fig. 5. The amplifier 20 is mainly constructed by an emitter follower transistor TR1 and a base-grounded transistor TR2. A matching resistor Ro is connected between the base electrode of the emitter follower transistor TR1 and the ground. The base electrode of this transistor TR1 functions as an input terminal of the amplifier 20. A coupling circuit of a capacitor C1 and a resistor R1 is connected between the emitter of the emitter follower transistor TR1 and the emitter of the base-grounded transistor TR2. The collector of the latter transistor TR2 is connected via another matching resistor Ro to the positive power supply +V. This collector functions as an output terminal of the amplifier 20. The collector of the transistor TR1 and the base of the transistor TR2 are commonly connected to the negative power supply -V. A voltage gain or this amplifier 20 can be determined by the ratio of the matching resistor Ro to the coupling resistor R1 (i.e., Ro/R1). An amplitude/frequency characteristic of this amplifier 20 can be controlled by the coupling capacitor C1. A typical operation of the delay line circuit arrangement 100 will now be described with reference to Figs 4, 5, and 6. Now assuming that the amplitude/frequency characteristic of each delay line element 10 represents decay in the high frequency range as indicated by symbol "Y1". Accordingly, the output delayed echo signal of each amplifier 20 is emphasized in the high frequency range as denoted by symbol "Y2". Then, the gain of the amplifier 10 is set to compensate for the insertion loss "ℓ" of the delay line circuit arrangement 100. When the received echo signals are supplied to the input terminal 35 of the delay line circuit arrangement 100, they are delayed by the predetermined delay time, i.e., 0.8 µs in the first delay line element 10A. During this delay operation, the amplitudes of the delayed echo signals are subjected to be decreased in the higher frequency band, compared with that of the input echo signals. In addition to the decrease of the amplitudes, the insertion loss "ℓ" of this delay line element 10A is provided due to the passive circuit elements. Thereafter, the delayed echo signals are amplified and also frequency-corrected in the first amplifier 20A. The same delay and correction (compensation) operations are successively repeated in the succeeding delay line elements (10B...10J) and amplifiers (20B...20I). As a result, the resultant amplitude/frequency characteristic over the entire frequency range, e.g., 0 to 10 MHz, represents a substantially flat line "Yo", as shown in Fig. 6. In Fig. 7, another circuit arrangement of an analogue delay line 200 according to the second embodiment is shown. It should be noted that the same reference numerals shown in Fig. 4 will be employed as those for denoting the same circuit elements shown in the following figures. In this circuit 200, two delay line elements 10A and 10B are combined into a single first delay line section 50A, and ten delay line sections 50A to 50J are cascade-connected in combination with nine amplifiers 20A to 20I. In other words, twenty delay line elements 10A through 10T and nine amplifiers 20A to 20I are cascade-connected to constitute the analogue delay line circuit arrangement 200. In this circuit 200, each of these amplifiers 20A to 20I can compensate for the degradation of the amplitude/frequency characteristic in the high frequency range and also the insertion loss that have been caused by each of those delay line sections 50A to 50J. It is, of course, possible to employ more than two delay line elements as the respective delay line sections 50A to 50I, so as to obtain the desirable maximum delay time Tdm. Fig. 8 shows a block diagram of an analogue delay line circuit 300 according to the third embodiment. This circuit arrangement 300 is similar to the above-described circuit arrangement 200. That is to say, the same delay line sections 50A to 50J are cascade-connected in combination with newly-employed controllable amplifiers 30A to 30I. A controller 60 supplies various control signals to the respective controllable amplifiers 30A to 30I, thereby the gains and the frequency characteristic of the respective amplifiers 30A to 30I being separatedly controlled by these control signals in accordance with the given time delay, the desirable compensation for the insertion loss and also the high frequency characteristic. Fig. 9 is a circuit diagram of the amplifier that has been practically designed by the inventor. The first transistor TR1 is of the type of 2SA495 (G) TM-Y (TOSHIBA). The second and third transistor TR2 and TR3 are of the type of 2SC372(G)TM-Y (TOSHIBA). All resistors are in ohms (Ω), and all capacitors are in micro Farad (µF). When the coupling capacitor Cx is selected to be 32 PF (pico Farad), the amplitude/frequency characteristic of this amplifier is plotted in a graphic representation shown in Fig. 10. From these characteristic curves, it can be understood that the high frequency compensation is effected by introducing this coupling capacitor Cx(32 pF). According to recent technology, such an amplifier is commercially available as a single chip integrated circuit. Therefore, such small IC chips can be assembled within the housing of the delay line circuit arrangement without requiring the additional space for these amplifier chips. It is generally known that as the cut off frequency f T of a commercially available transistor is about 100 MHZ in a standard performance, the maximum high frequency of the ultrasonic transducer can be extended to approximately 20 MHZ with a flat curve over the entire frequency range. In Fig. 11, an ultrasonic imaging apparatus 500 is shown in which a delay line unit 80 is employed according to the invention. Exciting pulses derived from the pulser/transmitter 3 (not shown in detail) are successively applied to transducer elements 1-1, 1-2, ..., 1-8 of a transducer 1 so as to produce echo pulses in a predetermined sequence. These echo pulses are transmitted toward an object 2. Thereafter, the echo pulses penetrate into interiors and boundaries of the object 2, where they are reflected as echo pulses. The reflected echo pulses are received by the same transducer 1 to produce echo signals having the medical information as the amplitude variations and the time elapse data. The received echo signals are amplified in a preamplifier unit 70. The amplified echo signals are processed by a delay line unit 80 in the desirable signal processing so as to derive a tomographic image signal. Then, the tomographic image signal is amplified in an amplifier 90 and converted by a digital scan converter 95 into desirable video signals. Finally, tomographic images of the scanned object 2 can be monitored on a TV monitor 98 based upon the video signals having better image qualities, e.g., higher resolution and no artifact. Fig. 12 shows a detailed circuit of the delay line unit 80 in which the delay line circuit arrangement according to the invention is operable in conjunction with voltage-to-current converting means. For the sake of simplicity, only the basic circuit is shown. The first transducer element 1-1 is connected via the first preamplifier 70-1 to the base electrode of the first transistor 82-1. The emitter of the transistor 82-1 is connected through the emitter resistor Re to the negative power supply (-15V). The collector of the transistor 82-1 is connected via the first tap of the first analogue switch 84-1 to the first junction between the matching resistor Rm and the first delay line element 10A, from which the delayed and compensated echo signal is derived. It should be noted that the delay line elements 10A, 10B to 10G are symbolized as a coil in Fig. 12. The second transistor element 1-2 is similarly connected via the second preamplifier 70-2 to the base electrode of the second transistor 82-2 whose emitter is connected through the emitter resistor Re to the negative power supply (-15V). The collector of the transistor 82-2 is connected via the second tap of the second analogue switch 84-2 to the second junction between the first amplifier 20A and the second delay line element 10B. The remaining circuit elements are connected in the same manner as the above-connection. Both the matching resistors Rm are biased by the positive power supply (+5V). A description will now be made of operations of the delay line unit 80 shown in Fig. 12. As previously described, the reflected echo pulses are received and converted by the transducer 1 into the corresponding echo signals. Voltage of the echo signal derived from the first transducer element 1-1 of the transducer 1 is amplified in the first preamplifier 70-1, and then applied to the base electrode of the first transistor 82-1. Since the first tap of the first analogue switch 84-1 has been selected, the following current path is established, i.e., the positive power supply (+5V) to the matching resistor Rm; the first tap of the first analogue switch 84-1; the collector-to-emitter current path of the first transistor 82-1; the emitter resistor Re and the negative power supply (-15V). Accordingly, a first delay current Id1 flows through the above-described current path, whose magnitude corresponds to the amplitude of the voltage of the echo signal derived from the preamplifier 70-1. In fact, as this current Id1 flows through none of the delay line elements 10A to 10H, it is not delayed. On the other hand, voltage of the echo signal across the second transducer element 1-2 is applied to the base electrode of the second transistor 82-2 after being amplified in the second preamplifier 70-2. Upon reception of the base voltage, the second transistor 82-2 conducts so that a second delay current Id2 flows through the second current path as shown in Fig. 12. Similarly, the voltage of the echo signal appearing at the eighth transducer element 1-8 is applied to the base electrode of the eighth transistor 82-8, with the result that an eighth delay current Id8 flows through the eighth current path as indicated in Fig. 12. As a result, the above-described eight delay currents Id1, Id2...Id8 are combined at the matching resistor Rm, so that the summed voltage across the resistor Rm is derived. That is to say, this voltage can be obtained by summing the respective delay currents Id1, Id2...Id8. Such a current summation is so-termed "a current summing method". As previously described in detail, according to the invention, the amplifiers can equivalently compensate for the abrupt decrease in the amplitudes of the delayed echo signals in the high frequency range and also for the insertion loss of the delayed echo signals. Therefore, there is no problem in the image quality of the resultant tomographic images even in the high operation frequency. Furthermore, the advantage of the present invention is that the entire circuit arrangement can be manufactured to have very small size by employing an IC chip for the amplifier. In addition to the previous advantages, the undesirable reflections occuring in the succeeding delay line elements can be eliminated by cascade-connecting the corresponding delay line elements and compensation amplifiers. Accordingly, no artifact appears in the resultant tomographic image. For example, the analogue delay line circuit according to the invention can be easily utilized in both the linear scan and the sector scan without requiring any complicated adjustment or tempering, although the maximum delay times of both scanning nodes are considerable different from each other, e.g., 1 µs for the linear scan and 5 µs for the sector scan. If the analogue delay line circuit 300 shown in Fig. 8 is operated in the linear scanning mode, and the gains of the amplifiers that are not necessary for the desirable compensation are sufficiently reduced, the so-called crosstalk noise produced from these amplifiers can be avoided. As apparent from Fig. 4, it is also possible to cascade-connect one analogue delay line element and one amplifier to establish a single delay line circuit arrangement. The transistors of the amplifier, as the active element, may be substituted by unipolar transistors such as FET's. To vary the gains of the amplifiers, an impedance change of FET under the control of the gate voltage may be utilized instead of the variable resistor.

Claims: An ultrasonic imaging apparatus comprising: a plurality of ultrasonic transducer elements (1-1 to 1-8) for converting received ultrasonic echoes into voltage output signals; a plurality of input terminal means (35A, 35B, 35C) for receiving said output signals from said transducer elements; analogue delay line means connected to said plurality of input terminal means and including a plurality of delay lines (10A to 10H) for delaying said output signals from said plurality of transducer elements (1-1 to 1-8) by a given delay time; a plurality of switching means (84-1 to 84-8), each having a plurality of input terminals and a single output terminal, for selecting one of said input terminals to be connected with said output terminal, said output terminal being connected to one of said plurality of ultrasonic transducer elements (1-1 to 1-8) and a plurality of amplifier means (20A to 20H) each connected in cascade between adjacent said delay lines (10A to 10H), characterized by further comprising: a plurality of voltage-current converting means (82-1 to 82-8) respectively connected to said ultrasonic transducer elements (1-1 to 1-8), for converting said voltage output signals from said transducer elements into current signals proportional thereto and supplying the current signals to said analogue delay line means through said plurality of switching means (84-1 to 84-8); wherein said plurality of amplifier means are provided for compensating for the reduction of the amplitudes of said current signals at the high frequencies of the used frequency range and also for compensating for the insertion loss thereof caused by the analogue delay line means by selecting or controlling the gains and frequency characteristics of said amplifier means; a plurality of taps derived between said amplifier means, said taps being connected to said input terminals of each of said switching means (84-1 to 84-8); resistor means (Rm), having one constantly biased end and another end connected to one end of said analogue delay line means (10A); and an output terminal means (45) connected to the output of the last amplifier means in the cascade, for adding said current signals delayed by said delay lines (10A to 10H) and producing the resultant signal from said output terminal means (45). An ultrasonic imaging apparatus as claimed in Claim 1, characterized in that said analogue delay line means include capacitive and inductive elements (C1:C2; L1:L2). An ultrasonic imaging apparatus as claimed in Claim 2, characterized in that said inductive elements are inductors (L1:L2) and said capacitive elements are made of stray capacitance (C1:C2). An ultrasonic imaging apparatus as claimed in Claim 1, characterized by further comprising analogue switch means (65), connected between said plurality of input terminal means (35A; 35B; 35C) and the transducer elements for selecting the given delay time of said compensated and delayed echo signals. An ultrasonic imaging apparatus as claimed in Claim 1, characterized in that said resistor means comprise input matching impedance means (Rm) connected to the first said input terminal means in the chain (35A) and output matching impedance means (Rm) connected to said output terminal means (45). An ultrasonic imaging apparatus (200) as claimed in Claim 1, characterized in that two or more analogue delay lines (10A, 10B; 10S, 10T) are cascade-connected to establish a delay line means (50A, 50J). An ultrasonic imaging apparatus (300) as claimed in Claim 6 characterized in that said plurality of amplifier means are amplifiers (30A:30B:30I) whose gains and frequency characteristics are variable, and control means (60) is coupled to said amplifiers (30A:30B:30I) so as to control the gains and frequency response at the high frequencies of the used frequency range thereof in accordance with the given delay time. An ultrasonic imaging apparatus as claimed in Claim 1, characterized in that said amplifier means includes an emitter follower transistor (TR1) whose base receives said delayed output signals, a transistor (TR2) whose base is grounded, and a coupling circuit having a resistor (R1) and a capacitor (C1:Cx) connected parallel to the resistor (R1) and interposed between the emitter of the first-mentioned transistor (TR1) and the emitter of the last-mentioned transistor (TR2). An ultrasonic imaging apparatus as claimed in Claim 8, characterized in that said coupling resistor (R1) is substantially equal to 220 ohms and said coupling capacitor (Cx) is substantially equal to 32 pico Farad. An ultrasonic imaging apparatus as claimed in Claim 1, wherein said analogue delay line means is formed by one analogue delay line (10A) connected to said input terminal means (35A) and said plurality of amplifier means is replaced by one amplifier means (20A), series-connected to said delay line (10A) so as to constitute a cascade-connected circuit. An ultrasonic imaging apparatus (500) as claimed in Claim 1, characterized in that said transducer elements (1-1; 1-2; 1-8) which form transducer means (1) are arranged in an array for transmitting ultrasonic beams toward an object (2) under examination upon receipt of high frequency exciting pulses, and for receiving ultrasonic echoes reflected from the object (2), thereby producing echo signals; transmitter means (3) have means for generating the high frequency exciting pulses and are coupled to the transducer means (1) so as to apply the exciting pulses to the transducer elements (1-1; 1-2; 1-8); receiver means are coupled to the transducer means (1) and include a delay line unit (80) for delaying the received echo signals and also compensating for a reduction of amplitudes of the delayed echo signals at the higher frequencies of the used frequency range and also an insertion loss thereof, and means (95) for processing the delayed and compensated echo signals so as to produce tomographic image signals, and display means (98) display tomographic images of the object scanned by the ultrasonic beams based upon the tomographic image signals. An ultrasonic imaging apparatus (500) as claimed in Claim 11, characterized by further comprising a plurality of preamplifier means (70-1, 70-2, 70-8) interposed between said plurality of transducer elements (1-1, 1-2, 1-8) and said plurality of voltage-current converting means (82-1, 82-2, 82-8) so as to amplify said voltages of the echo signals. An ultrasonic imaging apparatus (500) as claimed in Claim 10, characterized in that said voltage-current converting means are semiconductor elements. An ultrasonic imaging apparatus as claimed in Claim 11, characterized in that said amplifier means includes an emitter follower transistor (TR1) whose base receives said delayed echo signals, a transistor (TR2) whose base is grounded, and a coupling circuit having a parallel-connected circuit of a resistor (R1) and a capacitor (C1, Cx) and interposed between the emitter of the first-mentioned transistor (TR1) and the emitter of the last-mentioned transistor (TR2). An ultrasonic imaging apparatus as claimed in Claim 14, characterized in that said coupling resistor (R1) is substantially equal to 220 ohms and said coupling capacitor (Cx) is approximately 32 pico Farad.

Inventors: IIDA, TAKETOSHI C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1985
Main classification: G01S15
IPC codes: A61B8, G01S7, G01S15, H03H11