JPH0263013B2 - - Google Patents

Description

[Detailed description of the invention]

The invention comprises an ultrasonic transducer comprising a fixed array of transducer elements adjacent to each other, the adjacent transducer elements of the ultrasonic transducer being periodic in series in a plurality of adjacent groups. The present invention relates to an ultrasonic cross-sectional imaging system that transmits pulsed ultrasound waves to a heterogeneous object to be inspected and receives echo waves from discontinuities within the object. Conventionally, a method of mechanically moving an ultrasonic transducer has been used to form an ultrasonic cross-sectional image. This has some drawbacks. That is, if the transducer were to be moved by hand, the scanning process would be very long and dependent on the skill level of the operator. If the transducer is powered by a motor, a relatively heavy water tank is usually required. Furthermore, the extra travel distance in the aquarium results in a reduction in the maximum possible image frequency. To eliminate these drawbacks, ultrasound imaging systems that incorporate electronic scanning have been developed. In this method the ultrasound beam is shifted linearly in time. In a known ultrasound imaging system as described above (U.S. Pat. No. 3,881,466), a transducer device creates an unfocused ultrasound beam with lateral resolution determined by the width of the transducer element. be done. In this known system, there is a limit to the lateral resolution that can be improved by decreasing the width of the transducer element, set by the minimum width of the ultrasound beam. Although the cross-sectional images obtained with this known system are relatively sharp, there are still many applications where higher lateral resolution is desired in practice. A known method for obtaining high lateral resolution over the entire examined depth is so-called dynamic focusing, which is implemented using an array of transducer elements (see US Pat. No. 3,090,030). In this type of focusing, the location of the focus is determined by providing a delay that is variable over time between the transmitter signals applied to the transducer elements.
It is varied in time along the main axis of the beam over the depth to be examined. An important drawback of this known method is that it requires relatively complex and therefore expensive electronic circuitry. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a system which, with minimal equipment, can provide sufficient lateral resolution for medical diagnostic purposes over the entire examination depth. The feature is that the plurality of groups of transducer elements constituting the ultrasonic transducer are composed of even and odd numbered transducer elements that can be successively selected. It is constructed by alternately repeating the process of reducing the number of elements in one direction and increasing the number of elements in the opposite direction. According to the invention, the movement of the ultrasound beam from one group to the next is approximately half the center-to-center distance of two adjacent elements in the transducer element array (which is not the case in the prior art). (equal to the center-to-center distance). Therefore, the number of scanning lines is doubled, resulting in improved lateral resolution. Hereinafter, some embodiments of the present invention will be described with reference to the accompanying drawings. As shown in FIG. 1, a transducer apparatus 11 of a known ultrasound imaging system (US Pat. No. 3,881,466) includes an elongated fixed array of adjacent transducer elements 12. A group of A adjacent elements 12 are sequentially excited to generate a pulse. The position of each successive group of A elements is shifted a vertical distance of B elements from the position of the immediately preceding group. In this way, the ultrasound beam 13
is a series of dot-dashed rectangles 14 indicating the position of the beam 13 at each time point equally spaced in time.
As shown, move in the direction of arrow L. It is noted that the known transducer device 11 shown here generates an unfocused ultrasound beam 13, since the A elements in the transducer group are all excited at the same time when generating a pulse. Should. The unfocused radiation characteristic 22 of the ultrasound beam of FIG. 1 is shown in FIG. In FIG. 1, three arrows Q, L, and S define an orthogonal coordinate system. Arrow L is along the longitudinal axis of the radiating surface of transducer device 11. Arrow S is parallel to the main axis direction of the ultrasound beam 13. Arrow Q is perpendicular to the plane defined by arrows L and S. The positions of the cross-sectional views and front views shown in the accompanying drawings are determined by this coordinate system. FIG. 3 depicts, partially in cross-section, a preferred configuration of transducer 38 for practicing the present invention. Arrangement 38 includes a complete electrode 36 that is grounded and one surface 37 that serves as a radiating surface. Structure 38 also includes a fourth
As shown in the back view of the figure, it includes a piezoelectric layer 35 and electrode segments 31-34. From the above description of the structure 38, it is clear that the transducer elements according to the invention can have common parts, such as a piezoelectric layer 35 and a complete electrode 36. Structure 38 according to the invention
can be operated simply by providing an electrode segment on one side, applying a time-shifted transmitter signal thereto and obtaining an echo signal therefrom. Each electrode segment thus defines a transducer element according to the invention. The advantages achieved by the present invention, namely the high lateral resolution, are primarily due to the novel mode of operation of the transducer device. This will first be explained in detail with reference to FIGS. 2, 4, and 5. FIG. 4 shows a transducer group 21 according to the present invention.
The electrode segments 31-34 of FIG. To generate an ultrasound beam in accordance with the present invention, transmitter signals 41, 42, time-shifted with respect to each other as shown in FIG.
1-34. outer segment 31,
The transmitter signal for 34 has a phase lead. In this way, a loosely focused ultrasound beam 23 is created (FIG. 2). In a preferred embodiment of the invention, time shifts are created not only between the transmitter signals but also between the echo signals received by the individual elements of the transducer group. The transducer group 21 shown in FIG. 4 has four elements for transmitting and receiving, and the transmitted signal of the outer element and the time-shifted echo signal have a phase lead of 90°. I have it. According to the invention, the phase lead is determined relative to the period (360°) of a high frequency carrier signal (eg 2 MHz). The carrier signal is applied to the subsequent groups of electrode segments in the form of pulses with a repetition rate of 2 KHz, for example, and at a suitable phase angle. The effects of the transducer group 21 according to the present invention can be improved by the following items (1) to (3). (1) It has been found to be advantageous to select the combination of phase leads for the outer elements of the group as approximately 90° for the transmitter signal and approximately 45° for the echo signal, or approximately 45° for the transmitter signal and approximately 90° for the echo signal. .
As a result of using these different values of phase advance for the transmitter and echo signals, the radiation profile according to the invention (FIG. 2) can be additionally narrowed over a certain depth. (2) It is advantageous to weight the transmitter and echo signals. As shown in FIG. 5, the inner electrode segments 32, 33 are supplied with a transmitter signal having a larger amplitude _a0 . Similarly, echo signals received from inner segments are multiplied with a larger weighting factor than echo signals from outer elements. Preferably, the weighting ratio for both the transmitter signal and the echo signal is 2:1. (3) It is advantageous to create a loose focusing also in the Q direction shown in FIG. 1, for example by using a transducer arrangement (see FIG. 6) with a slightly curved emitting surface 37. It is. Q
Loose focusing in the directions can also be created electrically using a transducer arrangement, as shown in FIG. 7, in which each electrode segment is divided into three parts a, b, c in the Q direction. 7th
As shown in the figure, only the shaded portion of this segment is used for transmission and reception. The inner parts 32b, 33b are excited with a transmitter signal 41, and the remaining active parts are excited with a transmitter signal 42. Although such a system is electrically more complex than a transducer arrangement with a curved radiating surface, it requires a transducer arrangement with a flat radiating surface and is less expensive. In the known transducer device 11 shown in FIG. 1, the ultrasound beam 13 can be moved by the width of the transducer element 12 after each transmission and reception period. However, if at each point in time the ultrasound beam could be moved a smaller distance, for example by half the width of the element, the number of lines in the image would increase and the resolution would increase. Of course, the same effect could be obtained by halving the width of the elements, but this would double the number of elements and thus increase the complexity. As shown in Figures 8a, 8b and 8c, according to the present invention, the ultrasound beam is moved by half the width of the element. The successively selected groups 71, 72, 73 of transducers then alternately contain odd and even elements. Subsequent groups are formed alternately by decreasing the number of segments in one direction and increasing them in the opposite direction. The amplitude and phase of the transmitter signal or time-shifted echo signal are chosen such that the shape of the ultrasound beam remains substantially uniform, independent of the number of elements in the transducer group.
The table and the amplitude and phase relationships shown in the table give very similar beam shapes when using alternating four and three elements, for example.

【table】

[Table] In the embodiments described above in connection with FIGS. 1 to 8, when the ultrasonic waves are strongly focused, they are ideally focused in a spot shape, which is a single point (focal point), so the spherical surface It can be called target focusing. However, the second
According to the figures and the description associated with FIG. 6, only a loose focusing takes place, so that the ultrasound beam does not actually have a focal point, but a narrow width.
In a second embodiment of the invention, described below in conjunction with FIGS. 9-13, a different type of focusing is provided, namely along a line which should be called a focal line rather than a focal point. It is done. For this reason, the latter is called aspheric focusing. Regarding the second embodiment of the present invention, first, FIG. 9a,
This will be explained with reference to FIGS. 9b and 10. It is known that ultrasound beams can be effectively focused over considerable distances if the ultrasound waves are emitted with a conical wavefront (Swiss Patent No. 543313). Wavefronts of this type are emitted, for example, by conical ultrasound transducers. In accordance with the present invention, if the phase angle φ can be made to increase linearly with the distance between transducer elements 92-98 and the center of the transducer group, the transmitter signal 101- 10
4 to the time-shifted echo signal 20 of FIG.
2-208 can be approximated to a conical radiation surface. FIG. 10 shows the linear increase in phase angle φ. By shaping the emitting surface 37 in the Q-direction as shown in cross-section in FIG. 9b, a linear increase in the phase angle of the reflected ultrasound waves can be achieved. The dotted line 107 in FIG. 9a indicates the position of constant phase on the radiating surface of the transducer device. For simplicity, this figure shows a phase that changes continuously in the L direction instead of a phase that changes stepwise as in this example. In this example, the constant phase trajectory is a set of straight lines 107 rather than a circle as in the case of a conical wavefront. A better conical wavefront approximation is as follows:
This is obtained by the embodiment of the invention shown in connection with FIGS. a, 11b and 12. In this example,
The phase angle of the transmitter signal or time-shifted echo signal is a quadratic function of the position of the corresponding element at the center of the transducer group and a linear function at the ends. A corresponding phase angle distribution in the Q direction is obtained by shaping the emitting surface 34 as shown in FIG. 11b with respect to the cross section of the transducer device. Line 37 in Figure 11b
is preferably a hyperbola. This type of curve is circular in the central region 127 and straight at the ends. The improvement obtained with this embodiment is illustrated by the fact that the constant phase trajectory 106 shown in FIG. 11a now has rounded corners. It should be noted that the radiation group of the transducer in the embodiments shown in FIGS. 9a and 11a has a wider area than in the embodiment shown in FIG. be. This large area provides the larger aperture needed to obtain higher resolution. In the embodiments described above, as in the previously described embodiments, the inner part of the transducer's radiation group has a larger amplitude and the echo signals received there are given a greater weighting upon reception. Multiplyed with the factor, thereby increasing short range
Range field) can be improved. Regarding the shapes of the group 21 and the elements 31-34 as shown in FIG. 4 in order to obtain the loosely focused ultrasound beam 23 as shown in FIG. 2, first refer to FIGS. 18 and 19. explain. The transducer group provides effective loose focusing when its width W and length l are 15 to 30 times the wavelength. The radius of curvature R (FIG. 19) of the wavefront is approximately equal to half the depth of the object to be examined, preferably somewhat less. In the case of a transducer group containing four elements, the width of each individual element is such that the phase between the waves emitted from adjacent elements is
It is taken so that it is not much larger than 90°.
If the values of these radii of curvature and phase difference exceed the above-mentioned values, corresponding disturbances appear in the beam shape and the lateral resolution is accordingly degraded. However, loose focusing according to the invention can, at least in principle, be obtained with phase differences between 30° and 180°. Next, the shape of the transducer element will be explained with reference to specific examples (FIGS. 18 and 19). 1st
As shown in Figure 8, the inner two elements of the group transmit with a phase of 0° and the outer two elements transmit with a phase of 90°. From Figure 19 and the string theorem, the following equation (1) can be obtained. d ² ₁ = 2R・△ ………(1) where d ₁ is the lateral shift to obtain the desired 90° phase shift, R is the radius of curvature of the wavefront, and
This is the distance corresponding to a 90° phase shift. In this case, if the wavelength is λ, then Equation (2) is obtained. △=λ/4 ………(2) If R is 80 mm (approximately half the depth of the object being tested) and λ is 0.75 mm (this depth corresponds to a frequency of 2 MHz), then d ₁ is It becomes 5.48mm. The element is assumed to be at a distance d ₂ =6 mm from the center of the transducer group. This d ₂ value is the previously calculated distance
Approximately equal to d ₁ . FIG. 13 is a block circuit diagram of an ultrasonic cross-sectional imaging system that uses a seven-element transducer group for transmission and reception, as shown in FIG. 11a. The block circuit diagram of FIG. 13 includes a transducer structure 38, a timing generator 131, a timing signal 132 sent from the timing generator 131, a transmitter signal generator 133, and a transmitter as shown in FIG. A transmitter signal 134 is provided from a signal generator 133 through line 135 to an element selector drive switch 138, an element counter and decoder 136 for controlling a switch 138 connected to a timing generator 131, and sent from a group of transducers. echo signal 14
2. an echo signal receiver 143; a combined echo signal 144 at the output of the echo signal receiver 143; a time-sensing amplifier 145; a detector 146;
Signal processor 147, output signal 148 of processor 147, x deflection generator 151, deflection signal 154 provided by x deflection generator 151, Y stage function generator 152, stage function signal sent from Y stage function generator 152. 15
5, an oscilloscope 156 with three inputs X, Y, Z is shown. Timing generator 131 generates periodic timing pulses 132 to trigger the transmission of the ultrasound signal and generate the necessary sinusoidal signal. Transmitter signal generator 133 generates four electrical transmitter pulses 121-124 (see FIG. 14). Three signals 122, 123, 124
is +30°, +
It has a phase lead corresponding to the carrier signal phase of 100° and +180°. These transmitter signals are sent out on line 134. In element selector drive switch 138, transmitter signals are applied to seven supply lines. The transmitter signals on those lines are +180°, +100°, +
It has phases of 30°, 0°, +30°, +100°, and +180°.
Element counter and decoder 136 is connected to switch 1
desired 7 for transmission or reception through 38
drive each element. After each pulse, the shape in Figure 11a is shifted by one element in the L direction. At the same time, the transmitter signal is periodically interchanged with another phase on the supply line so that each element obtains a corresponding transmitter signal with the correct phase. The echo signal 142 is transmitted from seven switched elements to an echo signal receiver 143.
reach. The signals are then given different delays, multiplied with different weighting factors, and summed. The output signal 144 of the receiver 143 passes through a time-sensing amplifier 145 that compensates for attenuation in the tissue being examined. The signal is then rectified by detector 146 and provided through processor 147 to the z input of oscilloscope 156. Processor 147 compresses the dynamic range of the signal sent from detector 146. The x-deflection generator 151 generates a voltage proportional to the elapsed time since the last pulse was sent. Y stage function generator 152 generates a voltage proportional to the central axis position of the switched group of transducers. The structure and operation of the transmitter signal generator 133 will first be explained with reference to FIGS. 14 and 15. Timing pulse 132 triggers pulsed high frequency generator 161, which
The output signal 162 (pulsed carrier signal) of 1 is the phase
It is delayed in tap delay line 163 to obtain four signals with angles of 0°, 30°, 100°, and 180°. These signals are multiplied with corresponding weighting factors in weighting devices 164-167. FIG. 16 shows the echo signal receiver in detail. The echo signal 142 is transmitted through a weighting device 171-
177 with corresponding weighting factors.
They are delayed by phase shifters 181-185 and summed in adder 186. The basic principles of the preferred embodiment of element selector drive switch 138 in the system of FIG. 13 will first be described with reference to FIG. The device in Figure 13 is 7
Although a group of elements is used, for the sake of simplicity, the principle will be explained here for the case of a transducer group including four elements. The switching diagram shown in FIG. 17 can be used to trigger and shift groups of four transducer elements. The inner two elements of each group (e.g. elements 32 and 33 of the group) are triggered by a transmitter signal 41 as shown in FIG. ) is triggered by a transmitter signal 42 as shown in FIG. In FIG. 17, the transducer elements are shown with corresponding electrode segments 31, 32, 33, etc. By means of switching means 191, the transducer elements are periodically switched between four supply lines 192--
195. These four lines pass through switching means 196 to two supply lines 197.
and 198. These two supply lines 1
97 and 198 are supplied with transmitter signals 41 and 42 having amplitudes and phases as shown in FIG. FIG. 17 shows the switch positions for two successive transducer groups (solid lines), (dotted lines). The device controlling the switch means 191 does not require any explanation. In the switch means 196, in order to drive a new group, each switch (e.g. 213) is placed in the same position previously occupied by the upper switch (e.g. 212) to drive its previous group. The upper switch 211 is placed in the position previously occupied by the lowermost switch 214. If the electronic design of the switch means is suitable, the same switch can be used for transmitting and receiving. Separate switches for transmitting and receiving can be used. If it is necessary to use an electronic switch, it is sufficient to prepare another circuit similar to the one shown in FIG. 17, using separate supply lines for transmitting and receiving. Regarding the element selector drive switch 138 of FIG. 13 whose detailed configuration is shown in FIG. 17, as explained on page 25 of the specification, four elements are used for transmission and reception, and each transmission pulse is sent out. Each time, a group of four elements is moved one element at a time.Next, when the apparatus having the above structure uses four and three elements alternately as illustrated in FIGS. 8a to 8c, When applying the first pulse, the pulse is supplied to four elements as shown in Figure 8a, and when applying the second pulse, the pulse is supplied to three elements as shown in Figure 8b. Furthermore, when the third pulse is applied, the pulse is again supplied to the four elements as shown in Fig. 8c.When these first to third pulses are applied,
In the configuration shown in FIG. 17, the following open and close positions of the switch are taken. 1 When the first pulse is applied: All switches are in position 2 When the second pulse is applied: Switch to element 31; position (open) Switch to elements 32-34; closed (as shown) Switch; position (open) Switch 214; position switch 213; position, (as shown) Switch 212; position switch 211; position, (as shown) 3 When applying the third pulse: All switches are in position or higher By switching a switch such as
The element groups are sequentially moved as shown in Figures 8a, 8b and 8c. Here, when the first pulse is applied, the elements 31 and 34 are connected to the same supply line 198, and the other set of elements 32 and 33 are connected to the other supply line 197. During the second pulse application, elements 32 and 34 are connected to supply line 198 and element 33 is connected to supply line 197. Furthermore, when applying the third pulse, the elements 32 and 39
is connected to supply line 198 and elements 33,
34 is connected to the other supply line 197. We will now discuss in detail how the transmitter signal generator 133 shown in FIG. 14 is applied to alternating groups of four and three elements. Page 15 of the specification illustrates the phase angles and amplitudes used in this mode of operation. When using four elements (first pulse and third pulse
(when a pulse is applied), the signal line 121 (0° phase) is connected to the supply line 197 in FIG.
The relative amplitude of 21 is set to 1 (eg, 50Vpp). Also, the signal on line 123 is
98, its phase is changed from 100° to 90° as shown in FIG. 14, and the relative amplitude is chosen to be 0.5 (for example, 25 Vpp). When three elements are driven (when applying the second pulse), line 121 remains connected to supply line 197 and line 122 is connected to supply line 198 instead of line 123. This switching of lines 123 and 122 is performed according to a predetermined solid state.
This can be done using a state switch.
At this time, the relative phase of line 122 is changed from 30° to 45°, and its relative amplitude is 1 (for example, 50Vpp)
It is said that Also, in this example, line 124
is not used. 4 and 3 in the above steps
element groups are used alternately. Next, in this example, the operation of the echo signal receiver device 143 shown in FIG. 16 will be explained. A supply line 198 connected to the element selector drive switch 138 is connected to a weighting device 171, and a supply line 197 is similarly connected to a weighting device 172. Remaining weighting device 17
3 to 177 are not used in this example. Here, when four elements are used (when applying the first and third pulses), the weighting device 171
weights the signal with a coefficient of 0.5, weighting device 1
72 weights the signal by a factor of 1. Phase shifter 181 delays the phase by -45°. In addition, when three elements are used (second
(when a pulse is applied), the weighting device 171 weights the signal with a coefficient of 1, and the weighting device 172 similarly weights the signal with a coefficient of 1. Phase shifter 181 delays the phase by -22.5°. Weighting device 172
For example, changing the coefficients of
This can be achieved by changing the resistance value with a switch. Adjustment of the phase of the phase shifter can also be achieved, for example, by changing the capacitance with a solid state switch. Selection of weighting coefficients and phase delay are illustrated on page 15 of the specification. Although this example describes the case of a positive phase (phase lead), only a negative phase (phase delay, lag) is used in an actual echo signal receiver device.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of a transducer device of the conventional ultrasonic cross-sectional imaging system mentioned above. FIG. 2 shows a cross section of the radiation characteristics of a group of transducers compared to the loosely focused radiation characteristics of a group of transducers in the device of FIG. 3 is a cross-sectional view of a preferred embodiment of the transducer arrangement of the transducer apparatus of FIG. 1; FIG. FIG. 4 shows a back view of the transducer group of the arrangement shown in FIG. 3 for four transducer elements. FIG. 5 shows the waveform of the transmitter signal applied to the electrode segments of the transducer group of FIG. FIG. 6 shows the radiation surface in the structure of FIG.
2 is a cross-sectional view parallel to the QS plane of FIG. 1, the surface having a shape suitable for loosely focusing the ultrasound beam on the Q plane; FIG. FIG. 7 shows the reverse side of the example configuration of FIG. 3, in which a flat emitting surface is used instead of the concave surface of FIG. 6 to provide loose focusing in the Q direction. It's getting old. 8a, 8b and 8c are diagrams showing the shapes of the transducer groups 71, 72, 73 that are periodically and successively selected according to the present invention. Figure 9a shows the back side of a transducer group containing electrode segments and used in a second embodiment of an ultrasound imaging system. FIG. 9b is a cross-sectional view showing the shape of the radiating surface of the transducer group of FIG. 9a. FIG. 10 is a waveform diagram of the transmitter signals applied to the electrode segments of the transducer group of FIG. 9a. FIG. 11a is a back view of a transducer group having seven electrode segments used in a preferred embodiment of an ultrasound cross-sectional imaging system. Figure 11b is a cross-sectional view of the preferred configuration of the radiating surface of the transducer group shown in Figure 11a. FIG. 12 shows the 11th
FIG. 4 is a waveform diagram of a transmitter signal applied to the electrode segments of the transducer group of FIG. FIG. 13 is a block diagram showing an embodiment of a system for operating the configuration examples described in FIGS. 8a to 8c. FIG. 14 is a block diagram illustrating the transmitter signal generator in the system shown in FIG. 13. FIG. 15 is a waveform diagram of a timing pulse generated by the timing generator (FIG. 13) and a pulsed sine wave extracted from the timing pulse. FIG. 16 is a block diagram illustrating an echo signal receiver in the system shown in FIG. 13. FIG. 17 illustrates the principle of a preferred embodiment of the element selector drive switch in the system shown in FIG. Although the system shown in FIG. 13 includes groups of seven elements each, for simplicity the principle is shown in FIG. 17 for the case of transducer groups containing only four elements. ing. 18 and 19 show the shape of the transducer group and its elements.

Claims (1)

[Scope of Claims] 1. An ultrasonic transducer comprising a fixed array of transducer elements adjacent to each other, the adjacent transducer elements of the ultrasonic transducer being arranged in series in a plurality of adjacent groups. an ultrasonic cross-sectional imaging system that transmits pulsed ultrasonic waves to a heterogeneous object to be inspected and receives echo waves from discontinuities within the object, and the plurality of groups of transducer elements include successively selectable even and odd numbers of transducer elements, and the successive groups reduce the number of elements in one direction and reduce the number of elements in the opposite direction. An ultrasonic cross-sectional imaging system characterized by being configured by alternately increasing the number of elements.