JPS6157012B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The azimuthal resolution of an ultrasonic imaging device used for medical diagnosis and the like is determined by the probe that emits and receives ultrasonic beams. FIG. 1 shows a cross section of a transducer of a circular flat probe conventionally used in a mechanical scanning type ultrasonic imaging device and a schematic diagram of its ultrasonic beam. In this probe, if the diameter of the disc is D and the wavelength of the ultrasonic wave is λ, the width of the ultrasonic beam will be approximately the same width as the transducer up to the near field limit X _N determined by D ² /λ; At longer distances, the ultrasonic beam spreads with an aperture angle of λ/D. In contrast to circular plate probes with such characteristics, a concave convergence probe has been proposed that uses a concave transducer to narrow the ultrasonic beam at a short distance.
FIG. 2 schematically shows the cross section of the transducer of the concave convergent probe and the ultrasonic beam. By using a concave transducer in this manner, the ultrasonic beam can be narrowed in a specific region. Also, when receiving reflected echoes in this region, the same characteristics as when emitting ultrasonic waves are obtained. Therefore, it is common practice to use a concave convergent probe to increase the lateral resolution of an ultrasonic imaging device. An apparatus using the above-mentioned type of probe has the disadvantage that, although the resolution improves in a specific region, the resolution rapidly decreases when the device deviates from that region. Therefore, it is not convenient when the measurement area is wide. In order to improve this drawback, the planar disk vibrator is divided into ring shapes, and the received signals from these divided vibrators are delayed and then added to achieve a convergence effect similar to that of the concave vibrator. This convergence point is set by changing the above delay amount as time passes.
Attempts have been made to move the sensor to obtain high resolution over the entire measurement area. (Ueda, Sato, Maeda, Yamamoto: Journal of the Institute of Electronics and Communication Engineers, vol58-A (1975)
(See page 729) The same thing is done for rectangular oscillators.

The principle of a vibrator that can move these convergence points will be explained below, taking the rectangular vibrator array 1 as an example. narrow and long n arranged on the same plane with equal spacing
Consider rectangular oscillators 1, 2, ..., n. FIG. 3 shows the relationship between this arrangement state and the convergence points P ₁ , P ₂ , and P ₃ . Convergence points P ₁ , P ₂ , and P ₃ are placed on a straight line passing through the center of the transducer array and perpendicular to the transducer array. An arc is drawn around these points and passing through the centers of the vibrators located at both ends of the vibrator array. Distance l ₁ from the point where the straight line connecting the center of each oscillator and point P ₁ intersects with this circular arc to the center of each oscillator 1, 2, ..., n
₁ , l ₂₁ ...l _o1 , the delay times τ ₁₁ , τ ₂₁ , τ _o1 corresponding to the times τ 11 , τ 21 , τ o1 are given to the received signals from each oscillator and then added together, this oscillator array becomes the reflected echo from point P ₁ . It will converge to . Here τ _o1 = l _o1 /
c and c are the sound speeds in the medium, and in order to converge this oscillator array to point P ₂ , the same steps as above for each oscillator 1, 2,..., n are given as τ ₁₂ = l ₁₂ /c, τ ₂₂ = l ₂₂ /c,...
It is sufficient to perform the addition after giving a delay of τ _o2 =l _o2 /c. The same applies when point P ₂ becomes P ₃ . Here l ₁₂ , l ₂₂ , ..., l _o2 are l ₁₁ , l ₂₁ in P ₁ ,
..., is the length of the line segment at P ₂ given by the same construction method as that used to find l _o1 .

FIG. 4 shows an example of a wave receiving circuit in which the transducer array has a convergence effect. Here, 1 is a transducer row consisting of the elongated transducers described earlier, 2 is a circuit that delays the received ultrasonic signal from the transducer, and 3 is an adder. Here, the delay circuits have τ ₁₁ , τ ₂₁ ,...
It is obvious from the previous explanation that if a delay of τ _o1 is given, this oscillator array will be focused on the reflected echo from point P ₁ . _The delay times of the delay circuits connected to _the oscillators 1, _{2 ,} ..., n are determined as time _passes , _{respectively .}
When configured to change as τ _o2 →τ _oo , the convergence point of the received wave moves as time passes as P ₁ →P ₂ →P ₃ . Ultrasonic waves are emitted from the transducer array 1 into the measurement target, and the region where the ultrasound is reflected from the measurement target becomes the convergence point of the transducer array as the incident depth of the ultrasound increases with time. can be moved electronically according to the principles described above to improve resolution over the entire measurement area. This kind of thinking has been around for a longer time than the above-mentioned literature. (Iinuma, Hashiguchi; Proceedings of the 23rd Japanese Society of Ultrasonics in Medicine, P91, 1974-5, and Special Publication No. 12742, 1973) In fact, a device based on the above principle was prototyped and the results were reported. Naturally, the biggest drawback is that when the number of transducers is small and the width of the transducer array is narrow, the convergence effect in deep regions is reduced, and the convergence point is moved only in the near field limit X _N The effect is great.

An example that can explain this is shown in FIG. This was explained earlier when receiving the reflected echo when a 2MHz ultrasonic wave was emitted using a probe with a width of about 20mm, which has 40 elongated transducers each having a width of 0.4mm arranged adjacently at a pitch of 0.5mm. This figure shows the calculation results of the relationship between resolution and distance from the probe when the resolution is improved by moving the convergence point over time based on the principle. The convergence points are 5.0, 7.5, 10.0,
It was moved sequentially to 12.5, 15.0, 17.5, and 20.0 cm. Here, the resolution is expressed as the width in the azimuth direction in which the sensitivity decreases by -3 dB from the highest point.

In Figure 6, 120 0.4mm transducers with the same width as above are arranged adjacently at a pitch of 0.5mm to form a transducer array approximately 60mm wide, with convergence points set at 5.0, 7.5, 10.0,
The relationship between the resolution and the distance from the probe was determined by numerical calculation when the probe was moved sequentially to 12.5, 15.0, and 17.5 cm. Here the frequency of ultrasound is
At 2.0MHz, the resolution definition is the same as in Figure 5. As can be seen from this calculation result, increasing the number of transducers and widening the width of the probe can improve the resolution at long distances from the probe, but the depth of focus is shallow near the probe. Therefore, the resolution only improves near the convergence point, and the resolution drops sharply beyond this point, making it even worse in practice than when the width is narrow. This can be improved by increasing the number of convergence points, but this becomes a problem due to the complexity of the delay circuit, which not only makes the device large and expensive, but also makes it difficult to implement. Furthermore, as the width of the probe increases, the delay time to be given to the received signal also increases, which is disadvantageous in terms of performance in the actual configuration of the delay circuit. To give a concrete value, the transducer above is larger than the transducer in 40 probes.
The delay time that should be given to the received signal from the transducer in the center of the probe is approximately 7.5 cm.
While 420 nsec is sufficient, a probe with 120 transducers and a width of 6 cm requires a long delay time of 3800 nsec. If we tried to provide this with an LC delay line, which is often used in actual circuits, considering that the frequency of the received signal is about 2MHz, the signal attenuation would be large in terms of performance, which would be a problem. It is difficult to do so.

Next, the present invention, which solves the problems of the conventional moving convergence point, will be explained.

FIG. 7 is a diagram explaining the principle of the present invention. Here, reference numeral 1 denotes a probe constructed by arranging narrow rod-shaped vibrators adjacent to each other, similar to the conventional moving convergence point probe. Here, consider a case where the convergence point moves from point P ₁ to P ₂ and P ₃ over time. When converging to point P ₁ , the number of oscillators is determined so as to obtain the required resolution. In other words, determine the effective width of the probe,
Let its width be W. Also, if the distance from the center O of the probe to P ₁ is R, then the arc centered on point P ₁ and passing through both ends Q ₁ and Q ₁ ', which defines the effective width of the probe, is a straight line.
Find the point that intersects with the extension line of P ₁ O. Let the distance between this point and the center O of the probe be _L1 . This L ₁ is given by the following formula.

L ₁ = √ ² + (2) ² -R (1) Detection when the distance OP ₁ from the center of the probe to the convergence point and the effective widths of the probe Q ₁ and Q ₁ ' are each multiplied by k. Similarly, find the point where the arc passing through both ends of the tentacle and centered on the convergence point intersects with the extension of straight line OP ₁ .
If the distance between this point and the center of the probe is Lk, then Lk
is given by the following equation.

Lk=√() ² +(2) ² −kR =k{√ ² +(2) ² −R} =kL ₁ (2) In this way, increasing the distance from the center of the probe to the convergence point If the effective width of the probe is increased proportionally, the above-mentioned distance _L1 will also increase proportionally. Therefore, as the reflection point of the ultrasonic waves emitted from the probe moves over time, the convergence point of the waves received by the probe moves, and the effective effectiveness of the probe changes in proportion to this moving distance. The amount of delay that should be given to the received signal from each element of the probe in order to have a convergence effect equivalent to that of a concave transducer when the width is widened is determined in Figure 7 using the same method as in Figure 3. It can be found by applying it. Here, a delay was given to each element of the probe to have convergence equivalent to that of a concave transducer, but an axicon-shaped transducer (C.
B. Burckhard et al: ULTRASONICS vol.54,
(See No. 6, 1628), but only the case shown in FIG. 3 will be described below.

FIG. 8 shows the results of numerical calculation of the resolution (-3 dB width) as a function of the distance from the probe when the convergence point of the received wave is moved according to the above-mentioned principle. Here, the element width of the probe is 0.4 mm, and the pitch is 0.5 mm. The ultrasonic frequency is 2.0MHz, and the convergence points are 5.0, 7.5, 10.0, 12.5, 15.0, 20.0cm.
When moving the probe sequentially, the effective width of the probe is set to 2.0,
It is expanded to 3.0, 4.0, 5.0, 6.0, 7.0, and 8.0cm. As the convergence point is moved to a far distance, the effective width of the probe is increased, so the width of the ultrasonic beam is always constant, so a constant resolution can be obtained over a wide range. Also, here the convergence point is 17.5 from the probe.
When placed at points cm apart, the maximum required delay time is
2280 nsec, which is about half the value of the case shown in Fig. 6, which is easily achievable. According to the present invention, since the depth of focus and resolution are uniform over the entire area, uniform image quality can be obtained, and a major feature is that the convergence point only needs to be moved at a constant interval. In addition, since ultrasonic waves are attenuated in a living body, the reflected echoes become weaker as the distance from the probe increases. Therefore, in practical devices for biological diagnosis, the gain of the amplifier for the received signal is changed over time, but this is not sufficient. The present invention has the great advantage that as time passes, the number of transducers that receive waves in the probe increases and the received signal becomes stronger.

FIG. 9 is an example of a circuit implementing the present invention. Here, 1 is a probe having a large number of narrow width transducers arranged adjacent to each other, 6 is an amplifier, 2 is a delay line, 4 is an analog switch, 5 is a shift register, and 3 is an adder.
The received signal from the element is amplified by an amplifier and then guided to a delay line. The delay line is provided with delay time taps corresponding to the line segments shown in FIG. 7 in each element. Selection of these taps is performed by opening and closing analog switches. Because the convergence point is close to the probe and the effective width is small,
In a channel connected to an element that does not require a received signal, the analog switch can be grounded in addition to tapping the delay line so that the received signal is not applied to the adder. These analog switch groups are opened and closed by one or more shift registers having a serial input and a parallel output. In the figure, the analog switch is closed when the output of the shift register is 1, and opened when it is 0, and the received signal is applied to the adder. In the upper channel, the shift register is sent to the left three times before the signal from the oscillator is sent to the adder. In this way, in the vibrator near both ends of the probe, the input terminal of the analog switch is often grounded. The lower channel shows a state in which the received signal from the tap numbered 1 on the delay line is applied to the adder because it is located in the center of the probe. The analog switch moves the convergence point over time, so by applying a clock pulse to the shift register, the set numerical values are sequentially sent to the left, which sequentially switches the ON analog switches and moves them to the left. In the figure, the circuit consisting of the above-mentioned amplifier, delay line, analog switch, and shift register is shown as having two channels, but in actuality, a large number of channels are provided as required by the probe. All of the received signals from these many channels are led to an adder and summed.

Although the operation of the present invention has been described above using one embodiment, it is also possible to control switching of the analog switch by a ROM. An example thereof is shown in FIG.

In this figure, similar to the embodiment shown in FIG. 9, tap switching of the delay line and control of the effective width of the probe are performed by an analog switch 4. Switching of the analog switch is performed by a shift register 7. This shift register has parallel input and parallel output. The contents of this large number of shift registers 7 can be rewritten all at once by a control signal. Newly written data is transferred to a shift register 5 with serial input and parallel output, which is connected to a shift register 7 as shown in the figure. shift register 5
After the contents of are moved to shift register 7, the ROM
The contents of 8 are written sequentially. This operation is performed at intervals when the analog switch 4 is switched over time. The contents of ROM naturally have a convergence point in the order in which they are read.
Pre-calculated data is included so that the effective width of the probe will be a value in accordance with the principle of the present invention explained earlier.

The above explained the case of wave reception, but for wave transmission, the effective width of the probe is changed as the convergence point moves, and the ultrasonic beam width is kept constant regardless of the movement of the convergence point, similar to the case of wave reception. be able to.

As explained above, according to the present invention, it is possible to obtain uniform resolution over the entire observation area, and when implementing this, the depth of focus of the convergence point is uniform, so the number of switching positions is small, and the number of switching positions is small. Since the delay amount of the delay circuit can also be made smaller than that of the conventional method, it is possible to implement the method in a concrete manner. It still has great practical effects, such as compensating for the loss of sensitivity at long distances from the probe due to attenuation of ultrasound in the living body, and improving SN in this region.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of the cross section of a flat plate transducer and its ultrasonic beam, Fig. 2 is a schematic diagram of the cross section of a concave transducer and its ultrasonic beam, and Fig. 3 is a schematic diagram of a cross section of a concave transducer and its ultrasonic beam. Figure 4 shows the cross-section of the probe configured with the above-mentioned probe, and an explanatory diagram of the principle of determining the delay time that should be given to the signal of each transducer in order to give the probe a convergence effect. Figure 5 is a diagram of the principle of a circuit that provides a convergence effect by giving a delay to Figure 6 is a figure obtained by calculation. Figure 6 is a figure showing an example of the relationship between the distance from the probe and resolution when the convergence point is moved for a wide probe, calculated by numerical calculation. Figure 7. 8 is an explanatory diagram of the cross-section of the probe according to the present invention and the principle of determining the delay time to be given to the signal of each transducer. FIG. Figure 9 shows an example of the relationship obtained by numerical calculation.
The figures are diagrams describing an embodiment of the present invention, and FIG. 10 is a diagram depicting an embodiment of the present invention.

Claims (1)

[Claims] 1. A probe having a plurality of transducers, means for transmitting or receiving an ultrasonic beam using the probe, and means for electronically converging the ultrasonic beam. , an ultrasonic device having means for moving the convergence point of the ultrasonic beam in the depth direction, characterized by comprising means for changing the number of transducers involved in transmitting or receiving waves according to the convergence point. Ultrasonic device. 2. A patent characterized in that the means for changing the number of transducers is configured to increase the number of transducers as the distance from the probe to the convergence point of the ultrasound beam increases. An ultrasonic device according to claim 1.