JPWO1998057581A1 - Continuous wave transmission and reception type ultrasonic imaging device and ultrasonic probe - Google Patents

Abstract

(57) [Abstract] A transmitter (20) generates a high-frequency continuous wave that provides sufficient lateral resolution and serves as the transmitted ultrasound wave. A frequency modulator (15) alternates the frequency of the continuous wave in a rectangular wave pattern to perform frequency modulation. The alternating period of the frequency modulation is set to twice the delay time from the time a signal voltage is applied to a piezoelectric vibrator (1) until the ultrasound emitted from the piezoelectric vibrator is reflected from a focal point (5) and reaches the piezoelectric vibrator. A delay circuit (35) imparts a delay equal to the delay time to the transmitted wave to use as a reference signal, and a signal reflected from an object (9) of inspection is selectively detected by lock-in detection (55) of a mixed signal of transmitted and received waves. As a result of the increase in the number of transmitted and received waves per unit time, the S/N ratio is improved when, for example, imaging biological tissue with high resolution at the cellular level by irradiating the object of inspection with continuous ultrasound waves.

Description

[Detailed Description of the Invention] Continuous Wave Transmit-Receive Ultrasound Imaging Device and Ultrasound Probe Technical Field The present invention relates to a continuous wave transmit-receive ultrasound imaging device and ultrasound probe for measuring minute tissue characteristics in real time, such as inside internal organs, on the surface of organs, and on the surface of the body.

BACKGROUND ART Biopsy is a known method for diagnosing lesions in organs.

In a biopsy, an ultrasound imaging device is used to visualize organs within a body cavity, while a puncture needle is inserted into the lesion, a sample of tissue from the area of interest is extracted through the needle, and the extracted tissue is then differentiated to diagnose the disease. However, because biopsy involves removing the tissue from the body, fixing it, slicing it thinly, staining it, and then examining it, it presents several problems, including the need for several weeks for a diagnosis, the risk that the extracted tissue will change from its living state in the body, and the difficulty of obtaining three-dimensional images.

To address these issues, needle-shaped ultrasound probes have been proposed. These probes attach an ultrasound transducer to a puncture needle, which is inserted directly into the area of interest to measure the tissue characteristics of the area of interest or to image the surrounding biological tissue. Prior art needle-shaped ultrasound probes include, for example, a "probe with a recess in the needle and an ultrasound transducer attached to the wall of the recess" (JP Patent Publication No. 4-78299: First Prior Art) and a "probe with an interchangeable inner needle and ultrasound transducer" (JP Patent Publication No. 6-125: Second Prior Art). The first and second prior art ultrasound probes use ultrasound to measure acoustic properties, such as the speed of sound and reflectivity, of surrounding biological tissue.

Examples of ultrasound probes designed to image surrounding tissues based on the acoustic properties (sound speed, reflectivity) of the biological tissue surrounding the probe include a "probe in which an opening is formed in a portion of the outer needle, an ultrasound transducer mounted on the side of the inner needle is exposed through the opening, and the ultrasound transducer is used for scanning" (JP Patent Publication No. 5-9097: Third Prior Art) and a "probe in which an inner needle mounted with an ultrasound transducer is exposed from the tip of the outer needle" (Ultrasonic Imaging, Vol. 15, pp. 1-13 (1993)): Fourth Prior Art).

The third and fourth prior art techniques are designed to obtain images of a plane perpendicular to or including the needle axis, known as a B-mode image. As the ultrasound used for imaging becomes higher in frequency, absorption by biological tissue reduces the depth of ultrasound penetration and narrows the field of view. Therefore, the ultrasound frequencies used in methods for obtaining B-mode images are approximately 100 MHz or less. To avoid the problem of shallow ultrasound penetration and a narrow field of view that arises when using high-frequency, high-resolution ultrasound transducers at frequencies above 100 MHz, a method has been proposed for obtaining an image of a cylindrical (curved) surface around the needle, known as a cylindrical C-mode image (see, for example, JP 8-154936 A: Prior Art 5).

In the fourth and fifth prior art ultrasonic probes, an ultrasonic transducer equipped with an acoustic lens is installed inside the needle to improve lateral resolution. The ultrasonic waves generated by the piezoelectric transducer excited by a transmission voltage propagate through the acoustic lens material, are focused, and are reflected near the focal point of the acoustic lens, generating a reflected signal. The reflected signal then travels the reverse path back to the piezoelectric transducer and is converted into a voltage.

When using an acoustic lens to improve lateral resolution, pulsed waves are typically transmitted to ensure resolution in the depth direction (radial direction from the needle axis). When pulsed waves are transmitted, the delay time of the reflected signal reaching the piezoelectric transducer varies depending on the position of the reflector that reflects the transmitted signal. In other words, the delay time provides positional information in the depth direction.

In the fourth prior art, depth-direction images are obtained by using delay time to provide positional information in the depth direction. In the fifth prior art, a time gate is applied on the receiving side to set the imaging plane. That is, signals with a certain delay time are detected to image a cylindrical surface at a certain distance from the needle axis. In either case, a time gate is applied near the delay time to selectively detect reflected signals from within biological tissue. The time gate applied to the received signal separates the received signal from the transmitted signal in time, preventing the transmitted signal from entering the receiver.

The use of burst waves for transmitting waves is known in ultrasonic microscopes and other applications. When pulse waves do not sufficiently excite an ultrasonic transducer, burst waves can be used to improve the S/N ratio. Another known method is to use burst waves to improve depth resolution by interfering with the reflected signal and multiple reflections within the acoustic lens material.

In the method of improving the S/N ratio or depth resolution by using burst waves, the duration of the burst waves is set (in most cases sufficiently) shorter than the delay time between the transmission and the arrival of the reflected signal. A time gate is set on the receiving side to separate the transmitted and received waves, just as in the case of pulse waves.

Cellular-level lateral resolution is required to differentiate biological tissues based on their characteristics. As is well known, as the ultrasonic frequency increases, the wavelength shortens, improving lateral resolution, but the absorption of ultrasound by biological tissue also increases. This increased absorption significantly reduces the strength of the reflected signal, resulting in a lower S/N ratio. Figure 21 shows the amplitude attenuation due to absorption in the kidney and liver, clearly demonstrating significant absorption in the frequency range above 100 MHz.

Note that Figure 21 was obtained with reference to Figure 4.10 (page 176) in Chapter 4 of the book "Physical Principles of Medical Ultrasonics."

The needle-shaped ultrasound probe configurations of the first and second prior art technologies have the problem of only one measurement point, which does not provide sufficient information for diagnosis. Furthermore, the configurations for obtaining B-mode images of the third and fourth prior art technologies have the problem of being unable to achieve cellular-level resolution due to the difficulty in using high-frequency ultrasound above 100 MHz due to the high absorption of high-frequency ultrasound above 100 MHz.

The fifth prior art method of imaging the cylindrical surface around the needle in C-mode allows the use of ultrasound waves at frequencies between 100 MHz and 200 MHz, improving resolution to approximately 10 μm, but 10 μm resolution is roughly the size of a cell. To observe the characteristics of biological tissue with even higher resolution, it is desirable to use ultrasound waves at approximately 400 MHz. However, when using ultrasound waves at approximately 400 MHz, the absorption of ultrasound by biological tissue is also significant, so it is necessary to significantly increase the strength of the transmitted signal to more than compensate for the absorption of ultrasound and ensure a high signal-to-noise ratio.

Disclosure of the Invention Hereinafter, "delay time" in this invention is defined as "the time difference between the time when the piezoelectric transducer is excited by the transmitting voltage and the time when the ultrasonic waves generated by the excitation propagate to the focal length of the acoustic lens, are reflected, and then propagate back to the piezoelectric transducer and are reconverted into voltage."

The object of the present invention is to provide a continuous wave transmission-reception type ultrasound imaging device and ultrasound probe that can extract finer tissue characteristics of the interior of internal organs, organ surfaces, and body surfaces in real time by improving received signal strength and lateral resolution, thereby facilitating diagnosis. The novel features of the present invention will become apparent from the description and accompanying drawings in this specification.

A typical configuration of the present invention will now be outlined.

This system transmits continuous ultrasonic waves to the object of inspection to improve received signal strength and azimuth resolution. Generally, when noise N is constant, the S/N ratio is proportional to the square root of the number of transmitted and received waves. In other words, the greater the number of transmitted and received waves, the greater the signal strength. Therefore, this system differs from the previously described conventional technology (especially methods using burst waves) in that the duration of the continuous ultrasonic transmission is sufficiently long compared to the delay time between the transmitted ultrasonic waves reflecting from the object of inspection and reaching the piezoelectric transducer.

When using continuous ultrasound waves with a sufficiently long duration, it is not possible to time-gate the received signal, resulting in a problem whereby a transmitted signal approximately 100 to 1000 times larger than the received signal enters the receiver. To solve this problem, the present invention frequency-modulates the transmitted signal, constantly differentiating the transmitted and received frequencies and separating the transmitted and received signals. Details of frequency-based separation of ultrasonic transmission and reception and modulation of the transmitted frequency are discussed below.

We consider the application of the present invention to a needle-shaped ultrasound probe performing cylindrical C-mode imaging, similar to the fifth prior art. In the prior art, the imaging plane was set by applying a time gate to the received signal, as described above. In the present invention, the imaging plane is set by the focal area of the acoustic lens. That is, there is a certain focal depth near the focal point of the acoustic lens, and the reflected signal can be considered to originate exclusively from within the focal depth of the acoustic lens. Although the degree of freedom in setting the focal depth is less than with the time gating method of the prior art, the imaging plane can be determined by the shape of the acoustic lens.

In conventional pulsed wave technologies, depth resolution depends on the temporal resolution of the pulsed wave. This means that the frequency band of the pulsed wave must be as wide as possible. A wide frequency band causes chromatic aberration at the focal point of the acoustic lens, which contributes to degraded lateral resolution. In the present invention, continuous waves are used for transmission, resulting in a narrower band, reducing chromatic aberration and improving lateral resolution. This narrower frequency band of the receiver reduces white noise, which in turn improves the signal-to-noise ratio.

In conventional technology, it was possible to acquire images from multiple imaging planes at different depths by setting a time gate within the range of the focal depth. In the present invention, the imaging plane is determined, in principle, by the acoustic lens.

In this invention, the imaging plane is determined by the acoustic lens. When multiple imaging planes at different depths are required for diagnosis, i.e., when a stereoscopic image (approximately 2.5-dimensional) is required, which provides multiple imaging planes rather than a fully three-dimensional image, multiple ultrasonic transducers with acoustic lenses with different focal positions are installed within a single ultrasound probe, and images at multiple imaging planes at different depths are obtained using multiple ultrasonic transducers. In this configuration, to prevent mutual interference between the ultrasonic transducers, the timing of transmission and reception of each ultrasonic transducer is set to be different from each other, allowing the three-dimensional structure of biological tissue to be visualized.

In this invention, the frequency of the transmitted waves is modulated, alternating between at least two frequencies, e.g., frequencies f1 and f2. When receiving signals are acquired by alternating between frequencies f1 and f2, the signals obtained at frequency f1 and f2 are collected by separate data acquisition devices, allowing separate images at different frequencies (f1 image, f2 image). The acoustic properties of biological tissue generally depend on frequency. For example, tissues with high ultrasonic absorption exhibit a large difference in signal intensity between images obtained at low and high frequencies. Therefore, the distribution of ultrasonic absorption in biological tissue can be obtained from the difference image between the f1 and f2 images. The difference image shows clearer differences in signal intensity than the ultrasonic absorption distribution obtained at a single frequency, thereby enabling the depiction of higher-level tissue characteristics and facilitating diagnosis.

The present invention can be summarized as follows. The transmitted ultrasound waves are continuous waves with a high frequency sufficient to provide sufficient lateral resolution. The frequency of the continuous waves is alternating like a square wave and frequency-modulated. The alternating period of the frequency modulation is set to twice (2t) the delay time t from the time a signal voltage is applied to the piezoelectric transducer until the ultrasound emitted from the piezoelectric transducer reflects from the focal point (the subject of examination) and reaches the piezoelectric transducer. Furthermore, a delay equal to the delay time is applied to the transmitted waves to serve as a reference signal. A mixture of transmitted and received signals is selectively detected using lock-in detection to selectively detect the reflected signal from the subject. As a result of increasing the number of transmitted and received waves per unit time, irradiating the subject with continuous ultrasound improves the signal-to-noise ratio, for example, when imaging biological tissue with high resolution at the cellular level.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram showing the configuration of a first embodiment of the present invention.

Figure 2 shows the time variation of the transmitted voltage in the first embodiment of the present invention.

Figure 3 shows the frequency characteristics of the mixed signal of transmitted and received waves in the first embodiment of the present invention.

FIG. 4 is a block diagram showing the configuration of a device according to a second embodiment of the present invention.

FIG. 5 is a diagram showing the change in transmission frequency over time in the second embodiment of the present invention.

FIG. 6 shows the frequency characteristics of the mixed signal of the transmitted and received waves at the time indicated by the dashed line in FIG.

7 and 8 are graphs showing the time variations of the transmitting frequency and the receiving frequency in an example in which a continuous wave is transmitted in the second embodiment of the present invention.

FIG. 9 is a block diagram showing the configuration of a device according to a third embodiment of the present invention.

Figure 10 shows the time variation of the transmitted voltage in the third embodiment of the present invention.

FIG. 11 is a diagram showing the frequency characteristics of a mixed signal of transmitted and received waves in the third embodiment of the present invention.

FIG. 12 is a diagram illustrating the S/N improvement effect of the ultrasonic imaging method using continuous waves according to the present invention.

Figure 13 shows the dependence of lateral resolution and focal depth on center frequency in the present invention.

FIG. 14 shows the configuration of a fourth embodiment in which the ultrasonic imaging method of the present invention, which transmits continuous waves, is applied to a needle-shaped ultrasonic probe.

FIG. 15 is a cross-sectional view showing another configuration of the fourth embodiment of the present invention.

16 and 17 are diagrams illustrating an example of the operation of the needle-shaped ultrasonic probe according to the fourth embodiment of the present invention.

FIG. 18 is a diagram showing the schematic configuration of a fifth embodiment in which the present invention is used in a configuration other than a needle-shaped ultrasonic probe.

FIG. 19 is a diagram showing the underside of a sensor unit in the fifth embodiment of the present invention.

FIG. 20 is a plan view showing the configuration of an ultrasonic transducer divided into a plurality of blocks each consisting of a plurality of piezoelectric vibrators in a fifth embodiment of the present invention.

FIG. 21 is a diagram showing the attenuation characteristics of amplitude due to absorption by biological tissue.

Figure 22 shows an example of the configuration of an apparatus for measuring the dynamics of body fluids such as blood flow in a sixth embodiment of the present invention.

FIG. 23 is a diagram illustrating the relationship between the frequencies of transmitted and received waves in the sixth embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings. Note that throughout the drawings describing each embodiment, parts having the same function are designated by the same reference numerals, and repeated explanations will be omitted.

(First Embodiment) Figure 1 is a block diagram showing the general configuration of a continuous wave transmission/reception type ultrasound imaging device of the first embodiment. Figure 2 shows the time variation of the transmission voltage in the first embodiment. The transmission frequency is a sine wave with a constant frequency (f0). Figure 3 shows the frequency characteristics of the mixed signal of the transmission and reception waves in the first embodiment.

In Figure 1, the acoustic lens 3 of the ultrasonic transducer 10 is in direct contact with the subject, e.g., in vivo biological tissue 9. The piezoelectric vibrator 1 converts a continuous sine wave generated by the transmitter 20 into ultrasonic waves. The controller 21 controls the transmitter 20 so that the duration of the transmission by the ultrasonic transducer 10 is longer than the delay time. The ultrasonic waves propagating through the acoustic lens material 2 are converged by the acoustic lens 3, focused inside the subject, and reflected along the propagation path. The reflected waves are reconverted into voltage by the piezoelectric vibrator 1 and input to the differential amplifier 50, which serves as the receiver. 46 denotes a signal line for transmitting and receiving waves.

In the configuration shown in Figure 1, the focal point of the acoustic lens 3 has a focal depth defined by the ultrasonic intensity, i.e., the half-width of the sound pressure, and the reflected waves can be considered to be exclusively reflected waves from within the focal depth. Therefore, by mechanically scanning the ultrasonic transducer 10 in the X and Y directions within a plane perpendicular to the ultrasonic transmission and reception direction (the up-down (z) direction in Figure 1), a distribution image of the ultrasonic reflectivity of the plane 5 formed by the locus of the focal point of the acoustic lens 3 during scanning can be obtained.

In the configuration shown in Figure 1, transmitted waves from the transmitter 20 simultaneously enter the differential amplifier 50, which serves as the receiver, as well as the received waves. In the first embodiment, as shown in Figure 3, the frequencies of the transmitted and received waves are always the same. The transmitted waves are generally 100 to 1000 times louder than the received waves, interfering with the reception. In the configuration shown in Figure 1, the transmitted and received waves are equal in frequency, so the received waves are extracted as follows. Prior to measuring the ultrasonic reflectivity of the subject, the amplitude adjuster 30 and phase adjuster 40 are adjusted using a reference sample such as saline solution so that the output of the differential amplifier 50, which serves as the receiver, is zero. That is, a signal equivalent to the transmitted signal entering terminal A of the differential amplifier 50 is input to terminal B of the differential amplifier 50. By measuring the ultrasonic reflectivity of the subject, the reflected signal from within the living body can be detected exclusively as the output of the differential amplifier 50. In the configuration shown in Figure 1, the receiver detects the difference between a reference signal, which is an amplitude and phase adjusted version of the transmitted signal, and the output signal of the ultrasonic transducer.

The ultrasonic transducer 10 is mechanically scanned in the x and y directions, pausing at each scanning position for the required time to transmit and receive waves. The output of the differential amplifier 50 (the receiver) resulting from the transmitted and received waves is detected by the detector 60 and then integrated by the integrator 70 to achieve a sufficient S/N ratio. Furthermore, the frequency bandwidth of the differential amplifier 50 (the receiver) can be narrowed sufficiently to match the transmitted frequency, thereby reducing white noise. All ultrasonic signals reflected at the focal point of the acoustic lens, including those that are multiplexed and received later after reflection within the acoustic lens material 2, can be received, further improving the S/N ratio. The output of the integrator 70 is collected by the data acquisition device 80 and further processed by the signal processing device 90. The results of the signal processing are displayed on the display device 95.

(Second Embodiment) The first embodiment has the advantage of a simple configuration, but requires a sufficient dynamic range for the differential amplifier 50. Below, we will explain the second embodiment (Figure 4), in which the transmit and receive frequencies are always set differently, and transmission and reception are separated by frequency to ensure a sufficient dynamic range. Note that the configuration for setting the imaging plane based on the focal range and the effects of narrowing the receive frequency band to reduce white noise are all the same as in the first embodiment (Figure 1). Therefore, we will omit the details already explained in the first embodiment (Figure 1) and instead explain the transmission and reception separation method below.

Figure 4 is a block diagram showing the schematic configuration of a continuous wave transmission/reception type ultrasonic imaging device of the second embodiment, including a frequency modulator 15, a delay circuit 35 that applies a delay time t to the transmitted waves to generate a reference signal, and a lock-in amplifier (Lock-in-Amp) 55 that serves as a receiver. Instead of using a controller 21 to control the transmitter 20 so that the duration of the transmitted waves from the ultrasonic transducer 10 is longer than the delay time, the controller 21 may instead control the frequency modulator 15 so that the duration of the transmitted waves from the ultrasonic transducer 10 is longer than the delay time. Figures 5, 6, 7, and 8 are all diagrams illustrating examples of frequency modulation of the transmitted waves to separate the transmitted and received signals based on frequency. Figure 5 shows the time variation of the transmission frequency in the second embodiment, and Figure 6 shows the frequency characteristics of the mixed signal of the transmitted and received waves at the time indicated by the dashed line in Figure 5. Figures 7 and 8 show examples of ultrasound imaging operation using continuous waves transmitted in the second embodiment.

In the second embodiment, the ultrasonic transducer transmits waves using frequency modulation to provide a rectangular wave alternating between two frequencies, f1 and f2, which are located around the transducer's resonant frequency (center frequency), f0. In the second embodiment, if the time difference (delay time) between the application of a signal voltage to piezoelectric vibrator 1 and the time the ultrasonic wave reflects from the focal point and reaches piezoelectric vibrator 1 is t, the frequency modulation alternating period is set to 2t.

That is, f1 and f2 alternate continuously for a period of time t (delay time). In the second embodiment, when the transmitted frequency is f1, the received frequency is always f2, and conversely, when the transmitted frequency is f2, the received frequency is always f1. As shown in Figure 6, since the transmitted and received frequencies are always different, a delay time t is added to the transmitted signal to use it as a reference signal, and lock-in detection is performed on the mixed transmitted and received signals, allowing the reflected signal to be detected exclusively.

For example, if frequency f0 is 400 MHz and the difference between frequencies f1 and f2 is approximately 1 MHz, f1 and f2 can be sufficiently separated, and there can be little difference between the images generated by frequencies f1 and f2. The frequency modulation of the transmitted wave can be a sinusoidal wave instead of a square wave, but this can lead to problems with poor separation between the transmitted and received waves because the transmitted and received frequencies are close together during alternation.

Figures 5 and 6 show the simplest examples, but in general, if the alternating period of the transmitted wave frequency or the period of the periodic change in the transmitted wave is T and the delay time is t, then T = 2t/(2n-1) (n is a natural number), and the receiver references the transmitted wave signal delayed by (2n-1)T/2, or a time equal to t (the delay time), to achieve exactly the same effect as in the first embodiment.

Figure 7 is another example of frequency modulation of the transmitted wave, a graph showing how the frequency of the mixed signal of the transmitted and received waves changes over time. Even in the case of frequency modulation in which the transmitted wave frequency is continuously changed (Figure 7), a delay time t is added to the transmitted wave to create a reference signal, and lock-in detection of the mixed signal of the transmitted and received waves can be used to exclusively detect the reflected signal. The time variation of the reference signal frequency is the same as the time variation of the received wave frequency shown in Figure 7.

The example of frequency modulation of transmitted waves shown in Figure 7 has the advantage that the period of the frequency modulation of transmitted waves does not need to be set to match the delay time t, making it easier to set the frequency of the transmitted waves. The example of frequency modulation of transmitted waves shown in Figure 7 can also be modified to modulate the transmitted waves by changing the frequency in a step-like manner, as shown in Figure 8. The example of frequency modulation of transmitted waves shown in Figure 8 has the advantage of facilitating lock-in detection. In the example of frequency modulation of transmitted waves shown in Figure 8, the duration of each frequency of the transmitted and received waves must be shorter than the delay time t to prevent the transmitted and received waves from matching. The time change in the frequency of the reference signal with the delay time t added to the transmitted wave is the same as the time change in the received wave frequency shown in Figure 8.

(Third Embodiment) Figure 9 is a block diagram showing the general configuration of a continuous wave transmission-reception type ultrasonic imaging device according to the third embodiment. 8 denotes a piezoelectric transducer with a low resonant frequency, and 56 denotes a notch filter (receiver). The controller 21 controls the transmitter 20 so that the duration of transmission by the ultrasonic transducer 10 is longer than the delay time.

In the third embodiment, waves are continuously transmitted at a single frequency f0, and the ultrasonic transducer 10 is vibrated at a frequency sufficiently lower than f0. In the third embodiment, the low-frequency vibration of the ultrasonic transducer 10 causes a Doppler shift in the reflected waves, causing the frequency of the reflected waves to deviate from the frequency of the transmitted waves. Because the frequency of the reflected waves deviates from the frequency of the transmitted waves, as shown in Figure 9, a notch filter 56 using the transmitted wave signal as a reference signal can be used to remove the transmitted wave frequency and separate the transmitted waves.

Figure 10 shows the time variation of the transmitted voltage in the third embodiment, where the transmitted frequency is a sine wave with a constant frequency (f0). Figure 11 shows the frequency characteristics of the mixed signal of transmitted and received waves in the third embodiment, showing how the received frequency shifts due to Doppler shift and is distributed around the transmitted frequency f0.

In the case of a needle-shaped ultrasonic probe, the needle has a natural vibration, which can be used to generate low-frequency vibrations of the ultrasonic transducer 10. Sound waves transmitted from the ultrasonic transducer into the subject and reflected at the focal point undergo a Doppler shift, but sound waves that are not transmitted into the subject and are multiply reflected only within the acoustic lens material 2 (which becomes noise in the third embodiment) do not undergo a Doppler shift. Therefore, the advantage of using the Doppler shift is that multiply reflected sound waves (noise) can be removed by the notch filter 56, just like transmitted waves.

Next, we briefly consider the S/N improvement effect of the present invention by applying the method of the present invention to a needle-shaped ultrasound probe. For simplicity, we ignore the time required for signal detection, integration, A/D conversion, and transfer to a data acquisition device, and consider only the signal transmission and reception time. We assume that a needle-shaped ultrasound probe captures an image of 100 x 100 = 10,000 pixels in 1 to 2 minutes.

That is, the needle-shaped ultrasonic probe mechanically scans 100 x 100 = 10,000 points in the X and Y directions, transmitting and receiving waves at each of the 100 x 100 = 10,000 points, and capturing signals. Assuming one screen takes 100 seconds to capture, the time available for transmitting and receiving waves per point is 10 msec. Even with conventional pulse-transmitting methods, the S/N ratio can be improved by pausing the needle-shaped ultrasonic probe's scanning for a certain period of time, repeating transmission and reception, and adding up the received waves.

However, because transmission and reception are separated in time, the next transmission cannot be performed until reception is complete. Due to signal delays caused by reception tailing and multiple reflections, assuming a delay time t of 1 μsec, the transmission interval must be at least twice that, or approximately 2 μsec. Therefore, with conventional methods, the maximum number of transmitted and received waves within a 10-msec period is 5,000.

On the other hand, in the present invention, for example, when the transmission frequency is 400 MHz, the period is 2.5 nsec, so the number of waves that can be transmitted in 10 msec is 4,000,000.

If we approximate the S/N ratio as proportional to the square root of the wavenumber, then compared to the limit value of the conventional method, the S/N ratio improvement is approximately 30 times (√(4,000,000/5,000) = √(800) = 28.3). With the conventional method, 5,000 additions is not realistic, and the maximum number of additions is usually around 100. Compared to 100 additions, the S/N ratio improvement is 200 times (√(4,000,000/100) = 200). With the present invention, the higher the transmission frequency, the greater the number of waves per unit time, resulting in a greater improvement in the S/N ratio.

FIG. 12 is a diagram illustrating the S/N improvement effect of the ultrasonic imaging method using continuous waves according to the present invention.

In Figure 12, the horizontal axis represents the resonant frequency (center frequency) f of the ultrasonic transducer, and the vertical axis represents the S/N ratio. The S/N ratio when the number of transmitted and received waves is 1 (S/N = 1) is used as the reference (S/N = 10 for 100 additions), the limit of the conventional method (S/N = √5000 = 70.7), and the present method (the method of the present invention). In the conventional method, the S/N ratio is independent of the transmission frequency, but in the present invention, the S/N ratio increases with the transmission frequency.

In the present invention, as described above, the depth of the imaging plane (distance from the acoustic lens) is determined by the focal length of the acoustic lens, and the thickness of the imaging plane is determined by the focal depth d of the acoustic lens. Using the sound speed v, the center frequency f, and the lens F-number (the ratio of the focal length to the diameter of the acoustic lens), the lateral resolution is calculated from r = F(v/f), and the focal depth is calculated from d = ^2F2 (v/f).

FIG. 13 shows the dependence of the lateral resolution r and the focal depth d on the center frequency in the present invention (assuming v = 1500 m/s and F = 1).

As can be seen from Figure 13, at a center frequency of 400 MHz, the lateral resolution r is approximately 4 μm and the focal depth d is 7.5 μm. Compared to tissue sections prepared in biopsies, the imaging plane thickness, defined by the focal depth d, is slightly thicker but still quite practical. Of course, the imaging plane thickness can be reduced by using an acoustic lens with a smaller F-number.

(Fourth Embodiment) Next, we will describe a fourth embodiment in which the continuous wave ultrasound imaging method of the present invention is applied to a needle-shaped ultrasound probe. The present invention is not limited to needle-shaped ultrasound probes; it can also be applied to round-tipped rod-shaped ultrasound probes, and can also be applied to completely different device configurations, such as ultrasound microscopes.

Figure 14 is a block diagram showing the schematic configuration of a fourth embodiment in which the continuous wave ultrasonic imaging method of the present invention is applied to a needle-shaped ultrasonic probe. Figure 14 shows a cross-sectional view of the needle-shaped ultrasonic probe in a state in which the inner needle 6, equipped with an ultrasonic transducer 10, is exposed from the tip of the outer needle 7 inside biological tissue 9.

The ultrasonic transducer 10 consists of a piezoelectric vibrator 1 made of zinc oxide (ZnO) sandwiched between gold (Au) electrodes, a sapphire acoustic lens material 2, and an acoustic lens 3 with an F-number of 1. The resonant frequency of the piezoelectric vibrator 1 is, for example, 400 MHz. The acoustic lens 3 is in direct contact with the biological tissue 9. The inner needle 6 is moved by the drive unit 100 in both the rotational and axial directions around the needle axis, as shown by the arrows in Figure 14. As the needle moves, the focal point of the acoustic lens 3 traces a cylindrical trajectory within the biological tissue 9, and this cylindrical trajectory is the imaging surface (curved surface) 5. The control unit 105 controls the mechanical scanning of the inner needle 6 of the ultrasound probe by the drive unit 100, as well as the timing of ultrasonic transmission and reception. The control device 105 controls the wave transmitter 20 so that the duration of the wave transmission by the ultrasonic transducer 10 is longer than the delay time, but the wave transmitter 20 or the frequency modulator 15 may be controlled by a controller 21 (not shown).

The wave transmitting/receiving section, which is the main part of the present invention, may use any of the configurations of the first, second, and third embodiments. The configuration shown in Figure 14 uses a configuration similar to that of the second embodiment (Figures 5 and 6), but also shows an example configuration with the addition of new functions. In the configuration of the second embodiment (Figures 5 and 6), the difference between the alternating transmitted wave frequencies f1 and f2 is assumed to be approximately 1 MHz, and images are obtained using the reflected signals from the transmitted waves of frequencies f1 and f2, assuming that there is almost no difference between the images generated by frequency f1 and frequency f2.

In the example shown in Figure 14, for example, frequency f1 is 350 MHz and frequency f2 is 450 MHz, intentionally increasing the frequency difference, and images at frequency f1 and images at frequency f2 are stored in separate data acquisition devices 81 and 82. The acoustic properties (reflectivity, absorption rate, speed of sound, etc.) of biological tissue 9 generally vary with frequency, and the frequency-dependent differences in acoustic properties are thought to differ depending on the nature of the tissue.

Therefore, the difference image between the image at frequency f1 and the image at f2 accentuates the distribution of acoustic properties of biological tissue 9, more clearly showing the tissue properties. In the fourth embodiment, the image processing device 90 applies corrections to the image data at f1 and the image data at f2 as needed (the higher the transmission frequency, the weaker the received signal, so the image data is converted to image data at either frequency f1 or f2), and can render the difference image between the image at f1 and the image at f2. The image at f1, the image at f2, and the difference image are displayed on the display device 95. Of course, the image at f1 and the image at f2 can be displayed separately, or a simple additive image of the image at f1 and the image at f2 can be obtained and displayed as needed.

In the fourth embodiment, the depth of the imaging plane (distance from the acoustic lens) is determined by the focal length of the acoustic lens 3, and the thickness of the imaging plane is determined by the focal depth d of the acoustic lens. Therefore, in the fourth embodiment, the imaging plane is fixed, and it is not possible to image multiple planes in the depth direction to construct a three-dimensional image. This is inferior to conventional methods that allow multiple imaging planes to be set by time gating.

Figure 15 is a cross-sectional view showing another configuration of the fourth embodiment. For simplicity, only the cross section of the probe is shown in Figure 15; the rest of the configuration is the same as that shown in Figure 14. The configuration shown in Figure 15 allows imaging of multiple planes in the depth direction to construct a three-dimensional image. Multiple ultrasonic transducers (10, 10', 10"), each with an acoustic lens having a different focal length, are arranged on the side of the inner needle 6 near its tip. Each ultrasonic transducer images a field of view that is nearly overlapping with each other, obtaining images of multiple imaging planes (5, 5', 5") at slightly different depths. 45 denotes a bundle of three signal lines for transmitting and receiving waves. Each signal line in the signal line bundle 45 corresponds to signal line 46 (Figs. 1, 3, 9, and 14), and each signal line in the signal line bundle 45 is connected to a transmitting/receiving circuit (21, 20, 30, 40, 50 (Fig. 1); 21, 20, 15, 35, 55 (Fig. 3); 21, 20, 56 (Fig. 9); 20, 15, 35, 55 (Fig. 14)).

Using multiple ultrasonic transducers, it is possible to obtain three-dimensional images comparable to those obtained with conventional methods. Of course, if the focal lengths of each ultrasonic transducer (10, 10', 10") are the same, the imaging time for the same imaging plane can be shortened. When using multiple ultrasonic transducers, mutual interference between the transducers can be a problem. However, the imaging device of the present invention easily solves this problem. In other words, with conventional imaging methods, each transmitted and received wave is a pulse wave with a wide frequency band, making it difficult to prevent mutual interference while simultaneously transmitting and receiving waves.

In the fourth embodiment, the transmitting and receiving frequencies of each ultrasonic transducer can be easily set to different frequencies at any given time.

Figures 16 and 17 show a method for varying the frequencies of transmitted and received waves at each ultrasonic transducer (10, 10', 10") and illustrate the time variation of the transmitted and received frequencies. Figure 16 shows an example of continuous frequency modulation at different frequencies, and Figure 17 shows an example of frequency modulation by alternating each transmitted wave between different frequencies. In Figure 16, frequency modulation is repeated with a period T, but if the period T is sufficiently large compared to the delay time t, it does not affect the separation of transmitted and received waves. In Figures 16 and 17, solid lines indicate the timing of transmission at each ultrasonic transducer (10, 10', 10"). Dotted lines indicate the timing of reception, in which a delay time t is added to the transmitted wave to use it as a reference signal, and lock-in detection is performed on the mixed transmitted and received signals to exclusively detect the reflected signal. In Figures 16 and 17, the change in frequency of the received wave over time is the same as the change in frequency of the reference signal, which is the transmitted wave with a delay time t.

(Fifth Embodiment) Figure 18 shows the schematic configuration of a fifth embodiment in which the present invention is applied to a configuration other than a needle-shaped ultrasonic probe. This is an example in which an ultrasonic imaging method that transmits continuous waves is applied to a secondary sensor. For simplicity, only the probe is shown in Figure 18; the rest of the configuration is the same as that shown in Figure 14.

Figure 18 shows the external view of an ultrasound probe. The sensor unit 12, which has two-dimensionally mounted ultrasound transducers, is brought into contact with the surface of biological tissue 9, e.g., the body surface, to obtain an image of a plane below the body surface (imaging plane 5). Reference numeral 45 denotes a bundle of signal lines. Each signal line in the bundle of signal lines 45 corresponds to signal line 46 (Figures 1, 3, 9, and 14), and each signal line in the bundle of signal lines 45 is connected to a circuit for transmitting and receiving waves (21, 20, 30, 40, 50 (Figure 1); 21, 20, 15, 35, 55 (Figure 3); 21, 20, 56 (Figure 9); 20, 15, 35, 55 (Figure 14)).

Figure 19 shows the underside of the sensor unit in the fifth embodiment, illustrating the two-dimensional arrangement of the acoustic lenses 3 of the ultrasonic transducers 10. Figure 19 illustrates an example in which 50 x 50 = 2,500 ultrasonic transducers 10, each with an acoustic lens 3 of 1 mm diameter, are arranged in two perpendicular directions. Each acoustic lens 3 has an equal focal length, e.g., 2 mm, which defines the imaging plane 5 (Figure 18). It would be difficult to have all 2,500 ultrasonic transducers shown in Figure 19 transmit and receive at different frequencies.

FIG. 20 is a plan view showing the configuration of an ultrasonic transducer divided into a plurality of blocks each consisting of a plurality of piezoelectric vibrators in the fifth embodiment.

Since it would be difficult to have all 2,500 ultrasonic transducers transmit and receive at different frequencies, the system is divided into 5 x 5 = 25 blocks 13, each consisting of 10 x 10 = 100 ultrasonic transducers, as shown in Figure 20. Each block 13 measures 10 mm x 10 mm. If the frequency band to be used is 100 MHz to 200 MHz, frequencies different by 500 kHz can be assigned to the transmit and receive signals of the 100 ultrasonic transducers in one block, using a method similar to that used for the transmit and receive signals described in Figure 17.

The frequencies assigned to the 100 ultrasonic transducers in one block are all different. When assigning frequencies to the 100 ultrasonic transducers in each block, by making the relationship between the arrangement of the 100 ultrasonic transducers and the frequency assignments identical within each block (i.e., making the arrangement of the frequencies assigned to the 100 ultrasonic transducers in each block identical), the distance between ultrasonic transducers using the same frequency in adjacent blocks is at least 10 mm. In Figure 20, the circles in each block 13 indicate positions where a frequency f0 in the range of 100 MHz to 200 MHz is assigned, and these positions are spaced at 10 mm intervals, the same as one side of the block.

Indicating the position of the ultrasonic transducer Taking into account absorption by biological tissue 9, mutual interference between a specific ultrasonic transducer and an ultrasonic transducer more than 10 mm away is negligibly small compared to the signal from the focal point of an ultrasonic transducer 2 mm away from the specific ultrasonic transducer. In the fifth embodiment, the field of view is 50 mm x 50 mm, but the field of view can be expanded by increasing the number of blocks 13.

In the fifth embodiment, although the lateral resolution is limited by the array density of the ultrasonic transducers, it has the advantage of being able to capture images in a short time without the need for mechanical scanning of the sensor.

(Sixth Example) Next, we will explain an example of applying the present invention to a method for measuring the dynamics of bodily fluids (fluids), such as blood flow, inside an object being examined. When an object approaches an ultrasonic transducer at a velocity v, the relationship between the transmitted frequency F1 and the Doppler-shifted received frequency F2 is given by F2 = F1 (1 + (2v/c)), where c is the speed of sound and Df is the Doppler transition frequency. The blood flow velocity in capillaries is approximately v = 1 mm/sec. Assuming typical values of F1 = 400 MHz and c = 1500 m/sec, Df = 530 Hz. In other words, when the frequency of the received wave differs from the transmitted frequency before the delay time t, it is clear that a fluid with a certain flow velocity (blood flow or extracellular fluid flow) exists at the focal point of the transmitted wave.

Figure 22 shows an example configuration of an apparatus for measuring dynamics of bodily fluids, such as blood flow, in the sixth embodiment. In Figure 22, the lock-in detector (receiver) 55 used for detection has a frequency bandwidth of approximately 2 kHz. After lock-in detection of the received signal to increase the S/N ratio, the frequency is measured with a high-precision frequency analyzer 71, allowing the blood flow velocity (flow rate) at the measurement location to be detected. Note that the configuration shown in Figure 22 further adds a frequency analyzer 71 and a signal processing device 91 to the configuration of the second embodiment (Figure 4). The controller 21 controls the transmitter 20 or frequency modulator 15 to lengthen the duration of transmission by the ultrasonic transducer 10 compared to the delay time.

Figure 23 illustrates the relationship between the frequencies of transmitted and received waves in a method for measuring the dynamics of bodily fluids, such as blood flow. In the sixth embodiment, the transmitted and received frequencies are always set to differ by approximately 1 MHz, so frequency shifts of approximately Df = 530 Hz are not a problem. Therefore, when the transmitted wave is received within the bandwidth of a delayed reference signal, and the received wave frequency differs from the transmitted wave frequency before the delay time t, the presence of flow velocity (blood flow or extracellular fluid flow) is detected at the focal position of the transmitted wave. The time range in which flow velocity is detected is shown in Figure 23.

The detected flow velocity information is converted into color information, etc., using a known pseudocolor method by a signal processing device 91, separate from the signal processing device 90 shown in Figure 22, and displayed on a display device 95, superimposed on the image obtained on the imaging surface 5.

The above explanation has been given using the wave transmission and reception shown in Figure 7 of the second embodiment as an example. However, even if the wave transmission and reception shown in Figures 5 and 8 of the second embodiment is applied, the dynamic state of body fluids such as blood flow can be similarly detected.

Although the present invention has been specifically described with reference to the examples, it goes without saying that the present invention is not limited to the examples and can be modified in various ways without departing from the spirit and scope of the present invention.

The effects obtained by the typical configuration of the present invention will now be briefly described.

(1) The use of continuous waves significantly increases the number of transmitted and received waves, improving the signal-to-noise ratio.

For example, an approximate calculation using a 400 MHz needle-shaped ultrasonic probe shows that the S/N improvement effect is approximately 30 times greater than the limit value achieved by conventional methods.

(2) The receiving frequency band can be narrowed, and time-delayed signals can be received, further improving the S/N ratio.

(3) The use of narrow-band ultrasound eliminates chromatic aberration and improves lateral resolution.

(4) Differences in acoustic characteristics at different frequencies can be easily visualized.

(5) Even when multiple ultrasonic transducers are used for purposes such as obtaining a three-dimensional image, the frequencies of the transmitted and received waves of each transducer can be set to different values, preventing mutual interference.

Finally, we summarize the explanations of the symbols used in each figure. Reference numeral 1 denotes a piezoelectric transducer, 2 denotes an acoustic lens material, 3 denotes an acoustic lens, 5, 5', and 5" denote an imaging surface, 6 denotes an inner needle, 7 denotes an outer needle, 8 denotes a piezoelectric transducer with a low resonant frequency, 9 denotes biological tissue (subject), 10 denotes an ultrasonic transducer, 12 denotes a sensor unit, 13 denotes a block consisting of multiple piezoelectric transducers, 15 denotes a frequency modulator, 20 denotes a transmitter, 21 denotes a controller, 30 denotes an amplitude adjuster, 35 denotes a delay circuit, 40 denotes a phase adjuster, 45 denotes a signal line bundle, 46 denotes a signal line, 50 denotes a differential amplifier, 55 denotes a lock-in amplifier, 56 denotes a notch filter, 60 denotes a detector, 70 denotes an integrator, 71 denotes a frequency analyzer, 80, 81, and 82 denote data acquisition devices, 90 denotes a signal processing device, 91 denotes a signal processing device, 95 denotes a display device, 100 denotes a drive device, and 105 denotes a control device.

───────────────────────────────────────────────────── (Note) This publication is based on the publication published internationally by the International Bureau of Patents (WIPO). The effect of the international publication of the Japanese patent application (Japanese utility model registration application) related to this publication arises pursuant to Article 184-10, Paragraph 1 of the Patent Act (Article 48-13, Paragraph 2 of the Utility Model Act) and is unrelated to this publication.

Claims (1)

[Claims] 1. A continuous wave transmission-receive ultrasonic imaging device comprising a piezoelectric vibrator (1) and an acoustic lens (3) for focusing ultrasonic waves, at least one ultrasonic transducer (10) for transmitting and receiving ultrasonic waves to and from an object to be inspected (9), a transmitter (20) for providing a transmission signal to the ultrasonic transducer, a receiver (50, 55, 56) for receiving ultrasonic waves reflected from the object to be inspected, a scanning mechanism for scanning the ultrasonic transducer on a plane or curved surface perpendicular to the propagation direction of the ultrasonic waves transmitted and received by the ultrasonic transducer, and a controller (21, 105) for controlling the duration of the ultrasonic transducer's transmission so that it is longer than the delay time, which is the time difference between when the piezoelectric vibrator is excited by the transmission signal and when the ultrasonic waves generated by the excitation propagate to the focal length of the acoustic lens, are reflected, and then propagate back to the piezoelectric vibrator and are converted into voltage. 2. The continuous wave transmit-receive ultrasonic imaging device of claim 1, wherein the ultrasonic waves transmitted by the ultrasonic transducer are sine waves whose transmission frequency periodically alternates between at least two different, discontinuous frequencies, the alternating period of the sine wave frequency being equal to 2/(2n-1) times the delay time, where n is a natural number, and the receiver receives the reflected ultrasonic waves by referring to the transmission signals. 3. The continuous wave transmit-receive ultrasonic imaging device of claim 2, further comprising signal collecting means (81, 82) equal to the number of the different transmission frequencies, and collecting the received signals obtained for each different transmission frequency in separate signal collecting means. 4. The continuous wave transmit-receive ultrasonic imaging device according to claim 2, further comprising signal acquisition means (81, 82) equal in number to the number of said different transmit frequencies, means (90) for acquiring received signals obtained for each different transmit frequency in a separate signal acquisition means, and means (90) for constructing a difference image between images obtained at said different transmit frequencies using image data individually obtained using received signals obtained for each of said different, discontinuous transmit frequencies. 5. The continuous wave transmit-receive ultrasonic imaging device of claim 1, characterized in that the ultrasonic waves transmitted by the ultrasonic transducer are sinusoidal waves whose transmission frequency periodically alternates between at least two different, discontinuous frequencies, the alternating period of the sinusoidal wave frequency being equal to 2/(2n-1) times the delay time, where n is a natural number, and the receiver receives the reflected ultrasonic waves by referencing the transmission signal delayed by either (2n-1)/2 times the alternating period or a time equal to the delay time. 6. The continuous wave transmit-receive ultrasonic imaging device of claim 5, characterized in that it has signal collection means (81, 82) equal to the number of the different transmission frequencies, and collects the received signals obtained for each different transmission frequency in separate signal collection means. 7. The continuous wave transmit-receive ultrasonic imaging device of claim 5, further comprising: a signal collecting means (81, 82) equal in number to the number of the different transmit frequencies; a means (90) for collecting received signals obtained for each of the different transmit frequencies in a separate signal collecting means; and a means for constructing a difference image between images obtained at the different transmit frequencies using image data obtained individually using received signals obtained for each of the different, discontinuous transmit frequencies. 8. The continuous wave transmit-receive ultrasonic imaging device of claim 1, further comprising: a sine wave whose transmit frequency changes continuously and periodically; a period during which the sine wave frequency changes, where n is a natural number, equal to 2/(2n-1) times the delay time; and a receiver for receiving the reflected ultrasonic waves by referring to the transmit signals. 9. The continuous wave transmit-receive ultrasonic imaging device of claim 1, wherein the ultrasonic waves transmitted by the ultrasonic transducer are sine waves whose transmission frequency changes continuously and periodically, the period during which the sine wave frequency changes is equal to 2/(2n-1) times the delay time, where n is a natural number, and the receiver receives the reflected ultrasonic waves by referring to the transmission signal delayed by (2n-1)/2 times the alternating period or a time equal to the delay time. 10. The continuous wave transmit-receive ultrasonic imaging device of claim 1, wherein the ultrasonic waves transmitted by the ultrasonic transducer are sine waves whose transmission frequency changes continuously, and the receiver receives the reflected ultrasonic waves by referring to the transmission signal delayed by a time equal to the delay time. 11. The continuous wave transmit-receive ultrasonic imaging device of claim 1, wherein the ultrasonic waves transmitted by the ultrasonic transducer are sinusoidal waves whose transmission frequency changes discontinuously at regular intervals, the duration of the regular frequency being shorter than the delay time, and the receiver receives the reflected ultrasonic waves by referring to the transmission signal delayed by a time equal to the delay time. 12. The continuous wave transmit-receive ultrasonic imaging device of claim 11, further comprising signal collection means (81, 82) equal to the number of the different transmission frequencies, and wherein the received signals obtained for each different transmission frequency are collected in separate signal collection means. 13. The continuous wave transmit-receive ultrasonic imaging device of claim 11 further comprises signal collecting means (81, 82) equal in number to the number of the different transmit frequencies, collecting received signals obtained for each of the different transmit frequencies in separate signal collecting means, and means (90) for constructing a difference image between images obtained at the different transmit frequencies using image data individually obtained using received signals obtained for each of the different, discontinuous transmit frequencies. 14. The continuous wave transmit-receive ultrasonic imaging device of claim 1 further comprises signal collecting means (81, 82) equal in number to the number of the different transmit frequencies, collecting received signals obtained for each of the different, discontinuous transmit frequencies, and means (90) for constructing a difference image between images obtained at the different transmit frequencies. 15. The continuous wave transmit-receive ultrasonic imaging device of claim 14, further comprising: a number of signal acquisition means (81, 82) equal to the number of different transmit frequencies; and a received signal obtained for each different transmit frequency is acquired in a separate signal acquisition means. 16. The continuous wave transmit-receive ultrasonic imaging device of claim 14, further comprising: a number of signal acquisition means (81, 82) equal to the number of different transmit frequencies; a number of received signals obtained for each different transmit frequency are acquired in a separate signal acquisition means; and a means (90) for constructing a difference image between images acquired at each of the different transmit frequencies using image data individually obtained using the received signals obtained for each of the different, discontinuous transmit frequencies. 17. The continuous wave transmission-reception ultrasonic imaging device of claim 1, characterized in that the ultrasonic transducer transmits ultrasonic waves as sine waves with a constant transmission frequency, the ultrasonic transducer has a mechanism that operates in a direction parallel to the propagation direction of the ultrasonic waves transmitted and received by the ultrasonic transducer, and the receiver receives the reflected ultrasonic waves by referring to the transmission signal. 18. The continuous wave transmission-reception ultrasonic imaging device of claim 1, characterized in that the ultrasonic transducer transmits ultrasonic waves as sine waves with a constant transmission frequency, and the receiver detects the difference between a reference signal obtained by adjusting the amplitude and phase of the transmission signal and the output signal of the ultrasonic transducer. 19. The continuous wave transmission-reception ultrasonic imaging device of claim 1, characterized in having a sensor unit (12) in which the ultrasonic transducer is mounted two-dimensionally. 20. The continuous wave transmission-receive ultrasonic imaging device of claim 1, further comprising: a sensor unit (12) in which the ultrasonic transducers are mounted two-dimensionally; the sensor unit is divided into a plurality of blocks (13) each consisting of a predetermined number of ultrasonic transducers; and the transmission and reception frequencies of the ultrasonic transducers within each block are different. 21. The continuous wave transmission-receive ultrasonic imaging device of claim 1, further comprising means (55, 71, 91) for detecting the flow velocity of the fluid within the object of inspection from the signal received by the receiver. 22. A continuous wave transmission-receive ultrasonic probe for use in the continuous wave transmission-receive ultrasonic imaging device of claim 1, wherein the continuous wave transmission-receive ultrasonic probe is rod-shaped or puncture needle-shaped (6), the propagation direction of the ultrasonic waves transmitted and received by the ultrasonic transducer is perpendicular to the axis of the continuous wave transmission-receive ultrasonic probe, and the needle- or rod-shaped continuous wave transmission-receive ultrasonic probe scans the ultrasonic transducer in the axial direction or rotational direction around the axis while in close contact with the surface of a living body or inserted or pierced into the living body, thereby measuring biological tissue on a cylindrical surface around the axis defined by the locus of the focal point of the acoustic lens. 23. A continuous wave transmission-receive ultrasonic probe as defined in claim 22, wherein the transmission and reception frequencies of each ultrasonic transducer are different. 24. The continuous wave transmission-receive ultrasonic probe according to claim 24, wherein the ultrasonic transducers are multiple, the acoustic lenses of the ultrasonic transducers each having a different focal length, and the fields of view of the ultrasonic transducers in the axial direction and the rotational direction around the axis overlap. 25. A continuous wave transmission-receive ultrasonic imaging device comprising: an ultrasonic transducer (10) including a piezoelectric vibrator (1) and an acoustic lens (3) for focusing ultrasonic waves; transmitting and receiving ultrasonic waves to and from an object to be inspected (9); a transmitter (20) for providing a transmission signal to the ultrasonic transducer; receivers (50, 55, 56) for receiving ultrasonic waves reflected from the object to be inspected; means for scanning the ultrasonic transducer in a predetermined direction; and a controller (21, 105) for maintaining the duration of the transmission of the ultrasonic transducer longer than the time difference between when the piezoelectric vibrator is excited by the transmission signal and when the ultrasonic waves generated by the excitation propagate to the focal length of the acoustic lens, are reflected, and then propagate back to the piezoelectric vibrator and are converted into voltage. 26. The continuous wave transmit-receive ultrasonic imaging device according to claim 25, wherein the ultrasonic transducer transmits a sine wave whose frequency alternates periodically between two different, discontinuous frequencies, the alternating period of the sine wave frequency being equal to twice the time difference, and the receiver receives the reflected ultrasonic waves by referencing the transmitted signal delayed by half the alternating period or a time equal to the time difference.