Development of ultrasonic transducer technology

(2) Piezoelectric single crystal transducers: Nomura began research on piezoelectric single crystal materials in 1969, in the 1990s. Mid-term piezoelectric single crystal materials have attracted wide attention from researchers due to their excellent piezoelectric properties. At present, piezoelectric single crystal transducers are excellent research hotspots after composite transducers. For example, a new type of relaxed ferroelectric single crystal transducer represented by lead lanthanum zinc citrate-lead titanate and lead bismuth silicate-lead titanate has much higher piezoelectric coefficient and electromechanical coupling coefficient than the PZT ceramic material. The transducer array designed with piezoelectric single crystal material has much higher sensitivity and bandwidth than the piezoelectric ceramic replacement device. In 1999, Toshiba Corporation of Japan developed the 3.5MHZ PZNT91/9 ultrasonic transducer, which achieved high resolution and strong penetrating power, and was applied and clinically. In 2003,  the University of Southern California developed a high-frequency but elemental piezoelectric crystal transducer made of lithium tantalate material, which obtained a good penetration depth and image signal-to-noise ratio. However, the single crystal growth process is much more complicated than the ceramic preparation process. At present, it is not possible to produce piezoelectric single crystals at a price comparable to ceramics, and only a small number of transducers made of piezoelectric single crystals are applied and clinically.

2, Broadband transducer: early marked on the ultrasound probe such as 2.5, 3.5, 5, 7, 10MHz, etc. The operating frequency of piezoelectric cylinder component generally refers to its center frequency, its bandwidth is about 1MHz, this type of probe can be called single center frequency narrow band. The transducer is still private for a long time, and it has a large loss of high-frequency signal to the deep tissue echo, which affects the clarity and sensitivity of the ultrasound pattern. In the mid-1980s, based on the attenuation law of ultrasound in biological tissues and its influence on ultrasound images, a wide-band transducer was developed, such as a transducer with a center frequency of 3.5 MHz and an effective bandwidth of about 3 MHz. Superficial tissue uses high frequency to improve resolution, while deep tissue uses low frequency to form less attenuated echo signals, resulting in a clearer image display of deep tissue structures.In the 1990s, variable-frequency broadband transducers and ultra-wideband transducers were used in clinical diagnostics. Harmonic imaging technology is widely used in clinical practice, is also an imaging technology developed on the basis of broadband transducers. Since the broadband transducer can receive multiple harmonics generated by the incident ultrasound in the foundation of the tissue, it contains a large amount of human body information, can improve the axial resolution of the image, and can improve the sensitivity of the ultrasound imaging system.

3, Three-dimensional ultrasound imaging transducer: Compared with traditional two-dimensional ultrasound imaging, three-dimensional ultrasound imaging has the advantages of intuitive image display, accurate measurement of the volume and area of the target, and time required to shorten the diagnosis of the physician. Ultrasound imaging has been the focus of current applications and development. At present, there are mainly two methods for acquiring three-dimensional ultrasound images. One is to obtain a series of two-dimensional ultrasonic images with known spatial positions by using the existing one-dimensional phased line array, and then perform three-dimensional reconstruction on the images to obtain two-dimensional images mainly through mechanically driven scanning and magnetic field space. positioning scanning method. The mechanical drive scanning method is to obtain a two-dimensional image by fixing the transducer on a computer-controlled mechanical arm for fan-sweeping or rotating scanning. Due to complicated equipment and high technical requirements, the method of Pzt piezo crystals is currently used less; magnetic field spatial positioning .The scanning method is to fix the magnetic field position sensor on the conventional ultrasonic transducer, and measure the change of the spatial position of the transducer during the sampling operation; the random scanning can be performed like a conventional probe, and the motion track of the computer sensing probe is sampled. The method is flexible in operation and can perform a wide range of scanning. The disadvantage is that the system must be calibrated before each use, and the scanning process must be even and slow, which is greatly affected by human factors. In addition, the existing one-dimensional linear array transducer is composed of a plurality of small elements in one dimension, and electronic focusing in the imaging plane can be achieved. However, there is only one array element in a spatial position with a certain thickness from the imaging plane, and electronic focusing cannot be realized. In the future, three-dimensional reconstruction is realized, and the focus is usually achieved by using an acoustic lens in the thickness direction of the imaging plane, but the focus is fixed due to the focus of the lens. At the same time, the reconstruction of the three-dimensional image by the two-dimensional image is too long, and the resolution of the three-dimensional image is often lower than that of the two-dimensional image. Since the two-dimensional images are acquired at different times, the reconstructed three-dimensional images are difficult to realize real-time display of living tissues and organs. The piezo ceramic sensor is to use the two-dimensional area array probe to control the ultrasonic beam to focus in the three-dimensional space deflection direction, obtain real-time three-dimensional spatial data, and then reconstruct the three-dimensional image.