3-Dimensional Acoustic Imaging by Multifrequency Hologram Matrix

多频全息成像方法利用二维空间接收机（或发射机）阵列实现横向二维分辨率，利用频率阵列实现距离分辨率，从而获得真正的三维分辨率。首先，我们对三维多频全息成像进行了各种计算机模拟，并证实了该方法可以获得理想的真实三维分辨率的重建图像。即，不仅横向分辨率，而且距离分辨率都约为频率阵列中心频率波长的3.7倍。我们设计并订购了二维声学换能器阵列给日本电力株式会社。换能器的前尺寸为80毫米乘以80毫米。在由环氧树脂和金属粉末制成的底块上粘接一块质量良好的大PZT板。通过所谓的金刚石切割机切割，我们制作了16 × 16（=256）个接收传感器元件，孔径为1.0 mm × 1.0 mm，间距为4.5 mm。pzt材料的其余部分也由许多小的换能器碎片组成。将它们电平行连接，制成80毫米× 80毫米的大尺寸透射声换能器。这种换能器结构保证活塞振荡均匀，并向被测物体传输理想的平行平面波。谐振频率设计为600 KHz，多频全息图的频率孔径设计为450 ~ 550 KHz。因此，灵敏度或增益和相位特性在这个频率范围内是非常均匀的，相邻元件之间的相互耦合也非常小，即- 60db。这些特性对合成孔径法多频全息成像具有重要意义。引入一种快速响应的直接频率合成器，改进了传统的全息测量控制电路，使多频全息测量在较短的时间内成为可能。即每帧32毫秒，由小型计算机的软件直接控制。这种速度是活体医学成像所必需的。我们为每个接收元件设计和制造了多通道前置放大器，同时为矩阵的每个原始电路进行数字化电路，以使测量尽可能快。少

Multifrequency holographic imaging method uses a two-dimensional spatial receiver (or transmitter) array for lateral two-dimensional resolution and a frequency array for range resolution to get a true three-dimensional resolution.First, we made a variety of computer simulations of three dimensional multifrequency holographic imaging, and confirmed that this method gives us a desirable true three dimensional resolution of the reconstructed images. That is, not only lateral but also range resolutions are both about 3.7 times wavelength of the central frequency of the frequency array.We designed and ordered two-dimensional acoustic transducer array to Nippon Denpa Kogyo Company Limited. Front size of the transducer is 80 mm times 80 mm. A large PZT plate of a good quality is cemented on the backing block made of Epoxy resin and metallic powder. Cutting it by a so-called diamond cutter, we made 16 times 16 (=256) receiver transducer elements of a small aperture of 1.0 mm times 1.0 mm, whos … More e spacing is 4.5 mm. Rest part of the pzt material is also composed of many small fragments of transducers. Those were connected electrically parallel to make a big size of transmitting acoustic transducer of 80 mm times 80 mm. This type of transducer structure is guaranteed to make a uniform piston oscillation and to transmit an ideal parallel plane wave to the object. Resonance frequency was designed to 600 KHz, and the frequency aperture of multifrequency hologram is designed from 450 KHz to 550 KHz. Therefore, sensitivity or gain and phase characteristics over this frequency range is very uniform and also mutual coupling between the adjacent elements is very small, i. e. -60 dB. These characteristics are very important for multifrequency holographic imaging of synthetic aperture method.Introducing a direct frequency synthesizer of a fast response, we modified the conventional control circuit of holographic measurement, and made it possible to measure multifrequency hologram in a desirable short time. i. e. 32 ms per frame, being directly controlled by a software of a minicomputer. This speed is necessary for medical imaging of living body.We designed and fabricated multi-channel preamplifiers for every receiver elements and simultaneously digitizing circuits for each raw of the matrix to make the measurement as fast as possible. Less