Development of Sound Speed Microscope for Nano-scale Imaging of Biological Tissue Using Pico-second Ultrasonic Pulse

超声微成像对生物组织的观察具有无损、非接触等优点，可以定量获得声速、衰减等物理参数。我们一直在开发一种用于生物组织的超声波声速显微镜。将薄片试样置于玻璃基板上，将聚焦的宽带频率超声脉冲的反射解释为声速和其他参数。当对焦点光斑进行机械扫描时，可以观察到二维图像。目前正在研究其在手术组织病理学检查中的临床应用。这些数据对于评估临床超声成像的来源也很重要。然而，传统的横向分辨率仅为15微米，尽管更高的分辨率有望带来生物学和医学方面的重要信息。在这个研究项目中，我们的目标是开发一种分辨率为亚微米级的超声波显微镜。组装了100-500 MHz宽带脉冲扩频的超声显微镜。为了获得亚微米级的分辨率，采用了压电级。从大鼠海马中提取的培养神经细胞可以清晰地观察到强度和声速图像。此外，澄清了当声波从基材背面传播时，可以测量基材与组织界面处的反射波。因此，反射被可视化为声阻抗的二维剖面，以便在不制备薄切片标本的情况下观察组织。用声阻抗图像清晰地观察了大鼠小脑组织的结构。

Observation of biological tissue by ultrasonic micro-imaging has advantages of being non-destructive and non-contacting, making it possible to quantitatively obtain physical parameters like sound speed and attenuation. We have been developing an ultrasonic sound speed microscope for biological tissues. A thin sliced specimen is placed on a glass substrate, and the reflection of a focused ultrasound pulse with a wide band frequency is interpreted into sound speed and other parameters. A 2D image can be observed when the focal beam spot is mechanically scanned. Its clinical application to histopathological examination during surgery is being considered. The data are also important to assess the origin of clinical echographic imaging. However, the lateral resolution of conventional one used to be as poor as 15 micrometers, although one with a higher resolution was expected to bring important information in biological and medical point of view. In this research project, we aimed to develop an ultrasonic microscope with a resolution of sub-micron level. An ultrasonic microscope using a wide-band pulse spreading from 100-500 MHz was assembled. A piezo-electric stage was employed in order to obtain a resolution of sub-micron level. Cultured nerve cells taken from the hippocampus of a rat was clearly observed as both intensity and sound speed images.In addition, it was clarified that the reflected wave at the interface between the substrate and tissue can be measured when the acoustic wave was transmitted from the rear side of the substrate. Therefore the reflection was visualized as two-dimensional profile of acoustic impedance, in order to observe the tissue without preparing a thin sliced specimen. The structure of the cerebella tissue of a rat was clearly observed as an acoustic impedance image.