IntroductionUltrasonic microscopy is expected as a powerful tool for local tissue characterization. Its clinical application to histopathological examination during surgery is being considered. The data are also important to assess the origin of clinical echographic imaging. In clinical point of view, the microscopic observation should be as quick as possible to carry out the surgery in a short period.A tissue slice of 3-10 m in thickness placed on a glass substrate is often employed as a specimen to be observed. Sound speed is considered to be a good parameter to characterize abnormality of the tissue. In order to estimate the sound speed, reflections from the front and rear surfaces of the tissue slice are compared. Time lag between these two reflections is not easy to be estimated, because the thickness is less than the wavelength of the acoustic wave, of which frequency range is normally less than several hundreds of megahertz. Therefore, phase analysis in frequency domain is often carried out [1,2]. A monotonic burst wave is irradiated in most cases, and the attenuation and phase of the reflection are analyzed. A frequency scan is needed in order to perform this measurement, however, it needs a long time to switch the frequency. As the result, recent ultrasonic microscope needs more than one hour for obtaining one microscopy. In addition, the accuracy of measurement strongly depends on the frequency stability. The device to maintain the stability tends to complicate the system and bring a high instrumentation cost.In stead of the above frequency domain measurement, the authors proposed a time domain measurement using a sharp pulse wave that has a widely spreading spectrum. The reflection was directly recorded by a digital oscilloscope and then Fourier transformed into frequency domain. Both sound speed and thickness at each point were simultaneously calculated by phase analysis. A two dimensional microscopy was obtained by mechanically scanning the transducer. This report describes the result of a preliminary experiment to prove the feasibility of the pulse driven sound speed microscope for tissue characterization.
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