Optical Tomography Using Change of Ultrasonic Velocity by Light Absorption in Tissue

光学CT尤其有望应用于测量脑内血红蛋白的分布和肿瘤的空间诊断。然而，由于生物组织是光的高散射介质，因此重建光学层析成像是困难的。提出了一种利用光吸收引起的超声速度变化实现实用光学计算机断层扫描的方法。建立了检测光吸收引起的超声相移的实验装置。两个换能器面对面地放置在一个透明的丙烯酸容器中，作为发射器和接收器。当光照射含有吸收物的散射介质时，吸收区的温度升高，传播该区域的超声波相位发生偏移。使用射频锁相放大器测量接收机输出的相位和幅度。在光照下和黑暗中超声波相位的差异被测定并存储在个人计算机中。由于测量值是沿超声波传播路径的相移积分，因此为了重建光学层析成像，必须对样品周围的超声波进行扫描。在光照射下，即使在散射系数相当于人脑的介质中，超声波通过吸收物体时也会发生相移。利用光照引起的超声相移投影数据重建了散射介质中吸收区的光学图像。通过检测光照射引起的超声相移来识别隐藏在鸡肉中的吸收物的形状。

The optical CT is particulary expected to be applied to measurement of distribution of oxyhemoglobin in the brain and spatial diagnosis of tumors. However it is difficult to reconstruct the optical tomography because a biological tissue is highly scattering medium for light. A method using ultrasonic velocity change due to light absorption was proposed to realize practical optical computed tomography.The experimental setup for detecting the ultrasonic phase shift caused by the optical absorption was constructed. Two transducers were placed face to face in a transparent acryl container as a transmitter and a receiver. When light illuminates the scattering medium containing absorption objects, the temperature of the absorption area increases and the phase of ultrasonic wave propagating the area shifts. The phase and the amplitude of the receiver output are measured using the RF lock-in amplifier. The difference between the ultrasonic phase under light illumination and that in the dark is determined and is stored in a personal computer. As the measured value is phase-shift integrated along the propagation path of ultrasonic wave, the ultrasonic wave must be scanned around the sample in order to reconstruct the optical tomography image.The phase shift of ultrasonic wave was observed even in the medium with a scattering coefficient equivalent to that of the human brain when ultrasonic wave passed through absorption objects under light illumination. The optical image of the absorption region in a scattering medium was reconstructed from the projection data of ultrasonic phase shift due to light illumination. The shape of the absorption object hidden in the chicken meat was recognized by detecting the ultrasonic phase shift due to light illumination.