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

光学CT尤其有望应用于脑内氧合血红蛋白分布的测量和肿瘤的空间诊断。然而，由于生物组织是光的高度散射介质，重建光学层析成像是困难的。为了实现实用的光学计算机层析成像，提出了一种利用光吸收引起的超声速度变化的方法，搭建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容器中，作为发射器和接收器。当光照射含有吸收物体的散射介质时，吸收区域的温度升高，传播该区域的超声波的相移。接收器输出的相位和幅度是使用射频锁定放大器测量的。确定在光线照射下的超声波相位和在黑暗中的超声波相位之间的差异，并将其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，为了重建光学层析图像，必须扫描样品周围的超声波，即使在散射系数相当于人脑的介质中，当超声波在光照下穿过吸收物体时，也能观察到超声波的相移。根据光照射下超声相移的投影数据，重建了散射介质中吸收区域的光学图像。通过检测光照引起的超声相移来识别隐藏在鸡肉中的吸收物体的形状。

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.