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散射介质，因此光学层析成像的重建是困难的。提出了一种利用光吸收引起的超声声速变化实现实用光学计算机层析成像的方法，建立了检测光吸收引起的超声相移的实验装置。将两个换能器面对面放置在透明丙烯酸容器中作为发射器和接收器。当光照射到含有吸收体的散射介质时，吸收区的温度升高，超声波在该区域传播时发生相移。接收器输出的相位和幅度使用RF锁定放大器测量。在光照和黑暗中的超声相位之间的差异被确定，并存储在个人计算机中。由于测量值是沿超声波传播路径沿着进行相移积分的，因此为了重建光学层析图像，必须在样品周围进行扫描，即使在散射系数与人脑相当的介质中，在光照下超声波通过吸收物体时也会观察到超声波的相移。从光照射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，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.