Optical Tomography Using Echo Pulses Modulated by Light for Medical Diagnosis

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A new imaging method using the interaction between the near infrared light and the ultrasonic wave was investigated for application to medical diagnosis.A measuring equipment of the method using the ultrasonic transducer of 5 MHz resonant frequency was fabricated for construction of optical tomography. The method was applied to a biological tissue (a dog stomach) including absorbing objects. Ultrasonic echo pulses from the back boundaries of absorption region showed the time shift when the laser diode illuminated the sample. The image reconstructed from the velocity change revealed the shape of the absorption area in the dog stomach. Experimental results show that this method has the possibility for application to diagnosis of the spatial distribution of malignant tumor.Our equipment for optical tomography was insufficient for tissue characterization. An ultrasonic transducer of 20 MHz resonant frequency was employed instead of the transducer of 5MHz for improvement of spatial resolution. The ultrasonic echo pulse shift which was caused by the elastic coefficient change of the medium was detected under the light illumination of intensity within the skin exposure limit. Narrow boundaries of colored agar under chicken meat were observed in the image constructed from ultrasonic velocity change. The measuring equipment with improved spatial resolution is expected to apply to measurement of distribution of oxy-hemoglobin at the specified distance from the skin and spatial diagnosis of tumors.The high power laser diode was attached to the commercial ultrasonic diagnostic equipment. The array transducer of the equipment was employed to detect the ultrasonic velocity shift due to light illumination in the sample. Images of the sample were constructed from waveforms of the array transducer. The detail image of the absorption distribution in the sample was obtained.