Optical Tomography of Biological Tissue Based on Optically Assisted Ultrasonic Velocity-change Imaging

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We have already shown that the optical absorption distribution of the tissue mimic sample was able to be obtained by detection of ultrasonic echo pulse shift induced by light illumination. However, the experimental equipment using the single ultrasonic transducer is inadequate for the clinical application because echo data acquisition takes time to scan the transducer mechanically. We remodeled the commercial-type product of diagnostic ultrasonic equipment and obtained quickly the image of optical absorption distribution constructed by detection of ultrasonic velocity change using the ultrasonic array transducer.The array transducer was mounted above the phantom. The optical fibers attached to the high-power diode laser were arranged around the array transducer to illuminate the phantom. The echo pulses and their waveform data through the array transducer of the diagnostic ultrasound equipment were stored in a personal computer before light illumination. After light illumination, the echo pulses and their waveform data were also stored. The waveforms of echo pulses were interpolated to increase the sampling points and were divided into small areas with the width of transmitted pulse. The cross-correlation between the corresponding area of the waveform data stored before and after light illumination was calculated to measure the echo pulse shift induced by light illumination.Chicken meat phantoms in cluding the dyed agar or Au nano-rods were prepared as phantoms.The ultrasonic velocity change image constructed from the echo pulse shift gradually showed the absorption region of the phantom depending illuminating time of the light. On the other hand, the B-mode image did not display the optical absorption area in the phantom at all and did not change with time.Experimental results showed that our method had the possibility of application to early detection of malignant tumors and as a practical monitor of Au nano-rods for the drug delivery system.