| Research Abstract |
To improve spatial resolution of optical imaging of biological tissue, a new method using speckle patterns of ultrasound-modulated light was proposed. Results of experimental evaluations of the proposed method are summarized as follows.
1. A proposed method and an experimental system : To detect temporal variations of optical speckle patterns in ultrasound frequency, we used a conventional CCD still camera and introduced a principle of a strobe scope, that is, the shutter of the CCD still camera is open and the object observed is illuminated by pulsed light synchronized with exposed ultrasound. The experimental system used the cooled CCD camera of 16-bits contrast resolution. A pulsed laser diode (power =600 mW, pulse width = 0.1 ns) was used as the illumination light source, and a focused ultrasound transducer of 1 MHz in center frequency was used as the ultrasound source.
2. Transmission Imaging : One-dimensional transmission imaging of a breast tumor phantom was carried out. The light
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source, the phantom, and the CCD camera were placed in line and speckle patterns generated by transmitted light was used for imaging. An agar phantom (Thickness = 35 mm, μs'=0.5 mm^<-1>) was used as the phantom, and an absorbing object (10x10x10 mm^3) mimicking a breast tumor (μa= 0.048 mm^<-1>, μs'= 0.5 mm^<-1>) was buried at the center of the phantom. A profile of the absorbing object imaged by the proposed method is steeper than that of a conventional imaging result, indicating that the spatial resolution of optical imaging was improved by detecting the ultrasound modulated light.
3. Reflection Imaging : One-dimensional imaging was also tried using speckle patterns generated by reflected light. In this experiment, the light source and the CCD camera were placed at the same side of the phantom. The illumination beam entered the phantom at an angle of approximately 65 degrees, and an image of the speckle pattern generated on the same surface of the phantom was taken using a CCD camera placed in front of the phantom. Also in this case, spatial resolution was improved using the proposed method, and it was shown that this method has wide applicability.
4. Improvement of the system : In the above experiments, it took about 10 hours to measure a one dimensional profile because the pulse width of the light was needlessly small. A high-power light source (power =20 W, pulse width = 50 ns) was used, and measurement time was greatly reduced without S/N drop. Less
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