We are developing an ultrasound computed tomography (USCT) system for early breast-cancer screening. USCT has great advantages over mammographies because of its lack of X-ray exposure and compression pain. USCT can show both the reflection boundary (structure) distribution and the sound speed (hardness) distribution in a subject, which is estimated from the time-of-flight (TOF) information of transmitted ultrasound waves on the basis of an X-ray CT algorithm. Considering the nature of ultrasound waves, improving the image quality generally increases the calculation burden. To achieve both high-quality images and high throughput, we developed an iterative refraction calibration method. The measured TOF sinogram was iteratively calibrated by the difference between the fastest wave arrival time and the arrival time of the wave along the geometrically shortest path in a section. This method was applied to the data of a gel phantom and a dog’s tumor extirpated at Tokyo University of Agriculture and Technology, which was measured by a USCT prototype with a 10 cm-diameter ring array. As a result, we achieved a calculation speed seven times faster than that of a conventional bent-ray reconstruction with the same contrast as that of a sound-speed image.
In breast imaging by ultrasound CT, ultrasound is refracted owing to the difference of the sound speed between the breast and background water. The sound speed of a dense breast is higher than that of the water, while that of a fatty breast is lower than that of the water. In this study, we developed an oil-gel-based phantom for mimicking the wave refraction from the fatty breast to the dense breast. An oil gel was generated by adding SEBS (Styrene-Ethylene/Butylenes-Styrene, 10 wt%) to paraffin oil. The oil-gel-based phantom has a cylindrical shape and contains rod shaped inclusions which can be filled with salty water (3.5%). When temperature increases, the sound speed of water increases, while that of the oil gel decreases; the sound speeds of the oil gel were higher than those of the water at less than 20°C, while the sound speeds of the oil gel were lower than those of water at higher than 20°C. By controlling the temperature, the oil-gel-based phantom was able to simulate the refraction from the fatty breast (1476 [m/s]) to the dense breast (1559 [m/s]). For 43 days, the variation of the sound speed and attenuation of the oil gel in the reconstructed images were 0.7[m/s] and 0.03[dB/MHz/cm], respectively. This phantom with high temporal stability is suitable for multi-center distribution and may be used for standardization of data acquisition and image reconstruction across centers.