Yushi Tsubota

Highly precise acoustic calibration method of ring-shaped ultrasound transducer array for plane-wave-based ultrasound tomography

Ultrasound computed tomography (USCT) is promising for a non-invasive, painless, operator-independent and quantitative system for breast-cancer screening. Assembly error, production tolerance, and aging-degradation variations of the hardwire components, particularly of plane-wave-based USCT systems, may hamper cost effectiveness, precise imaging, and robust operation. The plane wave is transmitted from a ring-shaped transducer array for receiving the signal at a high signal-to-noise-ratio and fast aperture synthesis. There are four signal-delay components: response delays in the transmitters and receivers and propagation delays depending on the positions of the transducer elements and their directivity. We developed a highly precise calibration method for calibrating these delay components and evaluated it with our prototype plane-wave-based USCT system. Our calibration method was found to be effective in reducing delay errors. Gaps and curves were eliminated from the plane wave, and echo images of wires were sharpened in the entire imaging area.

Method for extracting microcalcifications with analysis of isotropy of scattered acoustic signals in ring-array transducer system

Ultrasound computed tomography (USCT) is a promising candidate for a radiation-free, painless, and quantitative modality for breast cancer examination. However, microcalcifications, which are one of the most important indicators of breast cancer in mammography examination, are difficult to detect by either USCT or conventional echography. In this study, we propose a new method for extracting point scatterers such as microcalcifications from ultrasound echo signals by measuring an isotropy of acoustic scattering. To measure the isotropy of the scattering, a wide aperture is required, thus, a ring-array transducer for USCT is advantageous over conventional probes. In the experiments measuring a gel phantom and a swine breast, the microcalcification-mimicking scatterers embedded in the samples were successfully extracted on the basis of the analysis of the isotropy of acoustic scattering. The result strongly supports the applicability of the proposed method to an extraction of microcalcifications in human breast for clinical use.

Animal study of high-speed iterative refraction calibration method for ultrasound computed tomography

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.

Oil-gel-based phantom for mimicking wave refraction of breast in ultrasound computed tomography

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.

Tissue boundary roughness index for evaluating malignancy of cancer in ultrasound-computed tomography system

Ultrasound-computed tomography (USCT) is a promising candidate for a radiation free, painless, and quantitative modality for breast cancer examination. We developed a USCT prototype with a ring-shaped transducer array (frequency: 1.5 MHz) that moves along the ring axis for scanning the entire breast. Distributions of speed and attenuation of ultrasound are computed from ultrasound transmitted through tissue. Although prior research shows that there is a correlation between malignancy and the speed of sound and attenuation values, in some cases both benign and malignant tumors have similar values. In this study, we propose to quantify the boundary roughness of a tumor from the spatial power distributions of reflected ultrasounds that are transmitted from different apertures of the ring transducer. In conventional ultrasound echography, the boundary roughness of a tumor is an important indicator of malignancy. However, roughness is judged by radiologists based on B-mode images and is a subjective index. We ...