Prof. Hiroko Yamashita Profile

Prof. Hiroko Yamashita

at Hokkaido Univ. Hospital

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Author

Publications (3)

Proceedings Article | 15 February 2021 Presentation + Paper

Optimized source estimation for full waveform inversion in ultrasound computed tomography

Atsuro Suzuki, Yushi Tsubota, Takahide Terada, Hiroko Yamashita, Fumi Kato, Mutsumi Nishida, Megumi Satoh, Ken-ichi Kawabata

Proceedings Volume 11602, 1160208 (2021) https://doi.org/10.1117/12.2580491

KEYWORDS: Computed tomography, Ultrasonography, Tumors, Error analysis, Device simulation, Computer simulations

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In full waveform inversion (FWI) for ultrasound computed tomography (CT), choosing the right sound source is essential for generating high-resolution images. We developed an optimized source estimation method for FWI to efficiently reduce the value of any cost function and evaluated its performance using simulation data and measurement data. In our optimized source estimation method, we obtain the sound source as α(k)·f, where f is a sound source (an arbitrary complex value), coefficient α(k) is cos(θ_k)+i·sin(θ_k), k is integer (0, 1, ⋯, N-1), θ_k is 2π/N·k, and the integer value of N is 360. We then determine the coefficient α(k) that minimizes the value of the cost function. In contrast to conventional source estimation, which only minimizes the value of the L2 norm cost function, our proposed source estimation can minimize the values of any cost function, such as the L1 norm or a hybrid of L1 and L2 norms. The advantage of our method is that it can be easily applied to FWI with various cost functions. In this preliminary study, we implemented FWI with the L2 norm cost function and compared the performance of our proposed method with that of the conventional method. In the simulation study, FWI with both the conventional and proposed source estimation methods improved the contrasts of inclusions of a numerical phantom compared to FWI with no source estimation. They both also improved the contrasts of inclusions of a measured oil-gel-based phantom compared to a bent-ray reconstruction method. The absolute mean errors between ROI and true values were 39, 11, and 11 [m/s] for the bent-ray reconstruction method, FWI with the conventional method, and FWI with the proposed method, respectively. In addition, FWI with both the conventional and proposed methods improved the contrasts of a patient’s tumor compared to the bentray reconstruction method. These results demonstrate that FWI with the proposed source estimation method can provide the same contrast and quantitative accuracy as FWI with the conventional source estimation method.

Proceedings Article | 16 March 2020 Presentation + Paper

Incident angle-dependent gain correction for reflection images on ultrasound computed tomography

Yushi Tsubota, Takahide Terada, Atsuro Suzuki, Wenjing Wu, Kazuhiro Yamanaka, Hiroko Yamashita, Fumi Kato, Mutsumi Nishida, Megumi Satoh, Ken-ichi Kawabata

Proceedings Volume 11319, 113190N (2020) https://doi.org/10.1117/12.2548555

KEYWORDS: Reflection, Ultrasonography, Breast, Skin, Computed tomography, Breast cancer, Transducers, Image processing, Calibration, Tissues

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Proceedings Article | 15 March 2019 Paper

High SNR emission method with virtual point source in ultrasound computed tomography

Wenjing Wu, Yushi Tsubota, Atsuro Suzuki, Kazuhiro Yamanaka, Takahide Terada, Kenichi Kawabata, Hiroko Yamashita, Fumi Kato, Mutsumi Nishida, Megumi Satoh

Proceedings Volume 10955, 109550I (2019) https://doi.org/10.1117/12.2506954

KEYWORDS: Signal to noise ratio, Virtual point source, Computed tomography, Breast cancer, Ultrasound tomography, Ultrasound transmission tomography

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