Kazuhiko Fukutani

Model-Based Reconstruction Integrated With Fluence Compensation for Photoacoustic Tomography

Photoacoustic (PA) tomography (PAT) is a rapidly developing imaging modality that can provide high contrast and spatial-resolution images of light-absorption distribution in tissue. However, reconstruction of the absorption distribution is affected by nonuniform light fluence. This paper introduces a reconstruction method for reducing amplification of noise and artifacts in low-fluence regions. In this method, fluence compensation is integrated into model-based reconstruction, and the absorption distribution is iteratively updated. At each iteration, we calculate the residual between detected PA signals and the signals computed by a forward model using the initial pressure, which is the product of estimated voxel value and light fluence. By minimizing the residual, the reconstructed values converge to the true absorption distribution. In addition, we developed a matrix compression method for reducing memory requirements and accelerating reconstruction speed. The results of simulation and phantom experiments indicate that the proposed method provides a better contrast-to-noise ratio (CNR) in low-fluence regions. We expect that the capability of increasing imaging depth will broaden the clinical applications of PAT.

Phase-separated Al–Si thin films

Phase-separated Al–Si films composed of Al nanocylinders embedded in an amorphous-Si matrix have been prepared by a sputtering method. By controlling the deposition rate, substrate temperature, and film composition, the average diameter of the Al cylinders can be varied systematically from less than 5to13nm with a cylinder density ranging from 1015 to in excess of 1016cylindersm−2. A three-dimensional simulation of phase separation in binary thin films was performed using a modified Cahn-Hilliard [J. Chem. Phys. 28, 258 (1958)] equation to understand the growth mechanism. The simulation studies indicate that the surface diffusion length and film composition are important factors which determine film morphology. Experimental and simulation studies are compared and discussed.

Photoacoustic mammography capable of simultaneously acquiring photoacoustic and ultrasound images

Abstract. We have constructed a prototype photoacoustic mammography system (PAM-02) capable of simultaneously acquiring photoacoustic (PA) and ultrasound (US) images. Each PA, US, and fused PA/US image can be acquired over a wide area of the breast using the scanning module of a US transducer, a PA detector, and optical prisms. The resolution of the PA images exhibits improvement from 2 to 1 mm compared to images acquired using our previous prototype. The maximum scan area of PAM-02 is 90 mm along the horizontal axis and 150 mm along the vertical axis. In a phantom experiment, the available depth was at least 45 mm. A representative example of the application of the PAM-02 prototype in clinical research at Kyoto University is presented and shows S-factor images, which are considered an approximation parameter related to hemoglobin saturation of tumor-related blood vessels. We confirmed the applicability of the system for anatomical and biological research.

Perpendicular recording media using phase-separated AlSi films

We have developed perpendicular recording media with nanosized cylindrical Co particles. The diameter and period of these particles are less than 10 and 12 nm, respectively. Al cylinders were grown in an a-Si matrix during AlSi film deposition due to the eutectic phase separation. The Al cylinders can be removed by chemical etching to form nanoporous templates. The pores were filled with Co by electrodeposition method via a metal underlayer as an electrode. These films show perpendicular anisotropy with Hc of 4.1 kOe.

Adaptive and quantitative reconstruction algorithm for photoacoustic tomography

Photoacoustic (PA) tomography is a rapidly developing imaging modality which can provide high contrast and spatial-resolution images of light absorption distribution in tissue. However, the quantitative reconstruction of absorption distribution is still a challenge. In this study, we propose an adaptive and quantitative reconstruction algorithm for reducing amplification of noises and artifacts in deep position due to light fluence compensation. In this method, the quantitative processing is integrated into the iterative reconstruction, and absorption coefficient distribution is iteratively updated. At each iteration step, the residual is calculated from detected PA signals and the signals calculated from a forward model by using the initial pressure which is calculated from the production of voxel value and the light fluence. By minimizing the residual, the reconstructed values are converged to the true absorption coefficient distributions. Since this method uses a global optimized compensation, better CNR can be obtained. The results of simulation and phantom experiment indicate that the proposed method provide better CNR at deep region. We expect that the capability of increasing imaging depth will broaden clinical applications.

Characterization of nanoporous Si thin films obtained by Al–Si phase separation

Nanoporous silicon films of ultrahigh pore densities and large surface areas have been fabricated by sputtering an Al–Si target and subsequent removal of the deposited film’s Al regions by etching in a sulfuric acid solution. The resulting nanoporous films are mainly composed of amorphous silicon and have cylindrical pores with an average pore density exceeding 1016pores∕m2. These nanoporous films can be crystallized by thermal annealing in a H2 atmosphere to improve their electrical properties. The electrical properties of the crystallized nanoporous films, which behave as p-type semiconductors, are very similar to those of electrochemically etched porous Si.

Advanced model-based reconstruction algorithm for practical three-dimensional photoacoustic imaging

In this study, we propose an advanced model-based reconstruction algorithm for three-dimensional photoacoustic imaging. The algorithm is based on accurate forward photoacoustic models and an optimization algorithm which minimizes the square of the error between the measured acoustic signals and the signals predicted by the forward models. The forward photoacoustic models incorporate system-configuration and detector-dependent factors such as frequency response and finite size effect. A conjugate gradient-based optimization algorithm is used for reconstructing images. In addition, we make use of the symmetry and locality of the photoacoustic waves in the computations of the forward photoacoustic models in order to reduce the memory requirements and computation time in three-dimensional image reconstruction. The results show that the proposed algorithm provides high-resolution and high-quality photoacoustic images.

Fabrication of Well-Aligned Al Nanowire Array Embedded in Si Matrix Using Limited Spinodal Decompositon

The morphology of a phase-separated binary film during spinodal decomposition, whose evolution is limited on the growth surface, is studied. The temporal evolution of the Cahn–Hilliard equation without the elastic energy term under frozen bulk approximation is solved to investigate the possibility of well-aligned nanostructures. The model simulation showed that the well-aligned phase pattern on the top surface can be formed when surface diffusion length was optimized and film thickness was larger than a certain value. These simulation results highly agree with the experimental observation of phase-separated Al–Si films. An additional simulation study indicated that well-aligned nanowires can be prepared from the initial growth stage when an initial phase pattern given on the substrate matched the periodicity of the phase pattern of deposited films.

Template-Assisted Growth of Nanowires Using Novel Nanoporous Films Fabricated by Inorganic Nano Phase Separation

Ultrahigh pore density nanoporous films with a pore diameter of less than 10nm and a pore density exceeding 10 16 pores/m 2 were developed. Nano phase separation of a eutectic Al-Si system was used for the fabrication of these nanoporous films. Co-sputtered AlSi films form Al nano-cylinders, perpendicular to the substrate and parallel to each other, embedded in an amorphous Si matrix during film growth due to phase separation. Removal of the Al nano-cylinders from the co-sputtered AlSi films by chemical etching gives us ultrahigh pore density nanoporous films. The nanoporous films consist of mainly oxidized silicon. Depending on the film compositions and the film preparation conditions, such as RF power and the deposition temperature, the average pore diameter can be varied systematically from less than 5nm to 13nm with the pore density from 10 15 to exceeding 10 16 pores/m 2 . Furthermore we have demonstrated a template-assisted growth of ultrahigh-density Ni nanowire arrays with an aspect ratio of ∼100 in the nanoporous films by electrodeposition. The fabrication method for nanowire arrays using the nanoporous films is quite simple and promising for the fabrication of nanostructured devices.

Dual illumination mode photoacoustic tomography for quantitative imaging

This study presents an algorithm to estimate the background optical properties for quantitative imaging, based on the initial pressure distribution obtained from photoacoustic tomography (PAT). The algorithm utilizes an alternative illumination mode of our original PAT system, which separately illuminates tissue from forward and backward directions toward an array transducer. The optical properties are determined by minimizing the difference between the following two ratios: one is the ratio of initial pressure distribution obtained from the illumination modes, the other is the ratio of the calculated light fluence distribution. We show that the algorithm estimates the background optical properties even if the initial pressure distribution is reconstructed under the limited-view conditions.