Shu Takagi

Simulations of Needle Insertion by Using a Eulerian Hydrocode FEM and the Experimental Validations

In this paper, simulations for needle insertion were performed by using a novel Eulerian hydrocode FEM, which was adaptive for large deformation and tissue fracture. We also performed experiments for the same needle insertion with silicon rubbers and needles, which had conical tips of different angles in order to investigate the accuracy of the simulations. The resistance forces in the simulations accurately followed those in the experiments until the conical portion of the needle was inside the rubbers, and the validation of the Eulerian hydrocode was revealed. However, the present simulation showed that after the conical portion was inside the tissue, the simulated resistance forces became lower than the experimental ones. The proportional increase of the friction forces and the roughly flatness of the tip force along the time were simulated. It was predicted that the tightening force along the needle side was underestimated.

4D Monte Carlo Dose Calculations for Particle Therapy Combined with the Spring Network Model of Lung Motion

To enable the evaluation of the impact of respiratory motion on charged particle therapy and to realize 4D treatment planning while keeping CT exposure as low as possible, we are developing a Monte Carlo dose calculation system combined with a computational biomechanical model of lung motion. The human lung is CT scanned for a single phase of a respiratory motion. The CT images are transformed into tetrahedral elements by automated segmentation. Then, the respiratory motion is simulated using a computational biomechanical model called the spring network model, and the calculated 3D shape of the lung for a given phase is transformed to a voxel data set. For each phase, assuming carbon-ion beam irradiation, biological dose distribution is calculated using the Monte Carlo particle and heavy ion transport code PHITS coupled with a microdosimetric kinetic model. The dose is mapped onto the reference data set to obtain the accumulated dose. The first version of the 4D dose calculation system we have developed so far can realistically reproduce the lung motion, successfully read the data set for each phase and calculate the accumulated dose. The number of the phases to be sampled and their weights can be set arbitrarily, without need of additional CT scanning. Our first simulations for a 70 MeV/u carbon ion beam with a diameter of 2 cm indicate that the dose distribution can significantly change with phase and that many data sets may be needed to accurately evaluate the dose to the surrounding normal tissue. The impact of the system we developed is two-fold: in the short term, it can be used to investigate different issues of 4D treatment planning; our goal is an entirely simulation-based 4D planning from a single CT scanning. The system is being developed further.

Multiscale Analysis of Heterogeneous Catalysis on a Silica Surface

Many studies have been reported about the catalytic recombination of oxygen and nitrogen on a silica surface, which is quite important for the reentry of a space vehicle. But, the reaction mechanism is not fully understood. Hence, in this study, we are constructing a catalytic reaction model using the ab initio calculations and the Monte Carlo calculations in order to reveal the reaction mechanism. First, desorption and surface migration of oxygen atoms on the α-quartz (0001) reconstructed surface were investigated using density functional theory. The result indicates that Langmuir-Hinshelwood (L-H) recombination frequently occurs at high temperature. Then detailed analysis of L-H mechanism was performed and the rate controlling factor was discussed.

Theoretical study on the shape instability of an encapsulated bubble in an ultrasound field

A theoretical study on the shape instability of a slightly deformed bubble encapsulated by a viscoelastic membrane in an ultrasound field is performed. The membrane effects of the inplane stress and the bending moment are incorporated into the traction jump condition at the bubble surface. The spherical motion of the bubble is numerically obtained by solving the Rayleigh-Plesset equation with the elastic stress. The deflection therefrom is linearized and expanded with respect to the Legendre polynomial. Two amplitudes for each shape mode are introduced because the membrane has mobility not only in the radial direction but also in the tangential direction. A simple expression for the natural frequency of shape mode is derived. Stability diagrams for the higherorder shape mode are mapped out in the phase space of driving amplitude versus driving frequency. The most unstable driving frequency is found to satisfy an integer multiple relationship with twice of the higher-order natural frequency. This finding is justified by a fact that the system with a boundary layer approximation is simplified into Mathieu’s equation. Liquid viscosity plays an important role in the shape stability due to the vorticity generation on the deformed membrane.A theoretical study on the shape instability of a slightly deformed bubble encapsulated by a viscoelastic membrane in an ultrasound field is performed. The membrane effects of the inplane stress and the bending moment are incorporated into the traction jump condition at the bubble surface. The spherical motion of the bubble is numerically obtained by solving the Rayleigh-Plesset equation with the elastic stress. The deflection therefrom is linearized and expanded with respect to the Legendre polynomial. Two amplitudes for each shape mode are introduced because the membrane has mobility not only in the radial direction but also in the tangential direction. A simple expression for the natural frequency of shape mode is derived. Stability diagrams for the higherorder shape mode are mapped out in the phase space of driving amplitude versus driving frequency. The most unstable driving frequency is found to satisfy an integer multiple relationship with twice of the higher-order natural frequency. This finding i...

Ultrasound-mediated gene transfection: A comparison between cells irradiated in suspension and attachment status

Sonoporation, in the presence of microbubbles, is a promising nonviral gene transfection method. Although the mechanism is not yet fully understood, shock waves emitted by cavitation bubbles have been known to play an important role in creating pores on cell membranes. This work investigates the gene transfection efficiency and influencing parameters of cells in two different statuses: attachment and suspension based on the fact that cells in suspension have more bubbles surrounding them and that shock wave has distinct effects on hit objects whether the object is attached to a rigid wall or not. Fibroblast cells (NIH3T3), both in attachment and suspension, and green fluorescent protein (GFP) plasmid were exposed to variations in acoustic pressure (0.6-1.2 MPa) and 10% duty cycle at fixed settings of 2 MHz central frequency, 5 kHz pulse repetition frequency and 1 minute insonation time, in the presence of 10% v/v microbubbles (Sonazoid, a commercialized product of ultrasound contrast agent). The transfect...

A Computational Cardiovascular Model for Characterizing Arterial pulses under Various Physiopathological Conditions

Arterial pulse is one of the most important biosignals monitored in clinics for diagnosing cardiovascular disease. Despite the many past efforts attempted to correlate characteristics of arterial pulse with cardiovascular diseases, the precise characteristics of arterial pulse that best predict the risk of cardiovascular disease yet remain a subject of considerable debate. This is due to the fact that arterial pulse is sensitive to a variety of factors, including both physical ones and physiopathological ones. Given a measured arterial pulse, the best way to uncover potential pathological factors is to isolate the respective effects of different factors, which is, however, difficult to realize in practice. In this context, we develop a computational cardiovascular model, aiming to provide a mathematical platform for quantitatively studying the respective contributions of various factors to characteristics of arterial pulse. Such a model is obtained by adopting a multi-scale modeling method, where the arterial tree is described by a wave propagation model coupled with a lumped parameter description of the remainder. In this study, the model is examined by simulating arterial pulse changes associated with arterial stenoses and aging, and hemodynamics in the left brachial artery as the artery is subjected to a varying cuff pressure during oscillometric blood pressure measurement. Simulated results demonstrate that the model can reasonably capture the main characteristics of arterial pulses under the conditions studied. Particularly, the cuff simulations reveal that the accuracy of oscillometric pressure measurement is affected not only by the local arterial properties but also by hemodynamic conditions in the rest of the cardiovascular system.

Nonlinear Phenomena of Acoustic Cloud Cavitation

Acoustic cavitation, which is normally consisted of bubble cluster, has an important role in various fields such as medical, environmental, industrial applications. The strong pressure appears in the case of bubble cluster like a cloud cavitation. As the dynamics of bubbles are strongly influenced by the thermal phenomena inside the bubbles, these phenomena are considered to numerically investigate the behavior the bubble cloud. The pressure wave focuses to the center of the cloud and the pressure inside bubbles extremely increases when the frequency of the pressure wave is near the first mode natural frequency of the cloud. Due to the strong nonlinearity of the pressure wave propagation in the bubbly liquid, a sinusoidal wave changes to the steep waveform in the cloud and it generates high pressure fluctuation near the center of the cloud even when the frequency is much lower than the first mode frequency.