Researchers, Developers and Staffs
Kohei  OKITA Kohei OKITA


Research Themes

Development of HIFU simulator for high-intensity focused ultrasound therapy
Numerical analysis of the focus control in microbubble-enhanced HIFU
Development of fluid dynamics solver using signed distance function for representation of objects in Cartesian mesh
Modeling of multiple fluid flows based on the diffuse interface model
Numerical analysis of the cavitation erosion and it's mitigation in neutron spallation source

Research Subjects and Results

Development of HIFU simulator for high-intensity focused ultrasound therapy

High-intensity focused ultrasound (HIFU) has been developed as a non-invasive cancer treatment. HIFU provides a heat coagulation of tissue around the focus. The objectives of the present study are to realize appropriate focus control using the array transducer, to support preoperative planning of HIFU therapy, and to support the development of HIFU devices by enabling computational prediction of ablation regions. To these ends, we develop an HIFU simulator. Figure 1 shows the flow of input data for the HIFU simulation. A volume model of the human body (a voxel data) was constructed from CT/MRI data on a living human body. The shape of a transducer defined using CAD is represented by the volume data of the Signed Distance Function (SDF). Then, a simulation of ultrasound propagation from the transducer through the voxel data is performed numerically. HIFU therapy for liver cancer using the array transducer was reproduced as shown in Fig.2. Although the results without the phase delay showed displacement and diffusion of the focal point due to the inhomogeneity of the body, a clear focal point was obtained by using the array transducer with an appropriate phase delay, which is provided by pre-computation of the ultrasound propagation from the target.


Figure 1: Schematic diagram of data flows to HIFU simulation


Figure 2: Numerical simulation of HIFU therapy for liver cancer using array transducer

Development of fluid dynamics solver using signed distance function for representation of objects in Cartesian mesh

A fluid solver using signed distance function (SDF) for shape representation was developed based on the immersed boundary method to simulate incompressible viscous flows. The forcing velocities near boundary are extrapolated by trilinear interpolation with taking into account a boundary condition using SDF. SMAC method is employed for solving basic equations for unsteady incompressible flows. The equations are discretized in space by 2nd-order central difference method, where the discretization near boundary is improved by SDF to satisfy a Dirichlet boundary condition for velocity. The fluid solver was verified in both steady and oscillating three-dimensional Poiseuille flows as shown in Fig.3. Figure 4 shows the grid dependency of the error of axial velocity for the oscillating flow. As the grid spacing decreases, L2 and L∞ norm of the error of the axial velocity profile respectively decrease by the order of 1.96 and 1.89 for the oscillating flow. Therefore, the fluid solver enables to analyze the Poiseuille flow using Cartesian mesh by 2nd-order of accuracy in space.


Figure 3: Schematic diagram of the numerical model for the Poiseuille flow through the cylindrical pipe which is arranged in the numerical domain of Cartesian mesh.


Figure 4: Grid dependency of the error of axial velocity (Oscillating flow, Reτ=1, angle of cylinder arrangement θ = arctan(3/4) = 36.9°)

Modeling of multiple fluid flows using diffuse interface model

A free energy model for immiscible multiple fluids based on the diffuse interface model was proposed. A triple junction problem was simulated for the verification of the model. A contact angle depends on the surface tension of each interface at triple junction. The numerical results were compared with the theory for various contact angles as shown in Fig.5. The represented contact angle agrees with the theoretical contact angle using the proposed free energy model for immiscible multiple fluids.


Figure 5: The represented contact angle of the blue fluid at triple junction as function of the theoretical contact angle. Symbols are compared for the resolution of the diffuse interface.

The microgravity driven motion of two immiscible droplets in the other immiscible fluid was simulated as shown in Fig.6. The red and blue fluids are respectively lighter and heavier than the surrounding fluid. The motions of the two droplets such as coalescence, rotation, covering and detachment were reproduced reasonably. The present free energy model enables to reproduce the three-dimensional motion of immiscible multiple fluids in the surface tension dominant flows.


(a) Rotating motion (sRG=1.0, sGB=1.0, sBR=1.0)


(b) Covering motion (sRG=0.7, sGB=1.0, sBR=0.7)

Figure 6: Gravity driven motion of the two immiscible droplets in the other immiscible fluid. The red and blue fluids are respectively lighter and heavier than the surrounding fluid.

VCAD Public Software

V-SDFlib
SPHERE:HIFU

Software Block Page


Selected Publications

【Papers】
K. Okita, K. Ono
Numerical Accuracy of Fluid Solver using Signed Distance Function for Shape Representation
Transaction JSME Ser.B in Japanese (submitted) (2011)

K. Okita, K. Ono, S. Takagi, Y. Matsumoto
Development of High Intensity Focused Ultrasound Simulator for Large Scale Computing
Int. J. Numerical Methods in Fluids, Vol.65, pp.43-66 (2011)

K. Okita, K. Ono, S. Takagi, Y. Matsumoto
Numerical Simulation of the Tissue Ablation in High Intensity Focused Ultrasound Therapy with Array Transducer
Int. J. Numerical Methods in Fluids, Vol.64, pp.1395-1411 (2010)

K. Okita
Numerical Simulation of the Tissue Ablation in High-Intensity Focused Ultrasound Therapy
J. JSCES, Vol.14,No.4, pp.11-12 in Japanese (2009)

K. Okita, H. Ugajin, Y. Matsumoto
Numerical Analysis of the influence of the tip clearance flows on the Unsteady Cavitating Flows in a Three-dimensional Inducer
Journal of Hydrodynamics, Ser. B, Vol.21, Issue 1, pp.34-40 (2009)

K. Okita, S. Takagi, Y. Matsumoto
Propagation of Pressure Waves, Caused by a Thermal Shock, in Liquid Metals Containing Gas Bubbles
Journal of Fluid Science and Technology, Vol.3, No.1, pp.116-128 (2008)

K. Okita, K. Ono, S. Takagi, Y. Matsumoto,
Modeling and Simulation of the Tissue Ablation in High Intensity Focused Ultrasound Therapy with Array Transducer
VPH2010, Brussels, Belgium, September 30 ? October 1, pp.178-180 (2010)

Matsumoto, K. Okita, K. Ono, S. Takagi
The Tissue Ablation in High Intensity Focused Ultrasound Therapy with an Array Transducer
WCCM / APCOM 2010, Sydney, Australia, 19-23, July (2010)

K. Okita, K. Ono, S. Takagi, Y. Matsumoto
Numerical Simulation of the Tissue Ablation in High Intensity Focused Ultrasound Therapy with an Array Transducer
ECCOMAS CFD 2010, Lisbon, Portugal, June 14?17, No.1132 (2010)

K. Okita, K. Sugiyama, K. Ono, S. Takagi, Y. Matsumoto
Numerical Study on the Effective Combination of Microbubble and Ultrasound in HIFU Therapy
10th International Symposium on Therapeutic Ultrasound, Tokyo, Japan, June 9-12 (2010)

N. Leduc, K. Okita, K. Sugiyama, S. Takagi, Y. Matsumoto
A TR-induced algorithm for hot spots elimination through CT-scan HIFU simulations
10th International Symposium on Therapeutic Ultrasound, Tokyo, Japan, June 9-12 (2010)

T. Shimura, K. Okita, S. Takagi, Y. Matsumoto
The Effect of the Elastic Body on the Focusing of Ultrasounds in Inhomogeneous Media
10th International Symposium on Therapeutic Ultrasound, Tokyo, Japan, June 9-12 (2010)

K. Okita, K. Ono, S. Takagi, Y. Matsumoto
Numerical Simulation of the Tissue Ablation in HIFU therapy with an Array Transducer
2nd Biosupercomputing symposium, Tokyo, Japan, March 18-19, p.140 (2010)

K. Okita, K. Sugiyama, K. Ono, S. Takagi, Y. Matsumoto
Numerical study on high intensity focused ultrasound therapy using array transducer
Physics Procedia, Vol. 3(1), pp.315-322 (2010)

Y. Matsumoto, K. Okita, K. Ono, S. Takagi
Numerical Simulation on High Intensity Focused Ultrasound Therapy I, II
2009 ASME International Mechanical Engineering Congress & Exposition, November 13-19, Lake Buena Vista, Florida (2009)

K. Okita, K. Sugiyama, K. Ono, S. Takagi, Y. Matsumoto
Numerical Simulation of High Intensity Focused Ultrasound Therapy with Volume Model of Human Body
9th International Symposium on Therapeutic Ultrasound, September 21-23, Aix-en Province, France, pp.371-374 (2009)

Y. Matsumoto, K. Okita, K. Ono, S. Takagi
Numerical Simulation of Therapeutic Ultrasound
FEF09, April 1-3, Tokyo, Japan (2009)

K. Okita, K. Sugiyama, K. Ono, S. Takagi, Y. Matsumoto
Numerical Study on High Intensity Focused Ultrasound Therapy using Array Transducer
International Congress on ULTRASONICS, January, 11-17, Santiago, Chile (2009)

K. Okita, H. Ugajin, Y. Matsumoto
Numerical Analysis of the Influence of the Tip Clearance Flows on the Unsteady Cavitating Flows in a Three Dimensional Inducer
Cavitation: Turbo-machinery & Medical Applications, WIMRC FORUM 2008, Warwick University, UK, 7-9 July, pp.246-250 (2008)

K. Okita, T. Tawara, K. Ono
Shape Representation by Signed Distance Function for Immersed Boundary Method
WCCM8 and ECCOMAS2008, June 31-July 5, Venice, Italy (2008)

K. Okita, K. Ono, S. Takagi, Y. Matsumoto
High Intensity Focused Ultrasound in Human Body
WCCM8 and ECCOMAS2008, June 31-July 5, Venice, Italy (2008)

K. Okita, K. Ono
Numerical Simulation of Immiscible Multiple Fluids Flow by Diffuse Interface Model
APS 60th Annual Meeting of the Division of Fluid Dynamics, Salt Lake City, USA, Nov.(2007)

K. Okita, K. Ono
Numerical Simulation of Multiple Fluids Flow by Diffuse Interface Model
6th International Conference on Multiphase Flow ICMF2007, Leipzig, Germany, July, S1_Fri_B_66 (2007).

K. Okita, S. Takagi, M. Futakawa, Y. Matsumoto
Pressure Wave Phenomena in Liquid Metals Containing Micro Gas Bubbles
APCOM’07 in conjunction with EPMESC XI, December 3-6, Kyoto, Japan, MS20-1-1 (2007)

K. Okita, A. Fujiwara, S. Takagi, Y. Matsumoto, M. Futakawa
Numerical Study on a Mitigation Strategy Using Micro Bubbles for Cavitation Erosion Caused by a Thermal Shock in Liquid Mercury
Sixth International Symposium on Cavitation, Wageningen, Netherlands, Sep 11-15 (2006).

Selected Publications Page