Finite Difference Time Domain Numerical Simulation On High-intensity Focused Ultrasound Field

Objective:High-intensity focused ultrasound(HIFU) has received widespread attention as a noninvasive, micro-trauma therapy technique, and applied in ESWL therapy, and HIFU tumor therapy in clinical treatment. However, some complications maybe caused in HIFU therapy, such as extracorporeal shock wave lithotripsy (ESWL) maybe cause hematuria, kidney haematoma, kidney rupture, and HIFU tumor therapy may cause skin burns, nerve injury and so on. One of the main factors for effecting HIFU therapy efficacy and causing these complications is the focal regional location, shape, size, pressure distribution and the focus former regional energy distribution. Study the focal regional location, shape, size, pressure distribution to provid theory for ESWL therapy plan, Study the focusing method reducing energy density non-target before the focal region, to solve injure of ESWL treatment and HIFU tumor therapy on human normal tissues, provide new ways and methodsMethods:In this paper, we used the high-intensity ultrasound nonlinear propagation FDTD simulation method introduced by Nagayoshi Morita, Reichenberger's ESWL experiment in water as an example. (1) Establish simulation model referring to the actual human tissue structure, research high-intensity focused ultrasound nonlinear propagation in human tissue and the effect of human tissue to the focal regional location, size, shape, sound pressure distribution in treatment process.(2) To solve the complications of ESWL and HIFU tumor treatment, apply the FDTD simulation method, study and propose a phase control ring focusing method which can reduce the ultrasonic energy density of shallow region and improve focus ultrasonic energy density.Results:1. Using Reichenberger's ESWL experiment in water as an example, establish a water simulation model, the simulation result is that the focal region is similar to rugby in 3D space, but the focal length (maximum sound pressure position) is not unanimous with equipment geometric focal length. 2. On the basis of Reichenberger water simulation model, establish the simulation mode with different thickness fat or muscle thickness, discuss the effect the different human tissues to the focal region, the max sound pressure, the focal length and so on. Simulation results show that the max pressure increases with fat thickening, decreases with muscle thickening, and the focus length decreases with the human fat or muscle thickening, the rugby-like focal regional long and minor axis decrease with fat thickening, the long axis increase with muscle thickening, the minor axis is invariable.3. Referring the actual clinical structure of the human body image of B ultra supersonic image, establish a simulate mode including subcutaneous fat, muscle, perirenal fat, numerical simulation ESWL treatment process ultrasonic nonlinear propagation. The result of simulation is that focal length is 118.94mm, which is less than the geometric focal length 120mm, max sound pressure is 34.19MPa, which is less than the measured sound pressure water 44.56MPa.4. A new phase control ring focusing method is proposed and simulated by FDTD method,the results show that if array sensor is in the same conditions, the max energy density in focus obtained by ring focus method is higher than point focusing method, increasing by 30%,but the max energy density of non-target before the focal region is the same, If the max energy density in focus is same, the maxi energy density non-target before the focal region obtained by ring focus method is smaller than point focusing method, reducing by about 10%.Conclusion:In this paper, the study uses Reichenberger's ESWL experiment in water as an example, establishes water simulation model and containing different human tissue models, simulates the focal region and the max pressure, focus location, the focal regional size, and studies the effect of human tissue on them, provided theory for ESWL therapy plan. Meanwhile, proposed a new phase control ring focusing method reducing energy density non-target before the focal region, to solve injure of ESWL treatment and HIFU tumor therapy on human normal tissues, provide new ways and methods. Furthermore, in this study the simulation model is different from actual human tissue structures, the effect of actual human tissue on formatting the focal region to be waited for further studies.