关键词: 高强度聚焦超声/
电阻抗层析成像/
温度监测/
表面电极电压
English Abstract
Noninvasive temperature monitoring for high intensity focused ultrasound therapy based on electrical impedance tomography
Guo Ge-Pu,Su Hui-Dan,
Ding He-Ping,
Ma Qing-Yu
1.School of Physics and Technology, Nanjing Normal University, Nanjing 210023, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11474166, 11604156), the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20161013), the Postdoctoral Science Foundation of China (Grant No. 2016M591874) and the Priority Academic Program Development of Jiangsu Higher Education Institutions, China.Received Date:08 April 2017
Accepted Date:07 June 2017
Published Online:05 August 2017
Abstract:As a new treatment modality with little thermal damage and few cell metastases to surrounding normal tissues, high intensity focused ultrasound (HIFU) therapy is considered to be one of the most promising technologies for tumor therapy in the 21st century. However, noninvasive temperature monitoring for the focal region exhibits great significance of precise thermal dosage control in HIFU treatment. By combining electrical impedance measurement and HIFU, an electrical impedance tomography (EIT) based temperature monitoring method using surface voltages is proposed to reconstruct the distribution of electrical conductivity inside the focal plane on the basis of the temperature dependent electrical impedance of tissues. In theoretical study, a comprehensive system of EIT measurement during HIFU therapy is established. With the consideration of acoustic absorption in viscous tissues, three-dimensional Helmholtz equation for HIFU is simplified into two-dimensional axisymmetric cylindrical coordinates, and the characteristics of temperature rising in the focal region are derived using Pennes bio-heat transfer equation. Then, by introducing the temperature-conductivity relation into tissues, the processing methods for electrical field and surface voltage in the focal region are constructed with constant current injection from two symmetrical electrodes. In simulation study, by applying the experimental parameters of the focused transducer, the distributions of acoustic pressure and temperature are simulated at a fixed acoustic power, and then the corresponding distributions of conductivity in the focal plane are achieved at different treatment times for centric and eccentric focusing. Furthermore, with the simulations of current density and electrical potential generated by the rotating current injection from 16 pairs of symmetrical electrodes, 32×32 voltages are detected by the 32 surface electrodes placed around the focal plane of the model. In conductivity image reconstruction, the modified Newton-Raphson (MNR) algorithm is employed to conduct iterative calculation. It shows that with the increase of HIFU treatment time, the electrical conductivity in the focal region increases accordingly and reaches a maximum value in the center due to the highest acoustic pressure and the most energy accumulation. It is proved that not only the position of the focal center, but also the conductivity distribution inside the focal region can be restored accurately by the proposed EIT based reconstruction algorithm. The favorable results demonstrate the feasibility of temperature monitoring during HIFU therapy, and also provide a new method of evaluating the noninvasive efficacy and controlling the dose based on electrical impedance measurements.
Keywords: high intensity focused ultrasound/
electrical impedance tomography/
temperature monitoring/
surface electrode voltage
