基于电阻抗层析成像的高强度聚焦超声温度监测技术

作为一种对正常组织无损伤且不易引起癌细胞转移的非入侵肿瘤治疗手段,高强度聚焦超声(HIFU)治疗过程中焦域的温度监测是实现剂量精准控制的关键.本文基于生物组织的温度-电阻抗的关系,将电阻抗层析成像(EIT)和HIFU治疗相结合,提出了一种利用组织焦平面的表面电压实现电阻抗重构的检测技术.建立了HIFU治疗和EIT综合系统模型,在考虑组织的声吸收条件下,对三维Helmholtz方程在柱坐标下的声场计算进行了二维简化,并引入Pennes生物热传导方程来计算HIFU焦域的声压和温升分布特性;引入生物组织的温度-电阻抗关系,基于麦克斯韦电磁场理论,建立了具有温度分布HIFU焦域的电流和电压计算模型,利用恒流注入的边界条件实现电场计算,获得焦平面的表面电压分布.在数值计算中,利用实验聚焦换能器参数,模拟了在固定声功率下组织焦域的声场和温度场分布,以及中心和偏心聚焦条件下不同治疗时刻的电导率分布;然后通过对称电极的循环电流注入,计算了组织模型焦平面内的电流密度和电势分布,获得了焦平面圆周分布的表面电极电压;进一步采用修正的牛顿-拉夫逊算法,利用32×32的表面电极电压实现了焦平面内电导率分布的重建.结果表明,基于温度-电阻抗关系的EIT电导率重建技术不但能准确定位HIFU焦域中心,还能恢复HIFU治疗中焦域的温度分布,证明了EIT用于HIFU治疗中温度监测的可行性,为其疗效评估和剂量控制提供了一种无创电阻抗测量和成像新方法.

Noninvasive temperature monitoring for high intensity focused ultrasound therapy based on electrical impedance tomography

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 21^st 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.