Kyoto University Graduate School of Engineering Department of Electrical Engineering Biological Function Engineering Laboratory



Numerical methods for analyzing electric, magnetic, and electromagnetic fields in biological objects and their application to biological function engineering

Biological activities of tissues and cells emit faint electric, magnetic, and electromagnetic fields, and they are also affected by external these fields via induced current, ohmic heating, electromagnetic force, and so on. Such phenomena are attracting a lot of attention from view points of both electromagnetic field utilization and safety issues in filed exposure. Typical beneficial applications are grouped roughly into three classifications; (i) medical diagnostic techniques, (ii) medical therapeutic techniques, and (iii) other engineering utilization. Examples of (i) are MEG, EEG, and MRI, those of (ii) are transcranial magnetic stimulation (TMS) and electroconvulsive therapy (ECT), and those of (III) are brain-machine interface (BMI), power supply to implanted electric devices, and so on. While, in order to ensure human safety in artificial electromagnetic environments, field estimation around sources (e.g., electric power equipment, home electric appliances, and mobile devices) and dosimetry assessment in human body have become more and more important. In these researches, numerical analyses based on Maxwell equations (and their approximated forms in particular conditions) are widely carried out to quantitatively investigate field distributions, which provides complementary methodology to physical experiments. The numerical methods have the following merits: being able to avoid ethical issues, having reproducibility, relatively low cost, and high degree of freedom in parameter settings. However, continuous efforts to progress numerical field calculation techniques are required in order to enhance the reliability of calculated results, to reduce calculation costs, and to refine reality of simulated physics.

In our laboratory, we are mainly investigating (a) exact analytical methods in multi-spheres model and (b) high-speed and large-scale methods in general shaped models based on the boundary element approach. In methods (a), calculated fields typically achieve more than ten digits of accuracy, which are utilized as reference values for calibrating measurement methods and numerical methods in (b). In methods (b), by developing surface charge simulation method (SCM) specified to voxel model analysis, and by developing a procedure to treat anisotropic media in the SCM, the applicability of the SCM has been greatly extended. The application of the fast multipole method and three-dimensional fast Fourier transformation and the use of graphics processing units (GPUs) also have enhanced the high-speed and large-scale calculation performances. For example, magnetically induced electric field in a whole body human model called TARO developed by NICT, which is composed of eight million tissue voxels, is calculated by a personal computer within several minutes by the SCM.

(Upper-left: EEG application,
upper-center: MEG application,
upper-right: TMS application,
lower-left: plane wave application to multi-spheres model,
lower-right: induced electric field in a whole body model)