Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 97

Biological tissues conduct electricity by means of water and electrolytes. Rather than considering electrical current in tissue (as in wires), we consider the current densityJ, a vector (Figure 2.27)

(2.25)

Figure 2.27 Ohm’s law in a circuit and a volume conductor.

σ is the tissue conductivity in siemens per meter (S m⁻¹). Some representative values are shown in Table 2.3.

Table 2.3 Tissue conductivity at various frequencies. Electrical properties from https://itis.swiss/virtual‐population/tissue‐properties/database after [4].

Tissue	Conductivity (S m−1)
10 Hz	1 kHz	100 MHz
Bone (cortical)	0.02	0.02	0.064
Brain (WM)	0.028	0.063	0.32
Fat	0.038	0.042	0.068
Heart muscle	0.054	0.11	0.73
Liver	0.028	0.041	0.49
Muscle	0.20	0.32	0.71

In practice conductivity may be anisotropic, e.g. along a muscle fiber as opposed to across it; or, at radio frequencies, it may be complex with real and imaginary components. For now we shall assume the simplest situation: isotropic, non‐complex but frequency dependent. Human anatomy, with irregular shapes and differing tissue conductivities, will exhibit much more complex behavior, with E‐field lines and current loops being altered by tissue boundaries and electrostatic charges induced on these boundaries according to Gauss’s Law.