Magnetic Mapping of DC Fields Related to Tissue Susceptibility in the Human Body

Several studies on dc magnetic fields outside the human body have been performed in the past (Kalliomaki, 1983; Farrell, 1983; Cohen, 1983; Swithenby, 1983; Varpula, 1983; Maniewski et al., 1988). As the major constituent of tissues in the body is water, it has usually been assumed that most tissues actually exhibit diamagnetic properties with the magnetic susceptibility of pure water, χ = −9 × 10−6 SI (Varpula, 1983). However, this is no longer true if a quantitative analysis is carried out to evaluate the magnetization in the earth field of a specific portion of the body as, for instance, the upper or the lower leg. Significant differences from a “water-like” diamagnetic behaviour are observed, which may provide interesting information on diverse contributions to the overall magnetization. These may originate from small amounts of paramagnetic components in specific tissues, or from slight differences in the susceptibility value of various constituents of the body itself. For example, it has been recently estimated (Maniewski et al., 1988) that the susceptibility of the lungs may be assumed as χ ∼ −4 × 10−6 SI, due to the “sponge-like” structure of this organ. Similarly, estimated values for the magnetic susceptibility of the arterial (χ ∼ −9.3 × 10-6 SI) and veneous blood (χ ∼ −7.8 × 10−6 SI), the bone (χ ∼ −10.0 × 10−6 SI), the muscle (χ ∼ −9.0 × 10−6 SI), and other tissues have been recently reported (Rassi et al., 1988). Such estimates, however, were based on in vitro measurements of the magnetic behaviour of oxi- and deoxihemoglobin in red cells, whereas no susceptibility value was available from in vivo measurements. In this paper we present the preliminary results of a detailed mapping of dc magnetic signals originating from body tissues in the earth field.