Magnetic Susceptibility Meter for In Vivo Estimation of Hepatic Iron Stores

An instrument is described for recording linear body scans of magnetic susceptibility in living rats. The sensor is a transformer which employs a toroid core with a large air gap. The magnetic properties of substances within the gap affect the reluctance of the core-gap magnetic circuit, thereby affecting the transformer characteristics. To minimize instability caused by temperature induced changes in core dimensions, the cores were made of the low expansion steel, Invar. The secondary voltage of the measuring transformer is compared with that of a similar reference transformer with differences in this comparison voltage processed by tuned amplification and phase-sensitive detection. The sensitivity of the instrument is limited by zero instability equivalent to ±0.06×10-6 EMU/cm3 over 30 seconds, with a precision of measurement of ±0.03×10-6 EMU/cm3 achieved by output filtering. Studies using live rats demonstrate that with this technique iron-loaded animals can be distinguished from control animals, because the high hepatic concentration of storage iron in the liver of the experimental group exhibits positive magnetic susceptibility. Consideration of the transformer characteristics suggests that similar, safe, and rapid measurements of magnetic susceptibility can be made in humans, although many other factors remain as variables which must be evaluated before such measurements can be used to quantitate iron stores in patients.