Multipole Analysis in Magnetopneumography-A Model Study

We compare several methods of calculating the magnetic dipole moment of the field of a magnetized human lung. This dipole moment is proportional to the total magnetic dust load in the lung, hence, it is an important parameter in occupational health studies of miners and welders exposed to magnetic respirable dust. To test the accuracy of the several methods explored in this paper, we use a uniformly magnetized ellipsoidal lung model to simulate the magnetic field of the lung. Two commonly used magnetic detectors are compared in this analysis: a single second-order gradiometer above the lung, and a system of two first-order gradiometers with one above and one below the lung. The field measured by the detectors is analyzed using a multipole expansion of the field. We find that an algorithm using dipole and quadrupole coefficients is the most successful at determining the dipole moment of the lung. This performs best with the two first-order gradiometers, and is acceptable with the second-order gradiometer 20-40 cm away from the center of the lung-the distance used in practical measurements.