  The starting point in quantitative susceptibility mapping (QSM) is a
theoretical model that is used to map susceptibility distributions from
magnetic field measurements. It requires regularisation techniques to avoid
artefacts in the resulting image. The underlying problem is that the model was
developed by starting with the so-called Lorentz sphere on a microscopic scale.
After averaging over a macroscopic sample, it is assumed that the magnetic flux
density vanishes in the center of the sample. For the macroscopic problem of a
homogeneous sphere in a uniform field, we show that at the surface the normal
component of the flux density is not continuous, which contradicts Maxwell's
macroscopic theory. In this paper, we propose a model consistent with
macroscopic magnetic field theory, in which we image magnetisation rather than
susceptibility. This model is well-posed. Some simple but representative
numerical examples show that it allows for high-resolution images.
