Spectral characterization of local magnetic field inhomogeneities.

The purpose of this study was the characterization of local magnetic susceptibility deviations by spectral analysis of their induced magnetic field inhomogeneities. Magnetic resonance spectra and related signal decay curves of local susceptibility deviations were simulated for different volume fractions and compositions of the object within the VOI. The size or composition of the object was varied at constant volume fraction, constant object size, or at constant 'magnetic strength' (defined as the product of the volume and the volume susceptibility of the object). Experimental spectra were acquired for individual metal spherical particles and a spherical air cavity. Where possible, spectra were used to characterize objects in terms of volume and composition. By simulations, a numerical relation was determined between the spectral broadening and the object's volume and composition. Comparison of spectra for various spherical objects showed the possibility of characterization with respect to size and composition. Experimental results confirmed the numerical results to a large extent, although the characterization was compromised by background signal decay, low volume fractions and limitations in signal-to-noise. In conclusion, spectral description of the field inhomogeneities related to small objects allows characterization of such objects with respect to size and composition. Practical applicability of the simulation results depends on background signal decay and volume fraction of the object.