In vitro relaxometric characterization of superparamagnetic contrast agents

Abstract

The relaxation behaviour of solvent protons of aqueous dispersions of single crystals, and of small agglomerates of superparamagnetic materials, has been measured at 0.47 T and 37 °C, and compared with that of larger systems like magnetic spheres or large agglomerates of magnetic grains. The longitudinal recovery of the nuclear magnetization, as well as its transverse decay, allow for mono-exponential fits only for dispersions of small particles, sized up to a few tens of nanometers. In this case the transverse relaxation rateR ₂ is independent of the echo time, and has a linear relationship to the iron concentration. For larger particles or agglomerates of a diameter lying within the micrometer range, the transverse evolution obtained with a Carr-Purcell-Meiboom-Gill (CPMG) sequence is characterized by a marked influence of the inter-pulse delay (TE), and has to be described by a multiexponential regime including a very fast initial decay. The initial fall in magnetization is one of the most prominent features of the large-size systems and, together with the parameters describing the subsequent multiexponential regime, is an important factor in the description of their effect. This parameter may be valuable in the comparison of such agents.