| Fe3O4 nanoparticles were synthesized by coprecipitation and then modified by dextran or citric acid, from which two water suspensions were obtained. The performance of both suspensions was characterized and compared with two commercialized contrast agents purchased from Shering Company, Germany. It is found that all the four samples are composed of superparamagnetic nanoparticles but the sizes of crystals, the hydrodynamic sizes of particle clusters, and the coating thickness of surface layer are quite different among them. Therefore, it will be interesting to compare the enhancement of the transverse relaxation time (T2) of protons among these four samples.Since the iron concentration of suspension is an important factor for T2, the precision and reliability of inductively coupled plasma emission spectroscopy, atomic absorption spectrophotometry, and themogravimetry techniques of iron ion concentration measurements have been compared. It is found that all the three methods have their own error but the inductively coupled plasma emission spectroscopy is the best. So in this study the iron concentration of suspensions was measured by this method.The influence of echo time, iron concentration, and measurement waiting time on proton T2 has been studied. It has been found that different samples behave differently. T2 enhanced by the self-made samples has a strong dependence on both the echo time and waiting time, which is caused by the larger cluster sizes of magnetic nanoparticles in suspension and the thinner coating thickness of surface layer, respectively. It is found that the concentration dependence of T2 may be well in agreement with theoretical prediction when waiting time is extrapolated to zero.The impact of molecular weight, cluster sizes, and the iron concentration on the waiting time dependnce of T2 in the self-made samples has been studied. The result shows that by decreasing the molecular weight, increasing cluster sizes, or increasing the iron concentration, the waiting time dependence is enhanced. This is because by changing these parameters the magnetic interaction among clusters in suspension is changed. The measured T2 as a function of cluster size has been compared with theoretical results. It is found that there are some differences between both, and this maybe because other factors are neglected, such as surface modification, magnetic core sizes etc.Finally, T2 enhanced by magnetic nanospheres with different sizes and different coating thickness has been studied. It is found that the echo time has some influence on T2 when the size of nanoshperes is large, and this is in accordance with recent theory. However, it is accidentally found that the coating thickness of nanospheres has no influence on T2, which is contradicted to recent theory, and requires a further study.
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