Our study has some limitations. First, the number of rabbits was too small to confirm the reproducibility of the observed aortic wall changes, and assessment of inter-individual differences was not possible. Second, the USPIO doses we used were too high for clinical applications; additional studies are necessary to identify appropriate human dose levels. Also, the imaging sequence was not optimized. (The use of a more T2*-weighted GRE sequence with longer echo times may enhance the sensitivity for iron-induced susceptibility effects. With such a sequence, even smaller iron accumulations can be detected. This may facilitate a further reduction in the contrast dose). Third, after the injection of D-and DM-USPIO particles, the signal from the aortic wall was very low and not uniform among the sites examined. ROI placement affects the results of imaging analysis, and ROI analysis of the aortic wall is subject to greater dispersions. In efforts to avoid this potential bias, we used the average result from three different ROIs in our SNR measurements. Fourth, the imaging effects we observed may be specific for WHHL rabbits.