The aortic walls of the control animals were smooth, and the signal emitted was relatively high. In contrast, signal strength decreased in a dose-dependent fashion in rabbits injected with D-or DM-USPIO. At dose 3, the SNR tended to be lower in rabbits injected with DM-than with D-USPIO (P , 0.1). At dose 2 DM-USPIO, the SNRs obtained from three ROIs on three different aortic images were significantly lower than those of the rabbit treated with an equivalent dose of D-USPIO (P , 0.05). At dose 1, there was no statistically significant difference (P . 0.1) (Figure 3) . These findings indicate that the uptake of either type of USPIO produced a decrease in SNR.