The effects of different pulse sequences and timing parameters on the increase in transverse relaxation rate produced by superparamagnetic iron oxide particles have been studied. Gradient-echo, single spin-echo, and multiple-echo images recorded at 2.0 T were used to evaluate the reduction in signal intensity per unit concentration of iron oxide for different echo times and in different media. For the same echo time, gradient-echo sequences were found to be more than twice as sensitive to the effects of the agent as single spin-echo imaging sequences, while multiple-echo sequences were much less affected than either. Using measurements of the relative effects on different sequences, the contributions of different relaxation mechanisms have been quantified.