MR lymphography with iron oxide particles: dose-response studies and pulse sequence optimization in rabbits.

Superparamagnetic iron oxide (SPIO) particles are a promising contrast agent for MR lymphography. The effect of SPIO on MR imaging of normal lymph nodes and the impact of the size of the dose have not yet been investigated in detail. Therefore, we performed dose-response and pulse sequence optimization studies. MR images of the iliac lymph nodes of 15 normal rabbits were obtained at 1.5 T with 12 different spin-echo (SE) and gradient-echo (GRE) sequences before and after SPIO administration. The contrast agent was injected into a femoral lymph vessel at five different doses (0.02-2.0 mumol Fe/animal). The dose that reduced signal intensity by half (ED50) was determined for each sequence, and images were evaluated qualitatively. Doses of 0.2 and 1.0 mumol Fe caused a complete signal loss throughout the lymph node. In this dose range, proton density-weighted SE sequences showed a profound signal loss (ED50, 0.132 mumol Fe), and lymph nodes were sharply demarcated. The GRE sequences (ED50, 0.027-0.070 mumol Fe) and the T2-weighted SE sequence (ED50, 0.014 mumol Fe) showed an even more pronounced signal loss but insufficient anatomic resolution. Underdosing (less than or equal to 0.1 mumol Fe) caused only a focal signal loss in the lymph nodes. Oversaturation (2.0 mumol Fe for SE sequences, greater than or equal to 1.0 mumol Fe for GRE sequences) led to image distortion and did not allow assessment of lymph node morphology. Our results show that optimal contrast enhancement of normal lymph nodes with SPIO can be achieved in the dose range of 0.2-1.0 mumol Fe on proton density-weighted SE sequences. Our results may serve as a basis for further development of noninvasive MR lymphography.