Objective: MR lymphography with interstitial injection of superparamagnetic iron oxide particles was optimized in normal rabbits by investigating the pattern of signal reduction in lymph nodes as a function of dose and time after administration. The optimized examination procedure was then used in a rabbit tumor model to study the potential of superparamagnetic iron oxide--enhanced MR lymphography in the detection of metastatic lymph node involvement.
Materials and methods: The popliteal and iliac lymph nodes of 18 normal rabbits were imaged to study the dose response and time course of the effect of the contrast agent. For the dose response study, six doses of 2-50 mumol of iron per extremity were administered to three animals per dose, and MR images were obtained before and 12 hr after administration. For the time course study, 20 mumol of iron per extremity was administered to four animals, and images were obtained before and 6 hr to 42 days after administration. VX2 tumor-bearing rabbits were examined 12 hr after administration of 20 mumol (10 animals) and 50 mumol (three animals) of iron per extremity. Superparamagnetic iron oxide was injected into the foot pad of the hind limb. T1-, T2-, and proton density-weighted MR images were obtained with a 1.5-T unit.
Results: In normal rabbits, a profound and homogeneous loss of signal intensity was found with doses of 2-5 mumol of iron per extremity in popliteal lymph nodes and with doses of 20-30 mumol in the iliac lymph nodes. Superparamagnetic iron oxide caused maximal loss of signal intensity in both popliteal and iliac lymph nodes 12 hr after administration. In tumor-bearing rabbits, different degrees of metastatic displacement of lymph nodes were discernible, and even small metastases (3 mm in diameter) could be visualized when using the optimized examination protocol and the proton density-weighted spin-echo sequence.
Conclusion: We conclude that interstitial MR lymphography with superparamagnetic iron oxide enables the detection of lymph node metastases and therefore is a promising technique for improved diagnostic imaging of lymph nodes in the staging of tumors.