OBJECTIVETo assess the characteristics of enhanced magnetic resonance image with ultrasmall superparamagnetic iron oxide (USPIO) in the inflammatory and tumor metastatic rabbit model, and explore its relevance with histologic ultrastructural findings.

METHODSTotally 36 New Zealand white rabbits were randomly divided into lymphadenitis group and metastatic group. Complete Freund's adjuvant was injected into the bilateral dorsal footpads of 18 rabbits to set up ipsilateral lymphadenitis model. The other 18 rabbits received a subcutaneous implantation of VX2 tumor cell suspension (1.5 x 10(7) cells/ml) in both thighs to set up metastatic lymph node model. Magnetic resonance scan were performed 24 hours before and after USPIO (90 micromol Fe/kg) injection. T2 values of each lymph node were measured and lymph node T2 enhancement rate was calculated as well. HE staining, Prussian blue staining, and electronic microscopy were performed to observe the pathological microstructure changes and the distribution of the iron particle in lymph node. Relationship between lymph nodes USPIO enhancement and its microstructures were further analyzed. Results Thirty-six lymph nodes in lymphadenitis group showed different degrees of reactive hyperplasia. Twenty-six lymph nodes in metastatic group were invaded by tumor cell. Non-enhanced scan showed mild difference between T2 signal intensity of the two pathological lymph node types. After USPIO enhancement, inflammatory lymph nodes showed distinct T2 signal reduction at the center, and metastatic lymph nodes showed homogenous and faint T2 signal reduction. Enhancement rate of benign and malignant lymph nodes were 57.39% and 29.45% respectively (P < 0.01). HE staining and Prussian blue staining indicated USPIO particles located mainly in the macrophages at inflammatory lymphatic medulla, while paracortical area and cortical area contained relatively much less USPIO particles due to less macrophages distribution. MRI findings were correlated with the pathological results. Electronic microscopy also verified that the majority of USPIO particles were located in the numerous cytophagic bubbles of macrophages. Lymph nodes metastasis including 4 lymph nodes with completed structure destruction due to entire tumor infiltration, 19 lymph nodes with partially lymph node structure destruction but reduced USPIO-contained macrophage numbers or reduced USPIO particles in macrophages, and 3 lymph nodes with only localized foci tumor metastasis at subcapsular area. Conclusions USPIO enhancement pattern of different lymph nodes is closely related to distribution and functional status of the intra-node macrophages. It may affect the accuracy of the lymph node property diagnosis based on USPIO enhanced image.