Lymph Node Imaging

Two clinical problems common to CT and MR imaging are: 1) distinguishing unenlarged metastatic lymph nodes from normal lymph nodes; and 2) differentiating enlarged metastatic nodes from benign hyperplastic nodes. Differentiation of metastases from fibrosis, lipomatosis and cysts is possible with resected lymph nodes in a 4.7T magnet using voxels of size 0.1 by 0.1 by 1.0 mm; however, gradient strength and switching capabilities are not adequate in clinical imagers to obtain the necessary spacial resolution. This inadequacy of clinical imagers is circumvented by the use of USPIO.

USPIO particles with a mean diameter of 80 nm may be injected into the interstitium of the foot pad of rats. After a suitable delay, marked loss of signal of normal lymph nodes is seen. Metastatic nodes show less uptake resulting in less decrease in signal, allowing differentiation of normal-sized, metastatic nodes from uninvolved, normal nodes. From experience with conventional lymphangiography, this route of injection is unlikely to opacify all the abdominal lymph nodes.

USPIO particles, with a median diameter less than 10 nm, will localize in lymph nodes following an IV injection. This material does not undergo uptake by the RE system as rapidly as larger particles, resulting in a longer plasma half-life in rats (81 minutes, vs 6 minutes). This factor and its small size allow transcapillary passage either into the interstitium and then to the lymph nodes or directly into the lymph nodes. In the rat model, IV injection of USPIO allows differentiation of normal lymph nodes from normal size metastatic nodes based on differences in signal characteristics. MR microscopy of excised lymph nodes, performed at 9.4T shows the USPIO to be associated with macrophages in the medullary sinuses.