Positive Contrast MRI of Magnetically Labeled Human Cervical Carcinoma Cells and Tumor Monitoring

Introduction: There are rapidly increasing interests in cellular and molecular magnetic resonance imaging (MRI) techniques using contrast agents for the visualization of the migration, and growth of various types of cells, including stem cells and tumor cells. Several MR contrast agents have been suggested for use in cellular MRI. Among them, superparamagnetic iron oxide (SPIO)-based contrast agents are currently being evaluated as potential markers for the monitoring of cellular growth, migration, or proliferation since SPIO’s can remain internalized for more than several weeks [1]. As a result the technique is applicable to monitoring tumor growth. However, SPIO-loaded cells cause signal reduction (negative contrast) associated with spin dephasing. This negative-contrast visualization is sometimes problematic since it can be confused with the signal voids caused by tissue inhomogeneity (low intensity). Several researchers have proposed positive-contrast imaging (PCI) methods using either spectral selection of an offresonance region near the labeled cells [2, 3], suppression of non-labeled regions by dephasing [4] or spectral selection using different echo times [5]. We were motivated to investigate a method to generate positive-contrast by using a susceptibility weighted echo-time encoding technique (SWEET) [6]. We demonstrate the SWEET method can be useful to detect the magnetically labeled human cervical carcinoma (KB) cells implanted in mice, and to monitor tumor growth in mice in vivo for a duration of 2 weeks.