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Figure 3. Three-dimensional T1-weighted gradient-echo MR imaging reconstruction (repetition time, 150 msec; echo time, 3.6 msec; flip angle, 34°; voxel size, 39 x 39 x 78 µm) shows tracking of immune cells with magnetically labeled lymphocytes homed to a human glioblastoma tumor (9L tumor model) xenograft in a mouse. Cell were labeled ex vivo by using a magnetic particle with membrane translocation signals (35). Approximately 10,000 cells are distributed throughout the elongated tumor (outlined in red) and cause low MR signal intensity, which is readily detectable. (Imaging was performed in collaboration with Helene Benveniste, MD, PhD, Center for In Vivo Microscopy, Duke Medical Center, Durham, NC.)