Control versus labeled mice: group 3 mice imaged at day 10 after cell transplantation. (A) Signal intensity in tumor of control mouse with unlabeled cells on either spin echo (SE) or gradient echo (GE) sequences are almost isointense with contralateral hemisphere. (B) Mouse receiving FE-PLL-labeled cells demonstrates clear ring at the tumor periphery on both SE and GE. (C-D) Post-Gd-DTPA contrast-enhanced MRIs to differentiate between slow blood flow in patent vessels at the tumors' edge and the presence of magnetically labeled cells. The hypointense area in a group 2 control mouse (C) that received killed labeled cells becomes isointense with surrounding brain after contrast, indicating the Gd-DTPA in the area increased the signal intensity, whereas the dark ring persists in the mouse with labeled cells (D).