MRI of labeled CD34⁺cells in tumors. (A) Magnetic resonance imaging (MRI) of tumor with unlabeled CD34⁺ cells, (B) MRI of tumor with labeled CD 34⁺ cells, (C) DAB-enhanced Prussian blue staining of tumor with labeled cells. Note low signal intensity along the margin of the tumor, which corresponds with the iron-positive cells on Prussian blue staining. (D) Enlarged inset from panel C, original magnification × 10. White arrows indicate the margin of the tumor and black arrows indicate the accumulation of iron-positive cells.