T2-weighted images of phantoms and injected muscle leg tissues and implanted CD133⁺ stem cells. MR images of phantoms formed by a set of test tubes containing a suspension of Endorem-labeled cells in gelatin: (A) 500, (B) 5000, (C) 20 000, (D) 50 000, and (E) 100 000 cells. Sample contains gelatin only. Inhomogeneities in the phantom images were caused by the moderate sedimentation of cells while the gelatin was setting. Sequence parameters were repetition time (TR), 2000 msec; effective echo time (TE), 42.5 msec; turbo factor, 4; number of acquisitions (AC), 16; field of view (FOV), 3.5 cm; matrix, 256 × 256; slice thickness, 0.5 mm; slice separation, 1 mm. Two sets of interleaved transversal images were measured to cover the whole muscle. For in vitro experiments, phantoms of sterile agarose 2% containing labeled cells were measured by a similar sequence with different geometry: FOV, 6 cm; matrix, 256 × 256; slice thickness, 1 mm. Only one slice was measured for phantoms. Prussian blue staining of samples containing 50 000 (F) and 100 000 (G) cells. (H) Several areas of hypointense signal were seen 24 hours after intra-arterial grafting by MRI (white quadrant). (J) Histologic examination performed on biopsy specimens of MRI-positive areas confirmed positive Prussian blue-stained cells that coexpressed the CD133 antigen around muscle vessels. Scale bars represent 100 μm (A-E); 25 μm (F-G, I, inset); 3 mm (H).