Hindawi Publishing Corporation Stem Cells International Volume 2013, Article ID 353105, 10 pages http://dx.doi.org/10.1155/2013/353105 Research Article Optimal Labeling Dose, Labeling Time, and Magnetic Resonance Imaging Detection Limits of Ultrasmall Superparamagnetic Iron-Oxide Nanoparticle Labeled Mesenchymal Stromal Cells Anders Bruun Mathiasen, 1 Louise Hansen, 1 Tina Friis, 1 Carsten Thomsen, 2 Kishore Bhakoo, 3 and Jens Kastrup 1 1 Cardiac Stem Cell Laboratory and Catheterization Laboratory, Rigshospitalet, Copenhagen University Hospital, Blegdamsvej 9, 2100 Copenhagen, Denmark 2 Department of Radiology, Rigshospitalet, Copenhagen University Hospital, Copenhagen, Denmark 3 Translational Molecular Imaging Group, Singapore Bioimaging Consortium, Agency for Science, Technology and Research (A ∗ -STAR), Singapore 138667 Correspondence should be addressed to Anders Bruun Mathiasen; abbe@dadlnet.dk Received 30 December 2012; Revised 9 February 2013; Accepted 11 February 2013 Academic Editor: Weian Zhao Copyright © 2013 Anders Bruun Mathiasen et al. his is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Background. Regenerative therapy is an emerging treatment modality. To determine migration and retention of implanted cells, it is crucial to develop noninvasive tracking methods. he aim was to determine ex vivo magnetic resonance imaging (MRI) detection limits of ultrasmall superparamagnetic iron-oxide (USPIO) labeled mesenchymal stromal cells (MSCs). Materials and Methods. 248 gel-phantoms were constructed and scanned on a 1.5T MRI-scanner. Phantoms contained human MSCs preincubated with USPIO nanoparticles for 2, 6, or 21 hours using 5 or 10 g USPIO/10 5 MSCs. In addition, porcine hearts were scanned ater injection of USPIO labeled MSCs. Results. Using 21 h incubation time and 10 g USPIO/10 5 MSCs, labeled cells were clearly separated from unlabeled cells on MRI using 250.000 ( < 0.001), 500.000 ( = 0.007), and 1.000.000 MSCs ( = 0.008). At lower incubation times and doses, neither labeled nor unlabeled cells could be separated. In porcine hearts labeled, but not unlabeled, MSCs were identiied on MRI. Conclusions. As few as 250.000 MSCs can be detected on MRI using 21 h incubation time and 10 g USPIO/10 5 MSCs. At lower incubation times and doses, several million cells are needed for MRI detection. USPIO labeled cells can be visualized by MRI in porcine myocardial tissue. 1. Introduction Stem cell therapy with potential to regenerate damaged myocardium is an emerging treatment modality for ischemic heart disease [1–3]. For future success of cardiac stem cell therapy, it is crucial to develop noninvasive tracking methods for determining the biodistribution and fate of the stem cells ater delivery. hus far, tracking of cardiovascular delivered stem cells in a clinical setting has been limited to direct cell label- ing with radioisotopes and tracking with gamma-cameras, single-photon emission computed tomography, or positron emission tomography [4]. Although providing highly sensi- tive visualization, these methods are limited by low spatial resolution and short half-lives of radioisotopes from minutes to hours, thus only permitting short-term tracking of the cells. Other drawbacks are exposure to ionizing radiation and nontarget signal leakage. Tracking of cells labeled with superparamagnetic iron- oxide (SPIO) or ultrasmall superparamagnetic iron-oxide (USPIO) nanoparticles using magnetic resonance imaging (MRI) ofers high spatial resolution in combination with high