To assess the utility of ¹⁹F and ¹H MRI for tracking mesenchymal stem cells (MSCs) after transplantation in a skin injury model in mice. We have selected SPIO nanoparticles and a ¹⁹F nanoemulsion, which are the most popular labeling agents for MRI, but as ambiguous reports of their value have been published, our goal was to test their utility in the context of repairing skin injury. The overall therapeutic goal of this study is to use mesenchymal stem cells to repair skin injuries. Specifically, our focus is on modeling acid burns that disfigure women attacked due to revenge or rivalry in third-world countries, as well as industrial injury victims in the Western world.The experimental design is been shown in Figure 1. MSCs were labeled with SPIO nanoparticles or a fluorine (¹⁹F) nanoemulsion as previously reported [1]. Acute skin injury was induced as described, with slight modifications [2]. MSCs were derived from 3 or 20 months old Luc+ transgenic mice, with graft recipients being 3 and 20 months of age. Prior to transplantation, MSCs were subjected to preconditioning with hypoxia or ascorbic acid (AA) treatment. MSCs (1 x 106) suspended in 50 µl saline were transplanted intradermally in allogeneic and syngeneic paradigms 1 day following acid burn induction. Imaging was performed every second or third day and included MRI (T2* for SPIO detection and ¹⁹F for fluorine with T2 for anatomical reference) and bioluminescence (BLI) as a reference for cell survival assessment. T2*-w axial images of the region with skin injury were acquired using an 11.7T Bruker Biospec scanner with a FLASH sequence: field of view (FOV)= 2.2x2.2 cm; slice thickness=1mm; TR/TE = 300/2.9 ms, number of averages [NA] =2; matrix size= 128×128. The same MRI scanner, with a 15 mm double-tuned 19F-1H surface coil, was used for fluorine imaging. 19F images were obtained using a RARE sequence with TR/TE = 1000/4.5 ms; slice thickness = 2 mm; a matrix size= 32 × 32; FOV= 2.0 × 2.5 cm2 ; NA=512; total experimental time 34 minutes. T2w scans were acquired with a RARE sequence using the same geometry as 19F MRI and the following parameters: TR/TE=5000/9 ms; NA=1. ¹⁹F signal was quantified using image J software by selecting pixels with a signal intensity above the set threshold and calculating for each image surface area with the ¹⁹F signal. BLI was performed using a Perkin Elmer Spectrum/CT ever 2-3 days after cell transplantation. Animals were sacrificed for immunohistochemical detection of hair follicle quantification 25 days after transplantation. MSCs died shortly after transplantation (within 10 days); however, despite their short survival, they facilitated improved healing of acid burns (Fig. 2). The SPIO nanoparticles and ¹⁹F nanoemulsion neither compromised the therapeutic activity of MSCs, nor their in vivo survival, as measured by BLI (Fig. 3). Donor age did negatively influence the healing properties of MSCs. Preconditioning (both hypoxia and AA) potentiates the healing capacity of MSCs. However, regardless of cell origin and the age of host, as well as the allogeneic or syngeneic source, the survival of transplanted MSCs is very limited and cell survival was not affected by any of the investigated factors. Susceptibility artifacts due to micro hemorrhages present in injured tissue compromised the quality of the 1H MRI to such a level that it was not interpretable, and it was impossible to separate SPIO-labeled cells from the background at any of the investigated time points (Fig. 4). In contrast, ¹⁹F MRI allows robust detection of transplanted cells within the tissue as characteristic background-free hot-spots (Fig. 4). We analyzed the entire process of the cell loss quantitatively, and we found a strong correlation between the loss of signal in BLI and ¹⁹F MRI (r=0.6, p<0.05) (Fig.5). Histological analysis confirmed that grafted MSCs did not survive long-term and that cell labeling had no effect on the repair of injured tissue. While visual inspection of the surface of all wounds revealed effective healing, with newly formed skin indistinguishable from the intact skin areas, the histological analysis with quantification of hair follicles detected differences between the transplanted and non-transplanted animals (p<0.05). Though, no differences between non-labeled and SPIO- and 19F-labeled cells was observed (p=0.5).¹⁹F MRI is a reliable tracer to track the location and survival of MSCs in the context of skin injuries. Considering that MSCs are already in clinical trials for wound-healing and ¹⁹F nanoemulsion is commercially available as a clinical-grade formulation, the presented method may be further translated to clinical studies.