Visualization of SPIO-labeled pancreatic islets and stem cells using a 3D ultrashort echo time sequences

Iron based labeling remains one of the most widely used approaches for the cells visualization by magnetic resonance imaging (MRI) due to its simplicity and efficiency compared to alternative methods. In the conventional MRI approach, superparamagnetic iron oxide (SPIO) labeled cells appear as hypointense regions on MR images. However, it is not always possible to identify SPIO labeled cells unambiguously because of the negative contrast of the SPIO particles on the dark background of other body components. Therefore, segmentation and following quantification of SPIO labeled cells remains challenging. MRI techniques called white marker techniques enable their visualization with positive contrast. In this study, we evaluated a 3D ultrashort echo time (UTE) MRI sequence for visualization of SPIO-labeled pancreatic islets and mesenchymal stem cells in vitro and in vivo at 7 T. Difference images were generated by subtracting longer-TE images from ultrashort-TE acquisitions in order to enhance visual contrast around labeled regions. SPIO labeled cells were detected in gel phantoms and after intracerebral transplantation in UTE MRI. Difference imaging generated visually enhanced contrast compared to ultrashort-TE images and improved visual distinction between labeled regions and surrounding tissue where conventional hypointense contrast was difficult to interpret.