One-stage Positron Emission Tomography and Magnetic Resonance Imaging to Assess Mesenchymal Stem Cell Survival in a Canine Model of Intervertebral Disc Degeneration

Intervertebral disc degeneration (IVDD) is a major health problem. Although mesenchymal stem cells (MSCs) have been used to promote IVD regeneration, the actual survival time of implanted MSCs in IVDs has never been studied noninvasively and continuously in vivo. To investigate survival of implanted MSCs in vivo, this study used a canine model of degenerated IVD and MSCs transfected with a mutant herpes simplex type-1 virus thymidine kinase and labeled with magnetic iron oxide nanoparticles (MION). One-stage positron emission tomography (PET) and magnetic resonance (MR) imaging were carried out 3 days and 2 weeks, 3 weeks, and 4 weeks after implantation of MSCs into IVDs with surgically induced degeneration. Pfirrmann disc degeneration grade determined from the MR images indicated that the repair progress of degenerated IVD stopped 3 weeks after MSC implantation. Meanwhile, MION signal strength, signal contrast ratio (%), and low signal area (mm2) did not change significantly from that seen 3 days after c...