Contrast agents are used in magnetic resonance imaging (MRI) to enhance contrast difference in tissues by changing relaxation rates between tissues with and without contrast agent. Contrast agents can be broken into two categories: T1–based agents and T2–based agents. T1–based contrast agents change the longitudinal relaxation time (T1) and T2–based contrast agents change the transverse relaxation time (T2). In this talk, we will focus on contrast agents based on super paramagnetic iron oxide (SPIO) nanoparticles. SPIO nanoparticles change T2 of surrounding tissues and are used to label different types of cells. In the talk, methods for detecting labeled cells and quantifying concentrations of labeled cells will be presented.