Magnetic resonance imaging (MRI) is among the most popular and useful diagnostic imaging tools due to its ability to generate high resolution three dimensional images without the use of ionizing radiation. The usefulness of MRI as a diagnostic relies on the ability to distinguish healthy and diseased tissue, however, this can often be challenging, particularly in small tumors. Therefore, contrast agents are commonly used to enhance tissue imaging. Currently approved contrast agents are metal-based, and often have significant toxicity concerns. Additionally, lower toxicity metal-free contrast agents have thus far proven to suffer from low sensitivity, short in vivo half-lives, and the need for very high concentrations of contrast agent. Due to these shortcomings non-metal based contrast agents have not been clinically adopted. These inventors have built nitroxide nanoparticles that overcome many of the current challenges in metal-free contrast agents.