MRI contrast agents improve visibility of internal structures by altering relaxation times. Positive contrast agents containing gadolinium, manganese or iron reduce T1 time, increasing signal on T1-weighted images. Negative contrast agents like superparamagnetic iron oxide decrease both T1 and T2 times, appearing dark. Gadolinium agents are most common; the gadolinium ion is chelated to reduce toxicity, and the coordination allows water molecules to rapidly exchange, accelerating relaxation. Side effects are usually mild. Relaxivity depends on factors like distance between water protons and gadolinium.