This study is being done in order to see how gadolinium-based MRI contrast (Magnevist) travels through the central nervous system (CNS) when injected through the spinal column, and to compare differences between two age groups (age 20-50, and 51-80). This will allow investigators to evaluate how much contrast has crossed into the tissue surrounding the brain. Such information will help investigators determine whether this kind of contrast can be used to model how other drugs might travel through the CNS and evaluate its use as an illustrative imaging marker that mirrors the glymphatic system.