Dexmedetomidine, an alpha2 agonist is currently the standard of care for sedation for diagnostic (MRI, CT and Nuclear Medicine) radiological studies at Boston Children's Hospital. Although it is FDA approved for sedation, its pharmacokinetics in children has not been carefully studied with prospective studies. The plasma concentration of dexmedetomidine necessary to achieve and maintain adequate sedation for motionless conditions for a MRI study has not been determined which is the purpose of this study.