Magnetic Resonance Imaging (MRI) is a non-invasive imaging test that shows detailed images of the internal structures of the human body that is being scanned. No radiation occurs with MRIs. MRIs use magnets to take pictures of internal structures and organs, such as the brain. People with metal in their body or who were exposed to metal shavings/fragments during their life (such as working in the military or in metal-working), should tell their doctor before having an MRI. Most people with a knee or hip replacement are able to have an MRI but be sure to let your ADNI clinician know.