- Magnetic resonance imaging (MRI) MRI is a diagnostic tool that uses a strong magnetic field and radio waves instead of X-rays to show detailed changes in brain structure and chemistry. For the procedure, the patient lies on a table in a narrow cylinder containing a magnetic field. A contrast material called gadolinium may be used (injected into a vein) to enhance the images. The procedure takes about an hour, and the patient can speak with a staff member via an intercom system at all times.