While a CT scan is particularly useful for assessing bony anatomy, MRI is best for imaging 'soft tissues' such as the brain and spinal cord. An MRI is the imaging modality of choice for brain and spinal cord tumors. The MRI works by assessing the alignment of hydrogen atom nuclei in a very powerful magnetic field. The use of a magnet rather than ionizing radiation is another advantage of MRI over CT. However, because of this magnet patients with heart pacemakers and some other implantable devices cannot undergo MRI testing. Additionally, some patients will find that the longer testing period of MRI (sometimes up to one hour) in a small confined space can induce feelings of claustrophobia and anxiety. Patients who are at risk for such feelings can be given appropriate medication prior to the procedure.