After the head ring is applied, a CT scan (some call it CAT scan) is obtained using the points of reference provided by the head ring. This takes about 10-20 minutes. A CT scan in some ways is just a sophisticated x-ray machine. However, the high resolution images which can be obtained, combined with the head ring allow the radiosurgery team to target any lesion in the brain extremely accurately. In some cases it may also be necessary to obtain an MRI (without the head ring in place) the day prior to treatment.