Fig 2. “Double donut” design is shown. General Electric Medical Systems Signa SP scanner consists of two very short superconducting cylindrical magnets separated by a 54-cm space to allow physician or nurse access. The space between the two magnet halves allows patient access from the top or sides, typically used with the surgeon and assistant on opposite sides of the patient's head. An optically linked frameless stereotactic tracking system is integrated into the top support piece. The scanner is designed for an operating-room environment, with electrical outlets and anesthesia gases integrated into the magnet covers. This system has a large fringe field, and all surgical instruments and accessory equipment in proximity to the system must be MR-safe or MR-compatible. (Photo courtesy of John Schenck, MD, PhD, and Robert W. Newman, MS, General Electric Medical Systems.)