The PTB 83-SQUID system for biomagnetic applications in a clinic

The PTB 83-SQUID (superconducting quantum interference device) system is described which is operated in a clinical environment. Integrated dc SQUID magnetometers with additional positive feedback (APF) are used in order to realize electronic first- or second-order gradiometer configurations. The dewar for the system has a flat bottom. It allows the detection of vertical (B/sub z/) and horizontal (B/sub x,y/) field components: 49 sensors (plus 14 reference magnetometers) are sensitive for B/sub z/ and 14 sensors (plus 6 reference magnetometers) for B/sub x,y/. The system is installed inside a standard shielded room (Vakuumschmelze type AK3b) in the Klinikum Benjamin Franklin, Steglitz, Berlin. A typical white noise level of 2.5 fT//spl radic/(Hz) has been measured in the first-order gradiometer mode. Due to the very low system noise level it became possible for the first time to detect the extremely weak neuromagnetic fields (5-10 fT peak amplitude) generated by the nerve roots deep in the lower back which are evoked by electrical nerve stimulation at the foot (200 Hz system bandwidth, 8000 responses averaged).<>