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 d e SQUID magnetometers with additional positive feedback (APF) are used in order to realize electronic first- or second-order gradiometer con figurations. The dewar for the system has a flat bottom It allows the detection of vertical (B-z) and horizontal (B-x,B-y) field components: 49 sensors (plus 14 reference magnetometers) are sensitive for B-Z an d 14 sensors (plus 6 reference magnetometers) for B-x,B-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/root Hz has been measured in the first-order gra diometer mode. Due to the very low system noise level it became possib le for the first time to detect the extremely weak neuromagnetic field s (5-10 fT peak amplitude) generated by the nerve roots deep in the lo wer back which are evoked by electrical nerve stimulation at the foot (200 Hz system bandwidth 8000 responses averaged).