Biomagnetic signals of interest lie between 1 and 100 pT for the human heart and below 1 pT for the human brain. For comparison, the static magnetic field of the earth amounts in Germany to about 50 µT and power line interferences in an urban environment have amplitudes between a few tens of a nT and approximately 1 µT. To be able to detect the tiny biomagnetic signals with a reasonable signal-to-noise ratio we have a magnetically shielded room (MSR) provided by Vakuumschmelze Hanau. The MSR, type AK3b, is approximately 3 m wide and 4 m long. It consists of two layers of mu-metal for the magnetic shielding and one layer of copper for the high-frequency shielding. The shielding factor - averaged over all three room directions - in the center of the room was determined as 35 at 0.02 Hz, 71 at 0.1 Hz, 561 at 1 Hz and 5 x 105 at 100 Hz. To further improve the low-frequency shielding performance, the MSR is equipped with an active compensation system. This consists of three orthogonal Helmholtz coils wrapped around the MSR. The currents fed into the individual coils are controlled by a 3-axes fluxgate magnetometer which detect low-frequency variations of the environmental magnetic field at the MSR position. The active field compensation system which suppresses frequency components below 10 Hz improves the shielding factor, e.g., at 1 Hz by about a factor of 10.
To perform biomagnetic measurements especially on the human brain (so-called magnetoencephalography, EEG), the MSR is equipped with an opening at the front side to optically stimulate the patient via a beamer and with a webcam to watch the patient when lying below the magnetometer system.
