It’s a World Record - realization of a unique magnetically shielded laboratory with a damping factor of more than one million
Whether for the direct measurement of brain waves or for the determination of neutron-torque, one needs a reliable degaussing of the magnetic shields in order to achieve the necessary low magnetic field strengths. A basis for this is a current-controlled HERO Power Precision Power Amplifier from Rohrer Mess- und Systemtechnik in Munich, tailored for the specific application.
Extreme requirements call for special, customized solutions.
The human brain still remains a mystery. Therefore it's a goal of the researchers to measure the location of brain activity directly and clearly. Previously, one could only estimate values by means of other variables, for instance by deducing from an increased blood oxygen content to a locally increased brain activity, which is not very accurate. "One solution would be to measure directly by MRI the extremely small magnetic fields in the nano-Tesla range that produce the brain waves. In addition, the neural effects occur in the millisecond range, i.e. at frequencies of less than 1 kHz", summarizes Dr. Rainer Körber, employee at the PTB (Physikalisch-Technische Bundesanstalt), Department of Biosignals.
This method to localize brain activity by using low field MRI would then enable an exact representation in the anatomical image and avoid the erroneous estimation of the EEG. The problem: The Earth's magnetic field (50 μT) and the various magnetic fields of electrical and electronic systems interfere and distort the measurements. Solution: a strongly magnetically shielded chamber. The PTB scientists have now developed a method which works with magnetic field strengths of less than 50 μT (millionths of a tesla). Via this method the hydrogen nuclei are selectively electromagnetically excited, which then give a signal until the return to the ground state. If one uses low field strengths, one can simultaneously detect brainwaves. Such a combination is not possible with conventional high-field MRI.
PTB have built a magnetic shielding cabin: it consists of two layers of Mu-metal, a soft magnetic nickel-iron alloy of high magnetic permeability, ideally suited to shield low frequency magnetic fields. A specially designed coil system to the Mu-metal layers creates an excellent degaussing of the corresponding chamber, with residual magnetic fields of less than 1.5 nT at a gradient of less than 20 pT/cm. The cabin installed in Berlin was built by the company Vacuumschmelze (Hanau). The degaussing is done with the amplifier by Rohrer.
The photo shows the inner layers of the magnetic shielding with Prof. Dr. Peter Fierlinger (left) and doctoral candidate Michael Sturm. (Source: TU München)