Recent developments showed that the new ActiveOne system can be used in the MEG/MCG too. Read story
During 1997 we developed a 64 channel amplifier system for MEG/MCG applications. This work was done for Dr. Peter van Leeuwen at the EFMT in Bochum where a BTI installation is used for MCG measurements.
We were asked to develop an ECG amplifier system. The three main demands for this application were :
No digital circuitry may be used inside the shielded room in order to prevent an increased noise level of the magnetic sensors
Only fiber optic connections between the front-end inside the room and the equipment outside the room were allowed in order to prevent the coupling of interference into the shielded room.
We had to provide 64 analogue outputs in order to interface with the 64 auxiliary inputs (with synchronized sigma delta 16-bit ADCs) of the BTI signal processor hardware.
To fulfill these demands a fully analog setup was chosen with a battery powered front-end and an analog optical fiber signal link per channel. The general configuration would be :
The system has an isolated front-end inside the shielded room and a receiver outside the room.
The front-end contains up to 64 electrode amplifiers and is powered by a 6V, 10Ah sealed lead-acid battery (approx. 8 hours continuous operation).
The amplified signals are transmitted to the receiver outside the shielded room by plastic optical fibers (one optical fiber per channel).
There are no digital circuits in the front-end in order to prevent interference to the MCG.
The optical fiber links are the only connection between the front-end and the receiver. The receiver is powered by the mains supply.
The receiver has up to 64 analog outputs on 37-pole D-type connectors. The connection between the receiver and the Magnes is by unshielded flatwires. The front-end is housed in a custom made plastic box (no magnetic materials), the receiver is housed in a standard 19" cabinet. A first prototype is currently under evaluation in Bochum It was proved in a series of tests in Bochum that the amplifier does not cause any significantly interference with the MCG signals. FET input stages were used in order to insure minimal DC (< 10 pA) and AC (<<1 pA) currents in the electrode wires. These small bias currents did not cause significant interference to the MCG signals, but nevertheless in the spectrum we could see that the magnetic sensors were able to detect them. The bias currents are now the main source of interference from the ECG amplifier to the MCG measurement; the influence of the currents in the front-end box could be kept below the level of the bias currents so long as the box so long as the front-end was not placed too close to the Dewar (minimal distance approx. 1 meter). Finally, a slow-start/stop power supply circuit was incorporated to prevent current transients in the shielded room (supply current is linearly increase/decreased during a 1 minute time period).