BioSemi.nl

I am trying to use a Biosemi for EIT work, and thus cannot use active electrodes. Can anyone tell me if any company makes passive electrode caps that will work with the Biosemi amps?

What would be the reason that active electrodes cannot be used for Electrical Impedance Tomography ?

Coen (BioSemi)

For EIT we need to apply specific currents between chosen pairs of electrodes. I do not think we can run arbitrary currents through the actuive electrodes and have them get through the electronics to the subject. The EIT hardware goes between the subject and the amplifier normally, but in the case of Biosemi, part of the "amplifier" is on the head. Thus I would like to know if I can use passive electrodes with the Biosemi. To do so I realize I will give up a lot of CMR, but am willing to live with that. The question really is, how and where do I attach the CMS and DRL if I do not use active electrodes?

Another related question is, how can I directly measure applied test signals? I tried to hook a signal generator (10 uV sine wave) to the input of the Biosemi amp, without any active electrodes. This is a bettery-powered signal generator, one end to channel 1, and the other to CMS. I got no signal through to the display. I also tried hooking DRL to CMS through a 1 K resistor; still no signal seen. How do I test this device?

My colleague and I are also curious how you can run the active electrodes off of 5 V with no return path for the 5 V? The Active 1 system used 3 wires to each active electrode, including 5V and 0V. How can you avoid the 0V connections on the Active 2 system?

Thanks,
-Jeff

I assumed an EIT setup with passive electrodes to inject current, and a separate set of active electrodes to measure the resulting potential distribution. You are correct that our active electrodes cannot be used for current injection.

The ActiveTwo inputs can be modified to work with (any) passive electrodes. However, the modification is permanent, the channels cannot be used with active electrodes anymore. With passive electrodes, the ActiveTwo system will show the usual problems seen with passive systems: extensive skin preparation, and/or a proper Faraday cage will be necessary to achieve acceptable signal quality. Therefore, we only recommend this options for a very small amount of special applications (for example: use for combined MEG/EEG measurements).

Active/passive electrodes, and the CMS/DRl circuit are two separate issues. The CMS/DRL works identical with passive electrodes (with a passive instead of an active CMS electrode), see http://www.biosemi.com/publications/pdf ... uction.pdf

The standard ActiveTwo inputs have a low input impedance specially matched to the outputs of the active electrodes, and therefore they cannot be driven directly by a standard signal generator. The easiest way to connect a signal generator is to place the active electrodes into two buckets with salt water, see viewtopic.php?t=12

It is nice to see that you appreciate the special two-wire design of our second generation active electrodes. Using a single wire for both the output signal and the supply current return did indeed involve some special electronic development work. I hope that you understand that because of commercial reasons, I cannot reveal into detail how we did it.

Best regards, Coen (BioSemi)

Coen, thanks for the informative response. Kind of as I suspected. Since my colleague will likely not want to have me modify the hardware, I will have to use some other system for me EIT preliminary work. Too bad, since the extra bandwidth would have been nice. You have a good product for EEG, though.
-Jeff