Spectra

Open Source Biomedical Imaging

May 02, 2019

Time Series Data: Functional Changes of Material in the Body

What is the underlying impedance data signal that Spectra uses, anyway? It’s pretty amazing and totally different from EEG and ECG, which you might be familiar with already. Since Spectra is measuring impedance changes, you are measuring how the dielectric qualities of the material between the electrodes change over time. Considering every material has it’s own dielectric curve, you can see changes like air entering and leaving the lungs, or the flow of blood through the heart valves. Spectra is measuring these types of material changes.

The video above shows the data Spectra collects when the electrodes are wrapped around the chest and the lungs expand and contract while breathing. When I take a deep breath, you can see a big change in lung volume, and when I take a shallow breath the change is much smaller. This information can’t be acquired with typical EMG/EEG methods, hence it’s a good example of how the EIT measurement technique offers unique information.

Other time series ideas include measuring fat, muscle, stomach, and bladder… or really anything that undergoes a material change where there are differences in conductivity over time. You could use it in a fish tank to see when a fish swims by, or to observe the dehydration process over time.

Getting a Good Signal on the Body

Electrical Impedance Tomography is susceptible to giving strange results unless you have good contact with the body and, preferably, use a four-electrode technique which implements common mode rejection. Spectra uses this four-electrode technique when imaging, which means you can get good signals when its used on the body. I tried my first eight-electrode system on my wrist but found I got too much noise without differential referencing, and then discovered that this four-electrode technique was the best standard for biological EIT imaging. If you’re interested in learning more, here is a good article on differential referencing.

Here are a set of electrodes I’ve used for lung imaging:

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