I kind of stalled working on this during the holidays - I now suspect the analogue front-end will need a bit more work, and I may have to figure out another way of doing the impedance measurement. Now, I have decided to give it a proper go, and enter it into the 2016 hackaday prize. This post will describe the theory behind my idea, and where I'm going with this. Since the hardware is boxed up, this is an easy way to get back to work on this. Here goes.

The idea of Electrical impedance tomography was new to me, and I first heard about it when this project was featured on hackaday (http://chrisharrison.net/projects/tomo/tomo.pdf): The idea that you could image the inside of the body in a non-non-invasive way with electricity as opposed to light, sound or radiation appealed to me. Surely this would be easier to build than an X-ray machine?

The basic technique (described in the above video) has been around for ages, but it's main limitation has been resolution - the maximum resolution is a function of the number of electrodes, and attaching more than 16 to someone is a pain, especially if you need to know their exact positions. Thus, it is mainly used for detecting changes (lung or heart movement, gestures, brain activation) as opposed to imaging.

Based on the theory of operation, my intuition was that the actual hardware required wouldn't be too expensive. A few papers on arduino-based EIT systems (e.g. 'A Low Cost Electrical Impedance Tomography (EIT) for Pulmonary Disease Modelling and Diagnosis.' by Chitturi et al) seemed to back this up.

However, they still used external chips to generate a signal and measure the various electrode voltages. And all these approaches were still limited by electrode number to very low resolutions. But one paper demonstrated imaging things inside a tank of (salt) water - there wasn't a need for the electrodes to contact skin for basic imaging to occur.

This leads to my idea - inside a tank is the object to be scanned. Two sets of electrodes are laid out in rings around the circumference of the tank. By rotating one set of electrodes in small increments and taking measurements at each position, you can effectively have thousands of 'virtual electrodes' to build up a much more detailed picture. This sacrifices the real time nature of EIT scanning in exchange for cheap, high resolution imaging. I think I am the first to attempt this, mainly because everyone with money just gets an MRI machine (which is superior by far).

There are a few drawbacks to my intended approach:

• If the water surrounding the limb being imaged is too conductive, almost no current will go through the flesh, limiting how much of the inside we can see. If it's not conductive enough, the signals will be too small.
• There is a huge variation in impedance between different parts of a body. So my idea of using the built in ADC with no amplification may be futile, and I'll have to add a decent op-amp or something.
• Electricity travels in 3D, while this system will only scan one slice at a time. However, since by adding another motor you can scan through a vertical distance, you can collect lots more data and try to reconstruct a 3D image.
• Going from the raw data to an image will be hard - lots to learn!

I have fixes in mind for some of the above, but much more experimenting is needed. Luckily, I should have access to some decent test gear at university, so I can test out some theories using signal generators and scopes far out of my price range :)

More to come after test week!