Impedance Plethysmograph

Hello Everyone,

I am new to the forum and in need of great help from you people. I am not good at electronics but I have no way of escaping this project as it is my final year project.

I need to build an impedance plethysmograph (IPG) which is a device that measures the tissue impedance. The current is applied to a portion of the limb with the help two electrodes, called drive electrodes, and the voltage across another pair of electrodes, called receiver electrodes, is measured. My idea of making the instrument is to use

1. Constant current source (sine wave generator with variable frequency (range 10kHz to 1000kHz) and current amplitude 1mA to 5 mA).
2. connect the output of the generator with the the drive electrodes, through a relay because I am making use of 2 sets of electrodes with four electrodes in each set, which is called tetra-polar arrangement. The relay will be controlled, to switch between the drive electrodes of two sets, with the help of 8051 micro controller.
3. connect the output of the receiver electrodes to the input of the data acquisition system (DAQ system). The DAQ further has other components, amplifier, rectifier, switching circuit, etc, to connect its (DAQ's) output to the PC.

I don't have much knowledge about the circuits so I want the circuit for each part like for sine wave generator, placing a relay between the generator and the drive electrodes, type of electrodes to be used, type of amplifier to be used and its circuit, type of rectifier to be used and the circuit, DAQ specifications...

I know I have exceeded my limits in asking for help in this thread but I had no option. Electronics is not my field of interest but I am stuck in it, and this is the last project and a must. So please please please help me with it.

Sounds like an interesting project. It's curious that you are asked to design the electronics.

My first question is what are you measuring with the "receiver electrodes" - voltage or current? In either case, what's the resolution you need - uVolts or uAmps, or ...?

If the requirements aren't TOO TOO difficult, an instrumentation amplifier might work for the measurement. You need to consider allowable offsets, noise, and dynamic range (maximum signal size).

I intend to measure voltage at the output electrodes, whose resolution will be mVolts.

Designing the electronics is really not my cup of tea. I have been trying since last month and did not get anywhere. How can this be made I could not even find a circuit to generate the required sine wave.

Isn't this what modern bathroom scales do? The ones that report fat and water percentages. Just wondering if a reverse engineering would give you ideas.

Have you Googled: impedance plethysmography circuit

Ken

What program of study is this for? That might help us steer you a little better. What, if any, coursework have you taken in electronics?

Yes I have googled it and there is a myriad of papers on the theory to build it and some have their own block diagrams and still some other papers have circuit diagrams with no values for the individual components. The are papers that lead you to IEEE Xplorer but I can't buy those papers.

For the course work, I am sure you will laugh at me but if it helps me solving this problem then here you go, I am doing engineering in electronics and communication, and this project has been given to me by a teacher of Instrumentation and Control department.

Hello,

Perhaps this PDF might be of interest:
http://homepages.cae.wisc.edu/~bme300/blood_resistivity_f08/reports/Final_Paper.pdf

Bertus

Check to see if your school library has a subscription to the IEEE Xplorer database, either directly or via collaboration with another institution.

If you are majoring in electronic engineering, then this doesn't seem like a completely unreasonable project, even if your emphasis is in communications (in fact, it's a much more reasonable project than if your emphasis were in something like computer engineering).

What are you using for your limbs? Humans? Living animals? If so, be sure that you have addressed all of the research protocol requirements. I don't know what country you are in, but I suspect you have some pretty stringent requirements. In the U.S., you have to jump through a bunch of hoops just to be able to ask students to fill out a survey, let along attach something to their body that will push electrical current through it.

Also, the legal stuff aside, be sure that you go overboard in addressing safety concerns if you are actually hooking this up to someone (even yourself). You mention 5mA and that strikes me as a pretty high current. I seem to recall that one of the concerns about surface body impedance measurements was that it required "high" currents approaching the 1mA range. So you might want to check into that some more.

I would suggest you start slow and simple. Come up with some kind of a somewhat reasonable stand-in for a living being. Perhaps a skin-on chicken breast or an uncooked sausage. Then first come up with a circuit that permits you to inject a controlled DC current into the thing and detect a voltage signal across the measurement probes. Then make it an AC signal. Then deal with the noise issues. Then figure out how to capture the data and get it to a PC. Break the problem down into small, manageable chunks and tackle each chunk as a separate, simpler problem.

Try searching on Body Fat Analysers?

I've come across a few models but they are always based on custom designed integrated circuits and it's hard to find a useful schematic.

Here's a pretty good reference article about the field. It doesn't say much about circuitry, but the citations might.

I tried to find an example of the bathroom scale circuits, but so far haven't had much luck. Maybe the patent database is worth a look.

I have broken it into parts and the very first part is what bothers me at the moment, the constant current supply with variable frequency (10kHz to 1000kHz) and the same current of 1mA to 5mA. Can you tell me how to make that?

Break this down even further. Make a circuit that does nothing but generate, say, a 100kHz sine wave at a fixed amplitude.

I think this might turn out to be the major breakthrough for the project (you know by now it is a bigi big thing for me at the moment). I thought of trying a different approach to the project and searched for projects on building your own ECG.

ElectroCardiGraph is somewhat similar to the Impedance Plethysmograph (IPG). In both electric current is applied to the human cells through the electrodes and both measure the voltages. Here is a link to project for building an ECG:
http://www.eng.utah.edu/~jnguyen/ecg/ecg_index.html

The maker of the project has also considered the amplitude of the current to be applied to the cells which will be 1mA to 5mA.

Do you think this can solve my purpose? I definitely see that it will provide me with the constant current supply but it makes use of 9V battery, so where will I get the frequency same hurdle again. Can a Dc to Ac convertor work here?

as4 said:

ElectroCardiGraph is somewhat similar to the Impedance Plethysmograph (IPG). In both electric current is applied to the human cells through the electrodes and both measure the voltages. Here is a link to project for building an ECG:

Click to expand...

Wrong. The ECG input amplifies the differential voltage of fields generated by the action potential of heart muscle cells. You do not apply an external current.

Ken

as4 said:

I have broken it into parts and the very first part is what bothers me at the moment, the constant current supply with variable frequency (10kHz to 1000kHz) and the same current of 1mA to 5mA. Can you tell me how to make that?

Click to expand...

We can, but the missing piece for most folks here would be the properties of the load - human tissue. At those frequencies, impedance and capacitance becomes a big issue. I've studied human resistance, impedance and capacitance, and it's certainly not a number you can just go look up. The reference I cited above makes the same point.

I built a cranial stimulation device for a friend that delivers constant current of 50-600µA as a square wave reversing polarity at about 0.5Hz (varying). Even that low current requires several volts to drive it through the ear lobe clips and across the relatively short distance from ear to ear. And frankly I don't know if even the 9V battery does the job. My circuit will cap the top end of the current, but it obviously cannot force more current thru than the straight 9V supply can muster. Driving 5mA from hand to foot, for instance, might require a fairly high voltage - one the subject might feel and not enjoy. I once touched my brother's ham radio RF amp. Knocked me across the room, with a burnt finger tip.

I reiterate my suggestion to reverse engineer the BIA technology used in bathroom scales. This has at least been approved for safe use, and many studies have shown that the results are quite good. The technology is moving from the bathroom into medical labs. If you have to deviate from the known and proven technology, you should have very good reasons and take precautions as you leave the safe zone.

Impedance seismography has been used to calculate system circulation and assess body liquid amounts. . Typically this has been employed to calculate the system circulation, specific "impedance cardiography.
1.detect thrombus filed in the deep veins of the leg.
2.screen patients who are likely to have thrombus in the leg.

I have been working on this project and thought it to be beneficial to share with you all the progress I have made so far. The list goes like this:

1.) making a dual power supply with the output as +5V, 0 and -5V. The power supply is required by the IC MAX038 which has been used to generate sine wave.
2.) making a circuit, using MAX038, that can generate a sine wave of 50khz and the output current is between 1mA to 5mA (approx. 2.16mA)
3.) making a switching circuit using the IC 4060 with a switching time output as 4 minutes approximately (3:57 min to be precise). This circuit operates a special relay which has four outputs. The four electrodes are connected to the four outputs of the relay.

I have tested the equipment on a potato and measured the output voltage changes using a CRO. I will be testing it on the arm muscles soon.

What need to do next is find a Data Acquisition System that can be used for the bio-medical purpose. Here the output voltage range will be in milli volts, so I need a DAQ that can sense such small voltage change and also switch the channel when the switching of the electrodes takes place. I would like to have a USB type DAQ so that the instrument is portable and does not need to be attached to the computer, at the time of operation, to store the data.
Can you help me find one?

Thanking you in advance

I think you would be better off designing an gain stage in your instruments output, and not trying to find a DAC with that low an input range.

Ken

I have an amplifier circuit using IC741 for the ac amplification. I will try to utilize it at the output.