P16071: MRI-Compatible EEG Cap
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Subsystem Design

Table of Contents

Team Vision for Subsystem-Level Design Phase

During the Subsystems Design phase, the team planned to begin testing to gain a better understanding of the feasibility of the two design paths (conductive ink cap and fully optical cap) that were selected during the Systems Design phase. This phase focused testing on the critical subsystems, including the optical modulator, foam electrode interface, and conductive ink wiring. In addition, the team planned to do more research into the primary feasibility concerns, including the manufacturing feasibility of conductive ink circuitry, the ability to depackage commercial electro-optic modulators, and the ability to operate a modulator reliably in the absence of a bias voltage.

Extensive preliminary testing was performed on a commercial electro-optic modulator in Dr. Preble’s lab; however, results and testing were constrained by the ability to only test on a noisy system not designed to accommodate the small signal amplitudes encountered during EEG acquisition. In addition, a commercial EEG was tested to compare the performance of a foam electrode interface to that of conductive gel. Unfortunately, the team was unable to obtain conductive ink samples in time for testing. Instead, additional literature research was performed to gather proof of concept data from other researchers, and the manufacturing feasibility of conductive ink was established during a meeting with Dr. Cormier.

Throughout this phase, the team was able to gain a deeper understanding of the advantages and challenges of each of the two proposed design paths, and the results of the preliminary testing will help inform the team of the detailed design requirements of each system.

Updated Engineering Requirements and Functional Decomposition

Engineering Requirements

 Engineering Requirements

Engineering Requirements

The Engineering Requirements Document can also be viewed here.

Functional Decomposition

 Functional Decomposition

Functional Decomposition

The Functional Decomposition can also be viewed here.

System Architecture

 System Architecture Diagram

System Architecture Diagram

Notch Filter

 Notch Filter Schematic

Notch Filter Schematic

 Notch Filter Simulation Results

Notch Filter Simulation Results

The input signal (red) is the combination of a 60hz and 30hz signal. The output signal (green) is representative of the 30hz signal as the 60hz signal has been filtered.

Low-Pass Filter

 Low Pass Filter Schematic

Low Pass Filter Schematic

The schematic does not contain any component values due to them relying on specifications of the input signal. In respect to the optical implementation, it is dependent on the detector and laser power. In the ink implementation, it is dependent on the conduction of the ink itself.

Optical Network

 Optical Network

Optical Network

Critical Sub-Systems

While all subsystems are important to the success of the project (or they would not have be included in the project), only a few have been deemed to be critical. To be deemed critical, there must be a significant perceived likelihood of failure. Subsystems which are highly important but mechanistic to implement were not considered critical since they will not require extraordinary precautions to be taken. To this end, the following subsystems have been identified as critical:

Test Results

Baseline EEG data with AD Instruments & Conductive Foam/ Conductive Mesh Testing

GOALS:

The Testing Document can be viewed here.

The Results Document can be viewed here.

Procedure:

Hardware:

Electrode positioning:

Left temple ­ Ground (green)
Right temple ­ Negative (white)
Behind right ear ­ Positive (black)
Foam Electrode

Foam Electrode

Foam and Fabric Electrode

Foam and Fabric Electrode

Baseline:

Meditrace foam electrodes
Abrasive gel
Alcohol swab
Signa creme

Foam Electrode:

Meditrace foam electrodes
Alcohol swab
AllSpec conductive foam

Foam and Mesh Electrode:

Meditrace foam electrodes
Alcohol swab
AllSpec conductive foam wrapped in conductive mesh

Experiments: All three tests were completed for each type of electrode (baseline, foam & foam/mesh). Each test was exported (channels 3&4 only) as a .txt file and a reference photo was snapped of the filtered data spectrum. The subject (Jason) was lying on the floor for each test of the tests to ensure a relaxed environment.

  1. Rapidly blinking for 10-20 seconds
  2. Eyes open for 15 seconds, eyes closed for 15 seconds
  3. Eyes open for 20 seconds, blinking for 20 seconds, eyes closed for 20 seconds

Power Density of Different Electrode Material

Conductive EEG Paste

Conductive EEG Paste

Conductive EEG Foam

Conductive EEG Foam

Conductive EEG Foam and Fabric

Conductive EEG Foam and Fabric

Pmax Ratio

Pmax Ratio

SNR of EEG

SNR of EEG-relevant frequency

SNR of EEG-relevant frequency

Amplitude Variation in EEG-Relevant Frequencies

2hz

2hz

6hz

6hz

10hz

10hz

22hz

22hz

50hz

50hz

Sample test results before and after process improvements for 100uVpp input voltage

Sample test results before and after process improvements for 100uVpp input voltage

A significant improvement was gained by changing the bias voltage to operate closer to the linear region of the sine wave and polishing the optical fibers to allow more light transmission. In each case, the signal generator was turned off at 3 seconds to collect noise data. The large SNR gain after these process improvements is evident by the figure.

Commercial Modulator Preliminary Testing

GOALS:

The Testing Document can be viewed here.

Procedure Connect the signal generator as input to the voltage divider. The output of the voltage divider is connected to the modulator. The power supply is connected to the modulator bias port. The Laser is run through the modulator into the photodetector, which then passes the signal to the amplifier and into Channel 1 of the AD Instruments recording device.

Initial data was taken with the bias voltage set to 6.75V, an amplifier gain of 10^6, a laser output at 1590nm and a power of 9.997dBm. The signal generator was disabled to quantify noise. The signal generator was then set to frequencies of 2Hz, 6Hz, 10Hz, 22Hz, and 50Hz, at each level being set to output voltages of 10mV, 20mV, 30mV, 40mV, and 50mV. The generator was then set to 10Hz, and the output voltage of the signal was swept between 40mV and 50mV in 1mV steps. The resulting signal was recorded in LabChart for each configuration for further analysis.

Calculations

Optical Subsystem Calculations

Conductive Ink POC Calculations

Sources:

http://web.mit.edu/~ppurdon/www/Bonmassar_IEEE_TMTT_2004.pdf

http://www.iaria.org/conferences2015/filesCOGNITIVE15/GiorgioBonmassar_Keynote_2015.pdf

EEG/(f)MRI measurements at 7 Tesla using a new EEG cap (“InkCap”)

http://www.nature.com/articles/srep09805?WT.ec_id=SREP-20150505

Risk Assessment

Risk Assessment
ID System Applicable Risk Item Effect Cause Likelihood Severity Importance Action to minimize
1 Optic Phase shift is too small to detect System failure Too much inherent noise in the system; inability to apply a bias voltage to the modulator 4 5 20 Research the system physics thoroughly before commiting to the design
2 Both U of R testing capabilities are exceeded before working system is developed Possible system failure Poor use of available test time; difficulty getting working system after minimal MRI testing 2 5 20 Find alternative test methods that do not rely on full-scale MRI; test as many items/ideas as possible within allotted time
3 Optic Budget is exceeded Customer requirement failure Inability to get used or discounted modulators; high cost of quality optical components (i.e. laser) 5 3 15 Ensure that we have well sourced cost estimates. Negotiate with companies to receive discountsthoroughly.
4 Both Insufficient time to complete project System failure Poor time planning; unanticipated tasks or component failures 3 5 15 Ensure that Gannt chart is frequently updated to reflect timelines, have backup designs in place in case of failures
5 Optic Modulators break or contain MRI-incompatible metal when depackaged System failure Insufficient research into depackaging; mishandling during depackaging process 3 5 15 Communicate with manufacturers to understand modulator packaging; depackage Dr. Preble's modulator first to get a feel for feasibility
6 Ink Cap corrupts MRI data Customer requirement failure Electrodes and/or ink corrupts data 3 4 12 Ensure that wires are laid out in an intelligent way to minimize loops. Ensure that the conductive footprint of electrodes is as small as possible
7 Ink MRI corrupts EEG data Customer requirement failure Loops, Eddy currents, etc 3 4 12 Make the data transmission wires as short as possible. Use signal processing to identify data collected during peak MRI activity and filter it
8 Optic Depackaged modulator has too much noise Engineering requirement failure Removing metal case introduces additional noise sources not tested for in POC 3 3 9 Try to successfully depackage Dr. Preble's modulator, and test the modulator before and after to monitor changes
9 Ink Electrode/wire heating exceeds established limits in MRI Engineering requirement failure Insufficient research into SAR minimization 2 4 8 Research SAR sources and minimization methods prior to designing electrodes and wiring
10 Optic Cap corrupts MRI data Customer requirement failure Electrode footprint from commercial modulators is too large 2 4 8 Research materials and footpring of commercial systems thoroughly, and depackage Dr. Preble's modulator for further examination
11 Ink Proper conductive ink system is unable to be manufactured at RIT System failure Insufficient research into InkCap manufacturing methods or RIT capabilities 1 5 5 Thoroughly understand manufacturing feasibility of system before committing to design
12 Both Parts do not arrive on time Deadlines are not met; ability to meet deliverables is compromised Parts are ordered too late; poor tracking of part shipments 2 2 4 Plan for a minimum of 2 weeks of shipping time when ordering parts; follow up with manufacturer on slow shipments
13 Ink Conductive ink degrades quickly EEG wire shorting Improper ink chosen; insufficient protection 1 4 4 Perform degredation tests on several brands of ink before selection for use in final design
14 Optic MRI corrupts EEG data Customer requirement failure MRI interferes with signal conduction through electrode 1 4 4 Use signal processing to identify data collected during peak MRI activity and filter it
15 Ink Budget is exceeded Customer requirement failure Budget is exceeded 1 3 3 Ensure that we have well sourced cost estimates for all components

Bill of Materials

The Bill of Materials can be viewed here.

Plans for next phase

Phase 3 Accomplishments

Test Plan

Plan Phase 4

Plan Phase 4

During the detailed design phase, the team needs to pursue more rigorous testing to firmly establish the feasibility of each design pathway, given the high cost and technological risks involved with the optical EEG system. In particular, the team should:


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