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P16071: MRI-Compatible EEG Cap
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Systems Design

Table of Contents

Team Vision for System-Level Design Phase

The team planned to:
The team planned to generate an appropriate functional decomposition and use it, in combination with the customer and engineering requirements generated in Phase 1, to explore top concept candidates for the high-level system design. This phase was intended to be heavily research-oriented, as the team worked to evaluate the feasibility of various concepts. The team also planned to begin some preliminary concept testing to guide the choice of prototype materials.

The team accomplished:

The team developed a functional decomposition of the intended device, and each member individually researched potential options for materials or concepts to accomplish each of the project’s most critical functions. Through research, and with the help of a Pugh Chart to compare concepts to the current state-of-the-art product, the team selected two main concepts to explore further: a conductive ink EEG cap and a fully-optical system.
After a conversation with Dr. Preble in Microsystems Engineering, the optical system was redesigned to address constraints involved with packaging a microengineered chip into a full fiber-optic system. Preliminary research into this new concept was initiated in parallel with research into a conductive ink alternative. Systems architectures and feasibility analyses were conducted on each system, allowing the team to better assess each concept’s advantages and disadvantages.
Finally, a preliminary test plan was developed to guide concept testing over the next few weeks, and a detailed risk assessment was performed to evaluate potential project risks and mitigation strategies. Conversations were initiated with various faculty members to explore the feasibility and testing opportunities of each concept.

Functional Decomposition

 Functional Decomposition

Functional Decomposition

The Functional Decomposition can be found here.

Benchmarking

Benchmarking
BrianAmp MR + BrainCap MR EGI - GES MR Micromaglink RT
# of Electrodes 32-128 32-256 64-256
Field Strength Safety 1.5 - 7T 3T 4T
Temporal Resolution 5 kHz/channel 8 kHz 20 kHz
Cost $50,000 Unknown Unknown
Signal Sensitivity (amplification) 0.5 uV/bit 0.024 uV/bit Unknown
CMRR >90 dB 50/60 Hz >90 dB 50/60 Hz 110 dB 50/60 Hz
Bit Resolution Per Channel 16 bits 24 bits 24 bits
Repositionability of Electrodes None Add-ons Available None

References:

BrianAmp MR + BrainCap MR
http://www.brainproducts.com/productdetails.php?id=5
EGI - GES MR
http://www.accessdata.fda.gov/cdrh_docs/pdf13/k131882.pdf
http://www.egi.com/research-division/research-division-research-products/ges-400-series
Micromaglink RT
http://compumedicsneuroscan.com/synamps-rt-64-channel-eegeperp/

The Benchmarking document can also be found here.

Concept Development and Selection

Morphological Chart

Morphological Chart

Morphological Chart

Selection Criteria

Selection Criteria
ID Criterion Description
1 Low cost Does not exceed cost of commercial products; meets customer cost expectations
2 EEG quality Design ensures that EEG data corruption is minimal during MRI recordings
3 MRI quality Design ensures that MRI data corruption is minimal
4 Technological feasibility Technology development is feasible within 2 semesters
5 MRI safety Device does not damage MRI or hurt patient during use
6 Universality of fit Device accommodates variety of head sizes
7 Electrode placement options Device allows for electrode re-positioning
8 Electrode count Number of electrodes with device
9 Ease of use Ease of electrode re-positioning, ease of cleaning, ease of storage
10 Application time Fast application time when patient is in MRI
11 Durability Ability to withstand daily use, travel, and handling

The Selection Criteria document can also be found here.

Pugh Chart

 Pugh Chart Functions

Pugh Chart Functions

 Pugh Chart DATUM: Brain Cap

Pugh Chart DATUM: Brain Cap

 Pugh Chart DATUM: Conductive Ink

Pugh Chart DATUM: Conductive Ink

The Pugh Chart can also be viewed as a document here.

A preliminary Pugh chart analysis with the DATUM as a the status quo competitor (BrainCap) did not make it immediately apparent which of the proposed alternatives is superior. Both the Conductive Ink and Optical Electrode systems received 7+, 2-, and 2 neutral rankings. The Onboard Storage Cap and Electrical to Optical Transmission system each only received 6+, in areas overlapping with the positive performance of the prior designs. Setting the Conductive Ink system as the DATUM instead shows that all three other alternative devices fail on cost, technological feasibility, and electrode count in comparison.

We therefore elected to discount the Onboard Storage and Electrical to Optical solutions, as they lose in several key areas compared to the conductive ink solution, and provide no relative advantage over the otherwise better-performing Optical Electrode system. Since the Optical Electrode system provides several key benefits over the conductive ink system, bringing a reduction in both EEG and MRI data corruption, our group has decided to pursue both options simultaneously. It is as of yet unknown whether the cost and technological difficulty associated with the Optical Electrode system will be insurmountable, but if they can be overcome then the solution would be more desirable than the Conductive Ink system. If not, then Conductive Ink represents the most attractive alternative based on this analysis.

Systems Architecture and Design

 System Architecture Diagram

System Architecture Diagram

Optic Specific Modules:

Light Emitter:
A laser around 1550 nm in wavelength will be used as the optical signal input. This wavelength was chosen because it optimizes the amount of loss in common optical transfer modules and materials.
Optical Splitter:
A 1x16 optical splitter is connected to the light emitter, to split the light into enough paths for each electrode, and into extra control paths for noise reduction purposes. The control paths will sit inside the MRI to experience the same noise as the electrode paths, but they will not be attached to the patient. The hope is to then subtract the received control signal from the received and amplified electrode signals, in order to get rid of MRI created noise.
Optical Electrode:
This electrode design utilizes an electro-optic modulator to phase shift an optical input based on the potential read on the body. The input for potential will be attached to either a conductive foam, or ink-based electrode, which can then be attached directly to the head. This electrode allows an optical signal to detect a biopotential on the head without ever converting to an electrical signal.
Optical Receiver:
The design will use a photodiode to initially receive the optical signal. The current idea is to 3D print a small fixture to attach the end of a fiber optic wire to a board mounted photodiode.

Ink Specific Modules:

Input Connectors:
The conductive ink EEG cap will only need a different method to connect the signals to the receiver. Possible connectors could be D-Sub connectors, or other sturdy connectors that would protect the leads, and provide a good connection.

General Receiver Modules:

Amplifier:
An amplifier stage will be needed for each incoming signal, to raise the received signal to the same level as the control path. This way, the control signal can be correctly subtracted from the received signal.
Subtractor:
This stage will subtract the control signal from the received signal using an instrumentation amplifier. This should mostly extract the information gathered from the electrodes, which will then be amplified by the instrumentation amplifier. (Risk of timing issues)
Filtering:
Low-Pass:
A low-pass filter will be used to filter out any high frequency noise that may have found its’ way on to the received signal. The filter will be able to cut off quite low, as the maximum frequency expected to be received is ~20 Hz. The type of low-pass filter is still up to decision.
Power Notch Filter:
A notch (or band-stop) filter will be used to filter out the 60 Hz noise created by power line noise. Additional notch filters may be needed if the harmonics of the 60 Hz noise become a problem.
Light Notch Filter:
Fluorescent lights typically flicker at twice the frequency of their power source, so 120 Hz in the US. Since fluorescent lights are common, there is a possibility that this frequency could end up as noise in the received signal. To prevent it, a 120 Hz notch filter will be used after the power notch filter.
MRI Environment Filtering:
This is simply a placeholder for possible filtering needed to deal with any additional problematic frequencies generated by the MRI. More research is needed to determine the necessity of this.
ADC:
The Analog-to-Digital Converter will sample the received signal, likely with a frequency around 1 kHz. Using a sampling frequency much higher than the expected received signal frequency will ensure proper detection of the signal. The ADC will have 8 channels, 1 for each electrode.
Protocol Conversion:
A protocol conversion chip will likely be needed to convert the output communication protocol of the ADC to a different protocol for transfer to a PC. As an example, converting a SPI output of the ADC to a UART protocol, so the signal can be sent over a USB.
Processing Software:
Software will be written to receive the incoming signal from a USB port or otherwise, and to translate the received bits into the biopotential read on the head.
Display Software:
Software will be written to display the biopotential levels for researcher use. The type of display has not been determined.

Feasibility: Prototyping, Analysis, Simulation

The overall feasibility document can be viewed here.

The estimated cost of the project can be viewed here.

Phase Shift equations have also been worked with as part of the Optical implementation which can be viewed here.

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 Conductive Conductive ink degrades quickly EEG wire shorting Improper ink chosen; insufficient protection 2 4 8 Perform degredation tests on several brands of ink before selection for use in final design
3 Optic Cap corrupts MRI data Customer requirement failure Electrode footprint from commercial modulators is too large 4 4 16 Make the electrode footprint as small as possible. Research materials and footpring of commercial systems thoroughly.
4 Conductive Cap corrupts MRI data Customer requirement failure Electrodes and/or ink corrupts data 4 4 16 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
5 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
6 Conductive 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
7 Optic Budget is exceeded Customer requirement failure Inability to get free lithography time; high cost of optical components 5 3 15 Ensure that we have well sourced cost estimates. Negotiate with the MicroE department for cost breaks on equipment
8 Conductive Budget is exceeded Customer requirement failure Poor budget planning; unanticipated costs 3 3 9 Ensure that we have well sourced cost estimates for all components
9 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
10 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 4 5 20 Find alternative test methods that do not rely on full-scale MRI; test as many items/ideas as possible within allotted time
11 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

The Risk Assessment document can be found here.

Plans for next phase

Phase 2 Accomplishments

During the next phase, the team plans to rigorously pursue several proof of concept tests to better establish the feasibility of each of the leading system concepts. These tests are outlined in the test plan, and they should all be completed by the end of Phase 3.

The testing plan can be found here.

In particular, the team should:


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