Chapter 12 Case Studies. Case Study - EEG Spike Detection Outline: Problem definition Data...

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Transcript of Chapter 12 Case Studies. Case Study - EEG Spike Detection Outline: Problem definition Data...

  • Chapter 12Case Studies

  • Case Study - EEG Spike DetectionOutline:

    Problem definition

    Data acquisition and preprocessing

    Alternative network paradigms and structures

    Results

  • AcknowledgmentsAt Johns Hopkins University Applied Physics Laboratory: Russ Eberhart, Roy Dobbins, Chuck Spaur

    At Johns Hopkins Hospital: Bob Webber, Ron Lesser, Dale Roberts

  • What is an EEG? EEG is an abbreviation for electroencephalogram.

    It is the recording of brain electric potentials varying in time at frequencies up to a few tens of Hz and measuring from a few microvolts to a few millivolts.

    For diagnostic purposes, it is usually taken by placing electrodes at standard locations. The most commonly used electrode arrangement is the International 10-20 montage.

  • EEG Electrode PlacementDiagram of top view of the scalp with the nose (front of scalp) up, illustrating the EEG electrode positions in the 10-20 International System. C is central, F is frontal, P is parietal, O is occipital, and T is temporal. Odd numbers designate leads on the left side of the scalp, even numbers designate leads on the right side, and Z designates zero or midline. (Many more channels may be used.)

  • Usually, electrodes are attached to the scalp. However, sometimes they are put directly on the brain:

  • EEG Spike Detection ProjectA team effort

    Always remember that medicine drives engineering and that the customer is always right

    Data were taken real-time from multiple channels (32-64 typical)

    Data rate is often 200250 Hz continuously, resulting in 10100 Mbytes per hour of data

    Accurate interpretation is critical: surgical procedures often depend on results obtained

    Replace expensive and time-consuming manual interpretation of EEG

  • Problems to be addressed Primary: On-line multi-channel analysis of EEG waveforms, including spike and seizure detection

    Secondary: Reduction in amount of data to be recorded and archived, especially paper records

  • Design Considerations Design process was iterative

    Raw data versus pre-processed parameters

    Network architectures

    Minimize system cost (< $10K 1990 dollars)

    Ambulatory system desirable

  • System Performance Specifications Multi-channel analysis capability

    Real-time analysis

    Minimal training required for each patient

    Spikes are defined by (4 out of 6) neurologists

    Recall and precision (each > 0.8) are performance measures

    Note: The first three were relatively easy to formulate; the last two were difficult.

  • Spike Detection Performance Metrics Recall: The number of spikes correctly identified by the system divided by the number of spikes identified by the neurologists

    Precision: The number of spikes correctly identified by the system divided by the total number of spikes identified by the system (includes false positives)

  • Contingency Matrix System DiagnosisGoldStandardDiagnosis

    Recall is TP/(TP + FN)

    Precision is TP/(TP+FP)

    PositiveNegativePositiveTP(true pos.)FN(false neg.)NegativeFP(false pos.)TN(true neg.)

  • Data Preprocessing and Categorization Design effort focused in two main areas* Preprocessing of data for NN input* Development of NN analysis tools

    Three main processing alternatives were considered1. Raw data using sliding windows (max. 200 ms spike results in 250 ms window)2. Preprocess raw data; present raw candidate spike centered in window3. Preprocess to produce parameters & spike center time

  • Methods Selected: 2 and 3 JHU Hospital software set to minimize false negatives(resulted in 2-3 false positives for each spike)

    Data of interest had an average of about 1 spike per second, so processing load was about 3-4 calculations per second

    Used the JHU Hospital spike viewer, which produced 9 parameters for each candidate spike

  • Spike Parameters

  • Input Data Scaling Had significant effect

    OK to normalize uniformly across all channels for raw data

    For parametric data, scaling across all channels didnt work; neither did scaling on each one individually

    For parametric data, success was achieved by scaling channels with same units togetheramplitudestimessharpnesses

  • ROC Curve for Output 1

  • ROC Curve for Output 2

  • The Universal Solution Myth Minimization of false negatives is more important than minimization of false positives in the U.S. It is the reverse in New Zealand.

    System performance specifications are customer and application dependent.

  • Case Study:Determining Battery State of ChargeUsing Computational Intelligence

    R. Eberhart, Y. Chen and S. LyashevskiyPurdue School of Engineering and TechnologyIndianapolis, Indiana

    S. Sullivan and R. BrostDelphi Energy and Engine Management SystemsIndianapolis, Indiana

  • The Situation Existing state of charge (SOC) estimation methods dont perform satisfactorily for many applications. Problems arise due to:Charge-discharge cyclesLoad profilesEnvironmental conditions

    Accuracy of existing systems was not better than about 10 percent.

  • Dynamic Load Profile

  • The ProjectDevelop a system to determine SOC for a string of two or more lead-acid batteries

    Goal: Achieve errors significantly less than 5% over charge-discharge cycles, load and environmental variations

  • Prior Technology * Amp-hour integration* High accumulation errors

    * Peukerts relation* Requires constant current discharge, and invariant temperature and environmental conditions * Discharge to 0% SOC required (hard on batteries!)

    * Families of capacity-voltage-current curves* Battery aging invalidates calibration

  • Electrochemical Principles * Need current, voltage, temperature, and amp-hours of individualbatteries

    Results in input vector with 4n inputs for n batteries

    * Makes online system difficult to implement computationallyand economically

    * Additional goal: Reduce dimensionality of inputs

  • Data Acquisition Approach Acquired numerous data sets

    Included constant and dynamic loads

    Varied temperature

  • Initial Input Parameters Discharge current of battery packTotal ampere hours usedAverage temperature of battery packMinimum battery voltageMaximum battery voltageAverage battery voltageVoltage difference between average and minimum battery voltagesMinimum battery voltage at previous sampling time

  • Initial Design Various approaches were tried

    Selected feedforward supervised training neural network

    Initial design used all 8 parameters in previous list in an 8-5-1 network configuration

    Sigmoidal activation function used for hidden and output PEs

    Each input was scaled between 0 and 1

    Levenberg-Marquardt algorithm chosen

    Second-order Butterworth filter implemented for test/run only

    Largest errors were at beginning and end of the cycle

  • Errors Worse Near 0 and 100 Percent

  • Final Design Reduced number of processing elements in neural networkto a 5-3-1 configuration (used inputs 1-4, 8)

    Used linear output processing element

    Trained on 2,500 patterns

    Tested on 24 data sets with various load and temperature profiles

    Average sum-squared error/pattern ~0.0006

    Errors in SOC estimation generally less than 1%, always less than 2%

  • Performance Results

  • Learnings/Conclusions 1. The methodology chosen met all project goals.2. Use only the data necessary to train the network; more is not always better.3. Match output processing element activations to problem.Computational methods may be useful for either training or testing that are not useful/needed for both.

    U. S. Patent 6,064,180 was issued May 16, 2000 for this technology.