EEG Electrode Localizer

Transcription

1 EEG Electrode Localizer submitted by: HyoJong Jang ( hj97@drexel.edu ) Pooja Konath Alvalappil ( pk495@drexel.edu ) Jesse Mark ( jam673@drexel.edu ) Coursework BMES 546 Fall 2015 Supervisor: Ahmet Sacan 1

2 Abstract Electroencephalography is a non-invasive state of the art technique in detecting electrical activity of the brain using electrodes attached to the scalp of the patient. EEG is used as a diagnostic tool to detect epilepsy, encephalopathy, brain death, and so on. Moreover, it is a powerful research tool for elucidating neural correlates of cognition. The most common method for positioning of electrodes is the international system. Different regions of the brain are associated with different functions, and therefore to assess a particular function of the brain, electrodes covering that specific area should be focused on during the reading. But oftentimes, it is difficult to relate the electrode positions with specific functional regions while using the actual EEG. The aim of this project is to create a tool which is able to both teach the user about which standard electrodes relate to which functional region, as well as be used post hoc to understand the quantitative percent of each region that is generating signal. Problem Description Our goal with this project was to create an easy to use interface in MATLAB that would be useful for the inexperienced and experts alike. We determined from the concept that using a version of the international positioning system would be ideal, and chose the specific number of electrodes (roughly 75 total) to allow for a wide range of coverage over the cortex while still remaining a reasonable and not overwhelming amount. In discussing what would be the most useful functions our program should provide, we came up with two answers which became the different modes. The user should be able to select an entire functional region of the cortex at once, either the left or right sides, to get a general picture of how our brains are laid out. The user should also be able to select specific electrodes themselves to determine both what they reference and how much of the underlying region they cover. This interaction is both practical when trying to learn or quiz yourself, and also has the ability to save and load one layout at a time at the push of a button. The excel file can then be shared between users in order to communicate visually and interactively, rather than being constrained to words or a static image. 2

3 Flowchart (Fig 1) Process flowchart Figure 1 demonstrates the flow of process through the program. When the program starts, all information will be loaded from electrodeposition.xlsx in OpeningFcn of the GUI. This file includes all center(x,y) position and radii of each electrode circle and each electrode labeled group number(1~10) and L/R(0,1,2 corresponding to left, right and center). Use of the program is cyclic and dynamically self-updating, and lets the user freely switch between the two modes of use whenever they wish to. Another version demonstrating this property 3

4 of our GUI is shown below. The methods of each section specifically will be discussed further on. Description of GUI Usage The GUI that appears initially when the program is run contains two push buttons at the top named Region Selection Mode and Electrode Selection Mode. These are the two modes used in this project. Mode 1 is selected as the default mode at startup. A list of nine functional regions of the cortex with 2 checkboxes for the Left and Right of each region are available. Upon clicking these boxes, the figure showing electrodes on the brain displayed on the right side of the GUI changes to highlight the electrodes corresponding to the particular selection made. Also, the electrodes are color coded to match the color of the functional region selected. For example, if the left box of prefrontal cortex is selected, then 10 electrodes on the left (including 3 electrodes in the center) are highlighted in a pink color. This mode thus gives an idea to the user on the electrodes associated with each region. Fig 2 shows an example of mode 1. (Fig 2) Region Selection Mode Upon clicking the Electrode Selection Mode push button, the check boxes will change to text area showing 0% as default value. In this mode, the user can select the electrodes on the figure on the right side with the mouse and update the percentage of each section with each click. The percentage of the functional areas covered by the electrode(s) selected will be displayed across that specific area. Selection of multiple electrodes at one time is possible. The user can deselect an already selected electrode by clicking on it a second time. Additionally, 2 more push buttons labeled as Save and Load buttons are provided in the second mode that appears at the bottom-right portion of the screen. These 4

5 buttons help the user to save the selected electrode layout and the corresponding percentages for future reference, and if desired also load the last saved data to the GUI. If the user makes a selection outside the boundaries of the electrode or buttons, then an error message Selected point not in electrode circle is generated. Mode 2 also has Electrode Selection Reset button which resets all the electrode selections made in mode 2 along with the percentages. Fig 6 shows an example of mode 2. Data Representation allcenter x,y positions of all circles 75 x 2 allradii radii of all circles 75 x 1 raw loaded data from electrodeposition.xlsx currentmode stores current mode(0 or 1 region or electrode selection) handlecheckboxl# handle string of each left checkbox# (dynamic var) handlecheckboxr# handle string of each right checkbox# (dynamic var) handlepercentl# handle string of each left percent text for dynamic var handlepercentr# handle string of each right percent text for dynamic var Group# total number of Group# (dynamic var) nlgroup# total number of selected left Group# (dynamic var) nrgroup# total number of selected right Group# (dynamic var) SelectedIdx maintains list of indices which was selected CircleColor pre defined color map for each region 10x3 currentmousex current x position when mouse is clicked currentmousey current y position when mouse is clicked ntotaleachregion total number of each region(l/r) selected 10x2 nselectedgroup# total number of each region#(all) selected (dynamic var) loadpercent store percent rate when load 5

6 Method & Results (Fig 3) Structure of excel file electrodeposition.xlsx In Fig 3, the green region contains the information the program uses for initialization and positioning, and also has practical information about the electrodes. The blue region contains the saving and loading functionality. When we save in Mode 2 (Electrode Selection Mode), we save each position as 1 (selected) or 0 (not selected) as well as the calculated percentage of the related functional area. Thus, when we use the GUI load button, this function will always load the latest saved data from the blue region. The center positions and radii of the electrode positions were calculated from imfindcircles in the Image Processing Toolbox. By using this function, we could avoid the tedious manual process for finding all center positions and radii for the circles. Below is the code which we used for automatic detection of circles. A = imread('eeg.png'); [centers, radii] = imfindcircles(a,[20 30],'ObjectPolarity','dark'); From this information, we could draw circles by using viscircles function in the Image Processing Toolbox. As we show the brain map image when the program starts, we 6

7 drew all circles in black as the initialization state. The left image below is the original image of our brainmap and the right image is the initialized image with blackened circles. (Fig 4) original image and initialized image This is a workaround we used because we were unable to remove the circles easily after they were drawn. Therefore, each time an electrode is selected or unselected, what really happens is that circles are being drawn on top of each other over and over (however, the reset button and related method wipe clear all the circles). This is one area which can be improved in future releases. The predefined colors for each region were intended to be distinct from each other and visible on most monitors. The figure below shows every electrode selected in Mode 1. The circles that remain black are outside the functional regions we used, but can still be selected and contribute to the total percentage of head coverage. They can be saved, but are not visible due to their color. (Fig 5) 7

8 In Mode 2 (Electrode Selection Mode), individual electrodes are selected in the brainmap image and the resulting percentage covered is calculated after each click. Below is an image of this mode in use. (Fig 6) Coverage calculation result example The specifics used in the above calculations of the percentage of each region covered have some idiosyncrasies that arose due to the particular layout we chose as well as how we defined our areas. The initial decision to split the brain into the left and right hemispheres was to allow for more information to give to the user. However, this brought up the issue of what to do with the midline electrodes. In the end, we made these electrodes count for both the left and right sides. This can be seen in the Region Selection mode where checking either side lights up the middle. The calculation of the percentages in the table will be explained using the Supplementary and Premotor cortex areas as an example (in orange above). All three electrodes of the left side (FC3, FC1, FCz) are selected, therefore 100% of this region is selected. Only one of the three electrodes for the right (FCz, FC2, FC4) are selected, so ⅓or 33% is covered. In total, ⅗electrodes of the entire cortex are selected, which turns into the 60% in the All column. The Total calculation is done in a similar fashion, with each left and right calculation being total # electrodes selected / total on one side (including midline). This, however, includes the black unaccounted-for electrode positions, so it is slightly difficult for the user to know when they are selected. This can be updated in future releases 8

9 by either finding what functional areas they cover, including them in the nearest region, removing them entirely from selectability, or creating an other category that contains them all together. We made our decision this time based on what we thought made sense to the project, but if this continued we would conduct user studies to find what makes the most sense and is the most desired format. The user can also deselect an already selected electrode. All the previous selected positions are stored in SelectedIdx and any new position selected is added to the end of SelectedIdx. This new position is then compared to the existing positions. Both the old and new positions are removed from SelectedIdx if there is a match. If so, the circle s color is changed back to black and the percentages are updated again. Selection detection is limited to inside the electrode circles in mode 2. To determine whether the clicked position is in the circle or not, we calculate the distance between the current position and each center position of all circles. Among all the circles (electrodes), there would be only one case where the calculated distance is less than the defined radius of circle, and we stored the index when this condition is satisfied and an error message is generated when it is not. Conclusions and Future Work One process we automated was using dynamic variables. The dynamic variables were used for setting a number of objects such as checkboxes and static text simultaneously when mode change or selection reset occurred. This is because changing the status of each object one by one is very tedious and redundant. By defining the handle of each object as a string including distinguished index, we could reduce the lines of code. In order to use the string of handles as a variable, we used sprintf to make a complete string of variables as the index varies in each for loop. Then we applied the eval function of the string to make the dynamic variable include each index. The creation of a program from scratch designed around the concept of a simple yet useful tool for EEG electrode placement brought together many of the concepts we learned in class. From the blueprinting stage, we wanted to pick a task that would be possible given our skills yet still present a challenge. The challenges started immediately. Simply matching each of the electrodes on our grid to functional cortex areas, and moreover in a way that made sense, covered a wide variety of functions, and didn t present overlaps was far more difficult than expected. Not many sources specifically link electrodes to the areas of interest, which in exchange makes our tool all the more novel and valuable for others to use. Given more time and enthusiasm, there are several areas which we could improve. One interesting update would be to allow for the choice of several different setups including 9

10 a variable number of electrodes on each. We used a 75 electrode system, but there are times when as small as 20 or as large as over 200 would be beneficial. The same concept of area selection or electrode selection could be imported, but the resolution would be different. While we re at it, so long as the image fits certain requirements we could use our automatic circle detection method and smart area mapping to allow for custom images to be imported. Right now we use an Excel file for all of our data storage, but we could also change it to allow for databases, or even csv files with a choice of format. And this was stated earlier, but allowing for more or even overlapping functional regions of the cortex would add more realism to our simulation. A clever use of colors and highlighting could present the user with an intuitive yet deep interface to explore the possibilities of EEG. Appendix After our in-class presentation, one additional function was added to the GUI. The additional function is the selection cancel function. The detailed method is described above in results. Below is a table of the functions used in our project. Function name Description Input Output Other action isselectedinelectrode Determine whether the current selected position is in the circle of electrode or not. Object, eventdata, handles Return 0 or 1( in the circle or not) SelectedIdx Display dialogu e box when clicked outside the circle CalculateRateRegion Calculate the rate related with brain region Object, eventdata, handles ntotaleachre gion, nselectedgro up Update the rates for each region drawcircle_checkbox draw circle when checkbox was clicked GroupNumber of Checkbox and L/R (0/1) Draw circles 10