Estimating the 3D position of indicators and markers has many applications in engineering, medical science, etc. For example, in the process of recording brain signals (EEG), reconstructing the 3D map of the electrodes over the head and estimating the exact position of each electrode, can lead to reconstructing the source of mentioned signals and also matching the information of these signals with other information such as MEG signals and MRI images.
One of the most important applications of determining the position of EEG electrodes is the source localization of signals caused by epilepsy. Using the exact position of the EEG electrodes and the signals received from them, the source of the signals related to epilepsy is estimated. Therefore, the presence of a device capable of accurately estimating the position of the EEG electrodes in the 3D space over the head will be very useful in locating the source of brain signals.
The 3D positioning device 3DPD v1.0 can reconstruct the map of the position of various markers in the 3D space. For this purpose, it is necessary to categorize the target markers with labels containing three-digit numbers that are provided with the device. Therefore, in the process of locating the source of brain signals using EEG electrodes, it is enough to stick these labels on these electrodes.
• Device hardware
1- Imaging frame: This device consists of 10 cameras on a circular frame, which is used to image the set of target markers (which we intend to estimate their 3D position). Each of these cameras is marked with a number from 1 to 10. Also, there is a push button on this frame to record images.
2- Device interface box: The interface equipment related to connecting the cameras to the computer is located in this box. To establish a connection between the cameras and the interface box, a special cable equipped with a DB50 connector has been used.
• Device software
The device software named 3DPD v1.0 has been developed to record images of the target markers and then process these images to estimate the 3D position map of these markers. As mentioned in the introduction section, to use this device, it is necessary to categorize the target markers with labels containing three-digit numbers and then record the images and process them by the device and 3DPD v1.0 software. These labels have specific numbers and are provided with the 3DPD device. To test the device, a human head model is used and labels containing three-digit numbers are placed on this model. This head model is provided with the device and is used to evaluate the positioning accuracy of the device.
• Imaging process
To determine the 3D position map of the target markers, after labeling them with three-digit numbers defined for the device, several of the target markers should be recorded from different angles. For this purpose, we keep the imaging frame above the set of target markers, so that this set is almost in the center of the frame and its distance from all the cameras is almost equal, to have the images of the markers set in all the cameras. then we can capture the image of the labels. The imaging process should be such that each of the labels on the target markers is recorded in at least two simultaneous images taken from at least two sets of 10 images. Therefore, during the imaging process, after recording each set of images, we must rotate the imaging frame a little and record the image again. We do this from several different angles and record the images. To calculate the 3D map of the complete set of labels, depending on the quality and position of the images taken, between 5 and 15 sets of 10 images will be needed, and if the images are not taken at the right angles or If they don’t have good quality, this number will increase.
Figure 1: Device Hardware
Figure 2: Device Software
Figure 3: Imaging Process
Figure 4: Estimated 3D Map of Markers
• Estimation of the 3D map of labels
After the imaging process is finished and enough images are recorded, the 3D map estimation process can be done. This process takes between 4 and 10 minutes, depending on the number of recorded image sets. After the process is finished, the three-dimensional map of the labels will be displayed, where you can see the X, Y, and Z positions of each label. Also, the table of obtained coordinates for the labels will be shown along with the STD error of their estimation.
• Hardware specifications:
ü 10 miniature cameras
ü Frame diameter: 38 cm
ü Frame weight: 1.5 kg
ü Cable length: 2.2 meters
ü Interface box dimensions: 25 x 15 x 35 cm
ü Computer interface: USB and Ethernet
· Software specifications:
ü 3D positioning accuracy: 1 mm RMS
ü Free head movements: allowed
ü Required number of image registration: 5 to 15
ü Time required for recording images (time required for patient participation): less than one minute
ü Time required for offline 3D positioning without patient participation: 4 to 10 minutes.
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