This post is to explain all necessary steps and stages in order to achieve an adequate acquisition of certain biomedical signals using, as you will see later, a computer hardware and development software.
For starters, let’s do a brief review and explain what exactly a biomedical signal: Biomedical signals there are many and with very different nature; they are analog signals that cover our body and we are extremely useful when it comes to diagnosing diseases, deficiencies in the functioning of certain organs or for clinical research, just to mention some practical examples. Examples of biosignals are electrocardiogram (EKG), electromyogram (EMG), EEG, electrodermal (EDA), etc. These biomedical signals are studied by the branch of science that is call biomedicine, defined (RAE, in spanish) as:
Set of disciplines such as biochemistry, molecular and cell biology and genetics, which play a key role in the current medicine.
Having explained what are the biosignals and why we are so important, we are now ready to measure them; as mentioned, this entry proposes a way inspired by the DIY (Do It Yourself) philosophy and explain how to carry it out.
First we need a Bitalino board, this is easily available through the manufacturer’s website. Although the manufacturer offers different versions and possibilities for your hardware, everything will look on this entry has been made with the Board Kit of BiTalino, which is sold at a price of €149.
This board allows us to acquire data at predetermined frequencies of 1, 10, 100 and 1000 Hz. Any of these frequencies, we will be able to simultaneously record 6 channels that offers BiTalino, namely:
- EKG. Electrical signal generated by the heart in its operation.
- Electrodermal, hereinafter EDA. In this case, what measure are changes in resistivity of the skin due to changes in sweats and other more complex biomedical parameters. This is the biosignal which used the famous polygraph, commonly known as ‘lie detector’.
- Electromyogram, or EMG. It’s another electrical signal present in skeletal muscles and thanks to which we are able to move them at will. You can find several satisfactory experiments that have made possible that people with severe paralysis will be able to pick up objects (even drink a glass of water) using a robotic arm guided by the electrical impulse that generates our brain when we have the will to move certain muscles.
- Accelerometer. It’s a typical acceleration sensor which is capable of measuring acceleration in any of the 3 axes of the space, although not simultaneously in this board.
- Light sensor. As you may guess, it’s a light sensor. With this sensor (and the previous) BiTalino tries to provide a whole kit to the biomedical signals, so that we are able to know the state of motion, measured by acceleration, and the amount of ambient light that the patient perceives as these signals are acquired him.
- Battery level. Its sixth channel is an indicator of the amount of remaining battery of the device.
It is necessary to specify that, for acquiring biosignals, the most appropriate frequency will be the 1000 Hz, resulting in 100Hz excessively low; While for other channels or other purposes all are perfectly usable.
Once everything has been briefly described, I will explain below how to use it.
This has been developed an application powered by LabVIEW software and its additional package Biomedical Toolkit, which provides a large number of tools for working with these biosignals and others. The purpose of the application can be divided into:
- Be able to record and display on screen in time real, any combination of 6 channels provided by BiTalino.
- Perform a filtering and conditioning of biomedical signals (in this sense only is has worked with EKG and EMG), display them on screen and, in addition, also provide analysis and information with clinical value where a person with knowledge of medicine is able to draw conclusions, as well as storing this information in files with a proper format.
We will see describe how to use it from the beginning, including pairing the device with your computer hardware. The Bitalino board and your computer can be linked together in different ways, even though it is designed BiTalino is to do it via Bluetooth (class II version 2.0). To do this is as simple as going to the control panel, bluetooth devices and add a device. A dialog box appears in Windows where we will have to enter the security code that brings BiTalino to bind it to our PC, it comes in the product instructions (default PIN is 1234). Once done this, and found that we have our computer on bluetooth, and the own BiTalino power already are able to run the program.
In doing so, we will see a window where you will have to configure 3 parameters:
- The COM port through which BiTalino is communicating with your computer, this is visible in the control panel, administrator of devices and looking for the new device we have just linked.
- The frequency of acquisition with the discrete values discussed above.
- A duration in seconds. This duration is used and is required for the second purpose of the application, and represents the length (in time and thanks to the frequency, in samples) of the package of biomedical data that the internal algorithms are going to process and display on screen in batch. This package can never be less than 10 seconds and some multiple of 30 seconds is recommended.
Once configured these 3 parameters, we just need to select the switches for the channels that you wish to activate and the operating mode. The operating mode is a variable that allows us to switch between the two options and responds to the objectives discussed above: one option is the Real time in which so many channels are displayed in the graph as they have been elected by the user and with the chosen acquisition frequency, when you want to stop the program by pressing the Stop button.
Using the second operating mode (by unchecking the option of RealTime) would enter into the processing and filtered mode and is intended to acquire, display and save biomedical signals (only EKG and EMG, because it uses Labview Biomedical Toolkit). This second mode requires the ‘Duration’ variable already commented, in seconds. If we use it, we will be acquiring signal during the time that we have defined in that variable, and then the program stops automatically. You can choose between EKG and EMG, or both at the same time. When you have passed that time, the selected biosignals will be displayed on the screen and, in addition to this, it will be display some additional information such as the analysis of the status of the person. The offered information differs between EKG and EMG because both are treated individually and processed with filters designed to optimize their characteristics and packaging so that the information provided is as accurate as possible; In addition they involve different physiological processes, so they are addressed independently at the time of analysis.
In the case of the EKG shows information statistic, histogram, graph of Poincaré, spectrogram, graphical analysis, etc.
For EMG, on the other hand, we do not show statistical results, but that all are of type graphics spectrogram, frequency of the signal and amplitude.
As it can be seen, this second mode (without Real time) is only designed for EKG or EMG; Although, by the mentioned above in relation to the context of acquisition of these signals, the program allows also recorded acceleration and/or the ambient light and save the results along with the acquired biosignal.
The last part of this post is about logging data. There is only the option for saving data on second operating mode. After the seconds that we defined in the variable ‘Duration’ have passed and the program displays on the screen the acquired biosignals and its biomedical parameters, it will appear a dialog window which invites us to save the file in the folder that you choose. This file uses an extension native National Instruments and widely used in applications using the LabVIEW software which is. tdms. It is a file format designed for the storage and analysis of large amounts of data and is widely used. This file is easily editable to using Microsoft Excel, importable in MATLAB, etc. It also allows to add comments (annotations) to the different channels that we have taken.
In addition to all the mentioned above, for a correct acquisition of biosignals it is essential to use some new pads each time, as well as clean the skin where they are to be placed, you can done with alcohol, and certainly do not add any liquid of any kind which may distort the signal acquired by sensors.
Then we can see two images that are intended to teach a location for the electrodes when taking an EMG, and EKG. There are different configurations and can be easily found over internet.
In the image on the left we see the placement for an EMG and which has been most widely used throughout this project. The neutral connector should be placed in an area of bone, by that elbow is a good place to do this. In the case of the EMG the polarity of positive and negative wires is undifferent; It is advisable that they are just above the muscle fibres which we want to observe and aligned with them.
The image on the right uses the placement in triangle configuration. This is to allocate the neutral electrode in a hip bone and the positive and negative terminals in both clavicles. In this case the order of positive/negative is irrelevant, even if it is not in other locations.