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Researchers are currently studying the functioning of the brain by monitoring two types of electromagnetism: electric fields and light. However, most methods of measuring these phenomena in the brain are very invasive.
MIT engineers have developed a new technique for detecting either electrical activity or optical signals in the brain, using a minimally invasive sensor for imaging. magnetic resonance imaging (MRI).
MRI is often used to measure changes in blood flow that indirectly represent brain activity, but the MIT team has developed a new type of MRI sensor capable of detecting electrical currents and the light produced by luminescent proteins. (Electrical impulses come from internal brain communications and optical signals can be generated by various molecules developed by chemists and bioengineers.)
"MRI offers a way to detect things from the outside of the body in a minimally invasive way," says Aviad Hai, a postdoc at MIT and senior author of the study. "It does not require a wired connection to the brain. We can implant the sensor and leave it here. "
This type of sensor could give neuroscientists a precise way of locating electrical activity in the brain. It can also be used to measure light and could be adapted to measure chemicals such as glucose, according to the researchers.
Alan Jasanoff, professor of MIT in Biological Engineering, Brain Sciences and Cognitive Science, and Nuclear Science and Engineering, and Associate Member of the McGovern Institute for MIT Brain Research, is the lead author of the document published in the October 22 edition of the Nature Biomedical Engineering. Postdocs Virginia Spanoudaki and Benjamin Bartelle are also the authors of the journal.
Detect electric fields
Jasanoff's laboratory has previously developed MRI sensors that can detect calcium and neurotransmitters such as serotonin and dopamine. In this article, they wanted to broaden their approach to the detection of biophysical phenomena such as electricity and light. Currently, the most accurate way to monitor electrical activity in the brain is to insert an electrode, which is very invasive and can cause tissue damage. Electroencephalography (EEG) is a non-invasive way to measure electrical activity in the brain, but this method does not determine the origin of the activity.
To create a sensor that can detect electromagnetic fields with spatial accuracy, researchers realized that they could use an electronic device, including a tiny radio antenna.
MRI detects radio waves emitted from the nuclei of hydrogen atoms in the water. These signals are usually detected by a large radio antenna in an MRI scanner. As part of this study, the MIT team reduced the radio antenna by a few millimeters to implant directly into the brain to receive radio waves generated by water in the brain tissue.
The sensor is initially set to the same frequency as the radio waves emitted by the hydrogen atoms. When the sensor picks up an electromagnetic signal from the tissue, its tuning changes and the sensor no longer matches the frequency of the hydrogen atoms. When this occurs, a weaker image appears when the sensor is scanned by an external MRI machine.
Researchers have shown that sensors can pick up electrical signals similar to those produced by action potentials (electrical impulses triggered by single neurons) or by local field potentials (the sum of electrical currents produced by a single group of neurons).
"We have shown that these devices are sensitive to potentials at the biological scale, in the order of millivolts, which are comparable to what biological tissues generate, especially in the brain," says Jasanoff.
Researchers performed additional tests on rats to determine if sensors could pick up signals in living brain tissue. For these experiments, they designed the sensors to detect the light emitted by cells designed to express the luciferase protein.
Normally, the exact location of luciferase can not be determined when it is deep in the brain or other tissues. The new sensor offers a way to increase the usefulness of luciferase and to locate more precisely the cells that emit light, say the researchers. Luciferase is usually introduced into cells with another gene of interest, allowing researchers to determine if the genes have been successfully incorporated by measuring the light produced.
Smaller sensors
One of the main advantages of this sensor is that it does not require any power supply because the radio signals emitted by the external MRI scanner are enough to power the sensor.
Hai, who will join the faculty at the University of Wisconsin in Madison in January, plans to further miniaturize the sensors so that more can be injected, allowing imaging of light or electrical fields on a large scale. larger area of the brain. In this article, the researchers carried out a modeling showing that a sensor of 250 microns (a few tenths of a millimeter) should be able to detect an electrical activity of the order of 100 millivolts, similar to the amount of current in a potential for neural action.
Jasanoff's laboratory wants to use this type of sensor to detect neural signals in the brain. It is also envisaged to use it also to monitor electromagnetic phenomena elsewhere in the body, including muscle contractions or cardiac activity.
"If the sensors measured hundreds of microns, as the modeling suggests in the future for this technology, imagine that you take a syringe and distribute a whole lot, then leave them there," says Jasanoff. . "This would provide many local readings through sensors distributed throughout the tissues."
The research was funded by the National Institutes of Health.
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