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SQUID-MEG
(superconducting quantum interference device - magnetoencelography)SQUID-MEG
This technique measures the magnetic fields generated by current flow in the human brain. The magnetic fields arise from neural activity, primarily due to postsynaptic currents in neurons. Liquid helium is cooled down to around 0 Kelvin, so that it remains superconductive and can be used to measure subtle magnetic field changes. Using different equations one can detect the inferred current flow in milliseconds.
This device is often used to detect events in epilepsy patients, the location of abnormality can be often detected as it has a better spatial resolution compared to EEG. The better spatial resolution is because the magnetic fields are less distorted by the skull and scalp. It has also been used in clinical research to measure connectivity between brain regions.
However, using cryogenic cooling implies that the sensors are a 2-3cm away from the head. As the signal of the brain decays with the square of the distance, if we double the distance we will reduce the signal to one-fourth. This reduces the signal to noise ratio.
Furthermore, it’s a one-size fits all helmets that are built for adults. As the sensors are in fixed positions, any movement degrades data quality. Lastly, as these machineries are made of liquid helium, they are very expensive to buy, and the scanners are hard to maintain
SQUID: superconducting quantum interference device - physics
SQUID: superconducting quantum interference device - physics
If you make a ring out of superconducting material, the flux which penetrates through this ring is also quantized through the fundamental constants of nature h (Plank) and e (elementary charge). If you keep this cold and in a magnetic field, the flux will not decrease but remain constant through time, this is due to the superconductivity property, which eliminates resistive losses.
Adding to the ring two very thin insulating barriers, creates an interferometer for superconducting currents. Referring to the double split experiment, the current can go two ways, and it will therefore interfere with itself. This is the way to measure very sensitive magnetic fields, of differences as small as 10 femtoTesla. (For comparison: a fridge magnet is 5mTesla, the earth magnet field is 30 microTesla, and the magnetic field produced by the brain is around picoTeslas.) This sensitivity is critical for detecting the brain's weak magnetic fields. For this reason, this technology can be applied to measure changes in the brain.
In clinical settings, this is being used to study things like epilepsia, dementia schizophrenia or head trauma. More sensitive measurements can lead to a better understanding, and in the future new treatment options. These techniques are also being developed to detect single photos, like NanoSQUIDS for spin detection and NEMS resonators.
Why is it still the most used technique?
Why is it still the most used technique?
SQUID MEG continues to be the preferred choice in many cases because of its established accuracy, full-brain coverage, and standardization in clinical and research protocols. SQUID-based systems offer reliable, well-calibrated, and reproducible measurements in clinical and research environments.
On the other hand, OPM MEG is promising, but currently better suited for specific applications, such as portable or motion-tolerant neuroimaging. For the moment, it still has lower sensitivity and still in the developmental stage, for this reason certain researchers still prefer SQUID MEG.
references images: [1] https://www.youtube.com/watch?v=JfJWOgJF_KA The future of measurement with quantum sensors - with The National Physical Laboratory
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