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Magnetic Resonance Sensor in a Sugar Cube of Space

One of the main issues with neuroprosthetics is size. They are intended to be inserted into the brain after all, to detect the electrical activity of individual neurons, or send activity back to them. It does not matter how good an interface technology is, if it is too big for implantation.

Functional magnetic resonance imaging (fMRI) is a very useful brain reading technique that has been utilised both increasingly and extensively over the past two decades, in order to determine the activity levels of entire brain regions. However, it requires a MRI scanner typically, which is an extremely large device, larger than the human body in fact, into which the person is inserted.

It works by detecting and tracking the blood flow around the brain rather than detecting the electrical activity of the neurons themselves. More active neurons means more oxygen removed from the blood as it passes that point. It is not as accurate as tracking the electrical activity as in an EEG or ECoG detection net, but it is non-invasive, and is still accurate enough to detect when individual conscious thoughts occur and to differentiate between them. FMRI scans the entire brain at once, so thoughts can be tracked from one brain region to another, and extrapolated in meaning from which areas are involved and in which order.

It is slower than an electrical system as well, as blood oxygen level changes happen after the fact. This is because the neuron discharges, then looks to refill its reserves. Still, fMRI is very definitely a viable method for reading minds, as innumerable studies continue to demonstrate. What is really needed, is a much, much smaller magnetic resonance scanner. Not something you put a person inside, but rather something that can be won on, or about the person.

Enter NIST (The American National Institute of Standards and Technology), and the German research institute Physikalisch Technische Bundesanstalt. Researchers from both have been working on the creation of a miniature atom-based magnetic sensor. It is far too big to be implanted, but magnetic detectors don't need to be. What it is however, is about one cubic centimetre in size.

The researchers used the prototype sensor pictured above, to measure alpha waves in the brain associated with a person opening and closing their eyes as well as signals resulting from stimulation of the hand. The measurements were verified by comparing them with signals recorded by a rather bulkier SQUID (superconducting quantum interference device). The new sensor performed only slightly less well than the SQUID, but has several advantages – notably its tiny size, cheapness to manufacture, and incredible ease of portability – slip it in your pocket and walk out.

The initial study has indicated that as well as the benefits in fMRI of a device constructed using these sensors – which would itself be small enough to put in a briefcase – the new mini-sensor would also be ideal for a second type of brain-computer interface: a magnetoencephalography (MEG) scan.

MEG works by measuring the magnetic fields produced by electrical activity in the brain. It's typically used for mapping processes much like fMRI, with a benefit of being significantly faster, but a side-effect of being slightly less accurate unless many sensors are used for a 3D surround picture of the brain. Otherwise you have clipping and occlusion problems as electrical activity closer to the detector drowns out activity further back. So again, you are increasing the size of the detector, as you increase the number of sensors required. However, with sensors of this size fully self-contained, it would not be unreasonable to expect a MEG scanner the size of a motorcycle helmet to be within the realm of possibility – currently they're the size of an armchair crossed with an operating table.

This is because the large, ad bulky SQUIDs used currently, are not stand-alone systems. They are superconductors that must be chilled to work. Specifically -269 degrees C, or 4 kelvin – four degrees above absolute zero. This is what accounts for the sheer size and mass of a MEG scanner. The new sensor on the other hand, operates at room temperature, so no cooling is required. It can conceivably be built into a device to be taken out into the field (or worn at home) whilst the SQUID most certainly cannot.

As one of the co-authors of the study, Svenja Knappe, put it: “We're focusing on making the sensors small, getting them close to the signal source, and making them manufacturable and ultimately low in cost. By making an inexpensive system you could have one in every hospital to test for traumatic brain injuries and one for every football team.”

Building on that, you could also have one in every school, or general practitioner’s office – or even in the home, for at-risk patients, or avid gamers. It is a long way off, but it is feasible, for the first time.

The mini-sensor consists of a container of about 100 billion rubidium atoms in a gas, a low-power infra-red laser and fibre optics for detecting the light signals that register magnetic field strength—the atoms absorb more light as the magnetic field increases. All of this enclosed in a single sealed device the size of a cube of sugar.

The brain experiments were carried out in a magnetically shielded facility at the Physikalisch Technische Bundesanstalt (PTB) in Berlin, Germany, which has an ongoing program in biomagnetic imaging using human subjects. The NIST sensor measured magnetic signals of about 1 picotesla (trillionths of a tesla). An improved version is undergoing initial trials now, with a higher sensitivity for light, which is expected to outperform the SQUID, putting the final nail in the coffin as it were.

References

NIST Mini-sensor Measures Magnetic Activity in Human Brain

Magnetoencephalography with a chip-scale atomic magnetometer (subscription required)
Alternatively: Biomedical Optics Express, Vol. 3, Issue 5, pp. 981-990

Dictionary entry: fMRI

Dictionary entry: MEG

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