As part of a season celebrating the work of Sir Peter Mansfield, SciBar follows up Rebecca's talk in July with Richard Bowtell from the University of Nottingham talking about New Ways Of Imaging The Brain At Work
There are a number of different technologies used in measuring brain activity but how do they actually work? Electroencephalography (EEG) and Magnetoencephalography (MEG) both work by measuring electrical signals in the brain that relate to the activity of neurons. Meanwhile, Magnetic Resonance Imaging (MRI), Near Infra-Red Spectroscopy (NIRS) and Positron Emission Tomography (PET) are sensitive to blood flow changes in the brain, caused by brain activity. These techniques that rely on blood-flow are slower than the electrical ones with a response time of around a second compared to milliseconds but they have better spatial resolution, meaning that the images that they generate are better.
MRI scanners are a very unnatural environment for the patient, having your head inside a magnetic coil, and have a low temporal resolution. The MEG on the other hand, uses an array of detectors around the head. The MEG generates an electrical field and the currents that flow in the brain's neurons also generate a magnetic field. Those detectors then measure those fields and the spatial pattern of field variation indicates location and strength of current.
The magnetic field in the human brain is very weak though, coming in at 10-12 Tesla. To put that into some kind of context:
Fridge magnet: 5x10-3 Tesla
Earth's magnetic field: 5x10-5 Tesla
Car, 50 metres away: 5x10-9 Tesla
Human heart: 10-10 Tesla
The strength of the reading in the brain also decreases as you move away from the source, so just 2cm away from the scalp it becomes 1.8x10-13 Tesla. Trying to detect this field is like listening for the footsteps of an ant at a rock concert.
The MEG needs a really sensitive field sensor and so Superconducting Quantum Interface Devices (SQUIDs) are used (sometimes, I'm sure that these scientists think up the acronym first!) These operate at -269oC, which is the temperature of liquid helium, however, the surface of the head wants to stay at 38oC. The other issue is that the housed in a magnetically shielded room, which is made from a layer of mu-metal (a kind of "super" iron rather than Limp Bizkit, who were nu metal) and a layer of aluminium.
The MEG scanner weighs 2 tonnes and features 275 sensors that are around 2 centimetres from the head's surface. While it can map a number of different responses such as moving a finger to looking at a picture, it's sensitivity to brain signals is limited. Also, the helmet is only designed for adults and even slight movements of the head can spoil the data. The scanner also costs around £1 million to buy, is expensive to run and is still an unnatural experience for people who have their head in it.
So, can we instead invent a brain scanner that can be worn like a hat? Yes, a wearable MEG scanner is now in development, which uses OPM sensors that are nearly as sensitive as the SQUIDs but can operate at room temperature and only cost £5,000 each. The sensors can be placed 6 millimetres away from the scalp but won't work if the static field is 10-9 Tesla (remember, the Earth's field is 5x10-5 Tesla). Hence, coil technology has to be used to reduce the field inside the room. A 3D printed head cast fixes senses with respect to the brain anatomy and means that they stay in their operating field range, even if the person moves their head.
Initial results are encouraging and it could potentially open up new opportunities for neuroscience and clinically focused investigations. The end goal is to create a device that people could walk around in.
On the other end of the scale, the 7 Tesla MRI scanner at Nottingham University is 3.3 metres long and weighs 30 tonnes. It has a 90 centimetre bore, is housed in a 230 tonne iron box and uses 430 kilometres of wire. It has 78 megajoules of stored energy, the equivalent of 2-3 Boeing 737s when they land. There are only 5 MRI machines in the UK that are as powerful, the one in Nottingham was the first, and around 40 worldwide.
SciBar returns to The Vat & Fiddle on the 27th of September at 7:30 where Graham Law will talk on Sleep Better: The Science And The Myths
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