Aktiivinen magneettinen suoja optisiin neuromagneettisiin mittauksiin

Optically-pumped magnetometers, based on optical measurement of the magnetization of an alkali vapor, are emerging as a promising alternative sensor for recording magnetic fields generated by the human brain. These sensors can only operate at very low absolute magnetic fields and therefore efficient shielding against the ambient magnetic field is required. The standard in biomagnetism is to use a passive magnetically shielded room to obtain sufficient shielding; however, the remanent magnetic field in a typical shielded room often exceeds the limits of these new magnetometers. In this thesis, a suitable ultra-low-field environment for the use of these sensors in Aalto University was set up. First, the magnetic fields inside the two- and three-layer magnetically shielded rooms of Aalto University were surveyed and then a portable active compensation system for further reducing the fields was built. Without compensation, the measured remanent magnetic fields were on the order of 100 nT and 5 nT in the two- and three-layer rooms while the gradients were about 40 nT/m and 5 nT/m, respectively. Both rooms had low-frequency field drifts of approximately 1 nT in a measurement period of 200 s. With the constructed compensation coil set, the static field could be reduced by a factor of about 10 in a head-sized volume. The feedback loop of the compensation system was also capable of locking the field to zero at the sensor site and could also remove the low-frequency fluctuations in the field. This study showed that neuromagnetic measurements with optically-pumped magnetometers should be feasible in standard shielded rooms by utilizing the constructed compensation system.Optisesti pumpatut magnetometrit, jotka perustuvat alkalimetallikaasun magnetisaation optiseen mittaukseen, ovat lupaavia vaihtoehtoisia antureita ihmisaivojen synnyttämien magneettikenttien rekisteröimiseen. Nämä sensorit toimivat vain hyvin alhaisissa magneettikentissä, joten tehokkaita menetelmiä tarvitaan ympäröivältä magneettikentältä suojautumiseen. Tyypillisesti biomagneettisia kenttiä mitattaessa vaadittava suojaus on saatu käyttämällä passiivista magneettisesti suojattua huonetta; jäännöskenttä tyypillisen suojahuoneen sisällä kuitenkin ylittää optisten magnetometrien vaatimukset. Tämän työn tarkoituksena oli luoda suotuisa magneettinen ympäristö Aalto-yliopistoon näiden anturien käyttöä varten. Ensiksi remanenssimagneettikentät Aallon kaksi- ja kolmikerrossuojahuoneissa mitattiin ja mallinnettiin, jonka perusteella suunniteltiin ja rakennettiin liikuteltava aktiivinen kompensointijärjestelmä näiden kenttien pienentämistä varten. Mitatut remanenssikentät olivat suuruusluokaltaan 100 nT kaksikerroshuoneessa ja 5 nT kolmikerroshuoneessa; gradientit olivat vastaavasti noin 40 nT/m ja 5 nT/m. Kentän matalataajuiset fluktuaatiot olivat noin 1 nT:n luokkaa molemmissa huoneissa mittausajan ollessa 200 s. Rakennetulla kompensointijärjestelmällä suojahuoneiden staattinen kenttä pystyttiin pienentämään kymmenesosaan pään kokoisessa tilavuudessa. Järjestelmän takaisinkytkentäsilmukka pystyi nollaamaan kentän anturin kohdassa ja poistamaan matalataajuiset häiriöt kentästä. Tämä työ osoitti, että rakennettua kompensointijärjestelmää käyttämällä optisesti pumpatuilla magnetometreillä voidaan tehdä herkkiä neuromagneettisia mittauksia tyypillisissä magneettisesti suojatuissa huoneissa

Magnetoenkefalografian mittaaminen lähempää aivoja: Teoria ja toteutus

Magnetoencephalography (MEG) is a noninvasive neuroimaging technique in which the magnetic fields of electrically active neuron populations in the brain are measured outside the head. The neuromagnetic field contains temporal and spatial information about the underlying neuronal sources. Measurements of this field can be used to make inference about brain function. The temporal resolution of MEG is excellent as the magnetic field gives an instantaneous measure of the neuronal activity at the physiologically relevant frequency range. However, the spatial resolution, or the amount of spatial information, is limited in the state-of-the-art MEG systems as the current superconducting sensor technology does not allow the field measurement as close to the neural sources as noninvasively possible but limits the measurement distance to a couple of centimeters from the subject's scalp. At that distance, the field amplitude as well as the number of spatial degrees of freedom in the field have decayed considerably. Recent developments in quantum optics have enabled sensors suitable for measuring the neuromagnetic field within millimetres from the scalp. With these optically-pumped magnetometers (OPMs), on-scalp sensor arrays can be constructed increasing the spatial resolution of MEG. In this Thesis, improvement in signal amplitude as well as in spatial resolution due to on-scalp field sensing is quantified both with simulations and measurements. Requirements for spatial sampling of the neuromagnetic field to achieve those improvements are theoretically investigated. To sense the neuromagnetic field on scalp, an OPM-based MEG system is constructed. The system uses active magnetic shielding with external coils to reduce interference in the measurements. Neuromagnetic responses to visual stimulation are measured using the OPM system and are compared to those obtained with a state-of-the-art cryogenic MEG system. The Thesis shows the potential of on-scalp sensor arrays implemented with OPMs to increase the spatial resolution and applicability of MEG.Magnetoenkefalografia (MEG) on kajoamaton aivokuvantamismenetelmä, jossa aivojen sähköisestä toiminnasta syntyvä magneettikenttä mitataan pään ulkopuolella. Tämä neuromagneettinen kenttä sisältää ajallista ja paikallista informaatiota sen synnyttävistä aivolähteistä. Tätä informaatiota voidaan käyttää tulkitsemaan aivoja ja niiden toimintaa. MEG:n ajallinen tarkkuus on erinomainen, sillä magneettikenttä tarjoaa välitöntä informaatiota hermostollisesta aktivaatiosta fysiologisesti merkityksellisellä taajuuskaistalla. MEG:n paikkatarkkuus, tai paikkainformaation määrä, taas on rajoittunut tämänhetkisissä MEG-laitteissa, sillä nykyinen suprajohtava anturitekniikka ei salli magneettikentän mittaamista niin läheltä aivoja kuin vain kajoamattomasti mahdollista: nykyisissä laitteissa magneettikenttä mitataan ainakin kahden senttimetrin päästä pään pinnasta. Tällä etäisyydellä sekä magneettikentän voimakkuus että kentän vapausasteiden lukumäärä ovat kuitenkin jo laskeneet merkittävästi. Kvanttioptiikan kehitys on mahdollistanut magnetoenkefalografiaan soveltuvan anturityypin, jolla pystytään mittaamaan magneettikenttä millimetrien päästä pään pinnasta. Optisesti pumpattujen magnetometrien (OPM) avulla voidaan siis rakentaa pään pinnalla sijaitsevia anturistoja, joiden avulla MEG:n paikkatarkkuutta voidaan parantaa. Väitöskirjassa tutkitaan sekä laskennallisesti että kokeellisesti parannuksia signaalien voimakkuuksissa sekä mittauksen paikkatarkkuudessa, kun magneettikenttä mitataan pään pinnalla. Työssä tutkitaan teoreettisesti vaatimuksia magneettikentän näytteistämiselle paikan suhteen näiden parannusten saavuttamiseksi. Väitöskirjassa rakennetaan MEG-laitteisto, jossa OPM-antureita käytetään mittaamaan aivojen magneettikenttä suoraan pään pinnalta. Laitteisto käyttää aktiivista suojausta magneettisten häiriöiden vähentämiseksi. Systeemillä mitataan näköaivokuoren neuromagneettisia vasteita ja näitä vasteita verrataan suprajohtavilla antureilla mitattuihin. Väitöskirja näyttää OPM:iin pohjautuvien anturistojen potentiaalin magnetoenkefalografiassa sekä yleisesti että sen paikkatarkkuuden parantamisessa

Optical Co-registration of MRI and On-scalp MEG

| openaire: EC/H2020/678578/EU//HRMEGTo estimate the neural generators of magnetoencephalographic (MEG) signals, MEG data have to be co-registered with an anatomical image, typically an MR image. Optically-pumped magnetometers (OPMs) enable the construction of on-scalp MEG systems providing higher sensitivity and spatial resolution than conventional SQUID-based MEG systems. We present a co-registration method that can be applied to on-scalp MEG systems, regardless of the number of sensors. We apply a structured-light scanner to create a surface mesh of the subject’s head and the sensor array, which we fit to the MR image. We quantified the reproducibility of the mesh and localised current dipoles with a phantom. Additionally, we measured somatosensory evoked fields (SEFs) to median nerve stimulation and compared the dipole positions between on-scalp and SQUID-based systems. The scanner reproduced the head surface with <1 mm error. Phantom dipoles were localised with 2.1 mm mean error. SEF dipoles corresponding to the P35m response for OPMs were well localised to the somatosensory cortex, while SQUID dipoles for two subjects were erroneously localised to the motor cortex. The developed co-registration method is inexpensive, fast and can easily be applied to on-scalp MEG. It is more convenient than traditional co-registration methods while also being more accurate.Peer reviewe

Potential of on-scalp MEG

| openaire: EC/H2020/678578/EU//HRMEGElectrophysiological signals recorded intracranially show rich frequency content spanning from near-DC to hundreds of hertz. Noninvasive electromagnetic signals measured with electroencephalography (EEG) or magnetoencephalography (MEG) typically contain less signal power in high frequencies than invasive recordings. Particularly, noninvasive detection of gamma-band activity (>30 Hz) is challenging since coherently active source areas are small at such frequencies and the available imaging methods have limited spatial resolution. Compared to EEG and conventional SQUID-based MEG, on-scalp MEG should provide substantially improved spatial resolution, making it an attractive method for detecting gamma-band activity. Using an on-scalp array comprised of eight optically pumped magnetometers (OPMs) and a conventional whole-head SQUID array, we measured responses to a dynamic visual stimulus known to elicit strong gamma-band responses. OPMs had substantially higher signal power than SQUIDs, and had a slightly larger relative gamma-power increase over the baseline. With only eight OPMs, we could obtain gamma-activity source estimates comparable to those of SQUIDs at the group level. Our results show the feasibility of OPMs to measure gamma-band activity. To further facilitate the noninvasive detection of gamma-band activity, the on-scalp OPM arrays should be optimized with respect to sensor noise, the number of sensors and intersensor spacing.Peer reviewe

Measuring MEG closer to the brain

Optically-pumped magnetometers (OPMs) have recently reached sensitivity levels required for magnetoencephalography (MEG). OPMs do not need cryogenics and can thus be placed within millimetres from the scalp into an array that adapts to the individual head size and shape, thereby reducing the distance from cortical sources to the sensors. Here, we quantified the improvement in recording MEG with hypothetical on-scalp OPM arrays compared to a 306-channel state-of-the-art SQUID array (102 magnetometers and 204 planar gradiometers). We simulated OPM arrays that measured either normal (nOPM; 102 sensors), tangential (tOPM; 204 sensors), or all components (aOPM; 306 sensors) of the magnetic field. We built forward models based on magnetic resonance images of 10 adult heads; we employed a three-compartment boundary element model and distributed current dipoles evenly across the cortical mantle. Compared to the SQUID magnetometers, nOPM and tOPM yielded 7.5 and 5.3 times higher signal power, while the correlations between the field patterns of source dipoles were reduced by factors of 2.8 and 3.6, respectively. Values of the field-pattern correlations were similar across nOPM, tOPM and SQUID gradiometers. Volume currents reduced the signals of primary currents on average by 10%, 72% and 15% in nOPM, tOPM and SQUID magnetometers, respectively. The information capacities of the OPM arrays were clearly higher than that of the SQUID array. The dipole-localization accuracies of the arrays were similar while the minimum-norm-based point-spread functions were on average 2.4 and 2.5 times more spread for the SQUID array compared to nOPM and tOPM arrays, respectively.Peer reviewe

A minimum assumption approach to MEG sensor array design

Objective. Our objective is to formulate the problem of the magnetoencephalographic (MEG) sensor array design as a well-posed engineering problem of accurately measuring the neuronal magnetic fields. This is in contrast to the traditional approach that formulates the sensor array design problem in terms of neurobiological interpretability the sensor array measurements. Approach. We use the vector spherical harmonics (VSH) formalism to define a figure-of-merit for an MEG sensor array. We start with an observation that, under certain reasonable assumptions, any array of m perfectly noiseless sensors will attain exactly the same performance, regardless of the sensors' locations and orientations (with the exception of a negligible set of singularly bad sensor configurations). We proceed to the conclusion that under the aforementioned assumptions, the only difference between different array configurations is the effect of (sensor) noise on their performance. We then propose a figure-of-merit that quantifies, with a single number, how much the sensor array in question amplifies the sensor noise. Main results. We derive a formula for intuitively meaningful, yet mathematically rigorous figure-of-merit that summarizes how desirable a particular sensor array design is. We demonstrate that this figure-of-merit is well-behaved enough to be used as a cost function for a general-purpose nonlinear optimization methods such as simulated annealing. We also show that sensor array configurations obtained by such optimizations exhibit properties that are typically expected of 'high-quality' MEG sensor arrays, e.g. high channel information capacity. Significance. Our work paves the way toward designing better MEG sensor arrays by isolating the engineering problem of measuring the neuromagnetic fields out of the bigger problem of studying brain function through neuromagnetic measurements.Peer reviewe

Requirements for Coregistration Accuracy in On-Scalp MEG

| openaire: EC/H2020/678578/EU//HRMEGRecent advances in magnetic sensing has made on-scalp magnetoencephalography (MEG) possible. In particular, optically-pumped magnetometers (OPMs) have reached sensitivity levels that enable their use in MEG. In contrast to the SQUID sensors used in current MEG systems, OPMs do not require cryogenic cooling and can thus be placed within millimetres from the head, enabling the construction of sensor arrays that conform to the shape of an individual’s head. To properly estimate the location of neural sources within the brain, one must accurately know the position and orientation of sensors in relation to the head. With the adaptable on-scalp MEG sensor arrays, this coregistration becomes more challenging than in current SQUID-based MEG systems that use rigid sensor arrays. Here, we used simulations to quantify how accurately one needs to know the position and orientation of sensors in an on-scalp MEG system. The effects that different types of localisation errors have on forward modelling and source estimates obtained by minimum-norm estimation, dipole fitting, and beamforming are detailed. We found that sensor position errors generally have a larger effect than orientation errors and that these errors affect the localisation accuracy of superficial sources the most. To obtain similar or higher accuracy than with current SQUID-based MEG systems, RMS sensor position and orientation errors should be (Formula presented.) and (Formula presented.), respectively.Peer reviewe

On-scalp MEG system utilizing an actively shielded array of optically-pumped magnetometers

| openaire: EC/H2020/678578/EU//HRMEGThe spatial resolution of magnetoencephalography (MEG) can be increased from that of conventional SQUID-based systems by employing on-scalp sensor arrays of e.g. optically-pumped magnetometers (OPMs). However, OPMs reach sufficient sensitivity for neuromagnetic measurements only when operated in a very low absolute magnetic field of few nanoteslas or less, usually not reached in a typical magnetically shielded room constructed for SQUID-based MEG. Moreover, field drifts affect the calibration of OPMs. Static and dynamic suppression of interfering fields is thus necessary for good-quality neuromagnetic measurements with OPMs. Here, we describe an on-scalp MEG system that utilizes OPMs and external compensation coils that provide static and dynamic shielding against ambient fields. In a conventional two-layer magnetically shielded room, our coil system reduced the maximum remanent DC-field component within an 8-channel OPM array from 70 to less than 1 nT, enabling the sensors to operate in the sensitive spin exchange relaxation-free regime. When compensating field drifts below 4 Hz, a low-frequency shielding factor of 22 dB was achieved, which reduced the peak-to-peak drift from 1.3 to 0.4 nT and thereby the standard deviation of the sensor calibration from 1.7% to 0.5%. Without band-limiting the field that was compensated, a low-frequency shielding factor of 43 dB was achieved. We validated the system by measuring brain responses to electric stimulation of the median nerve. With dynamic shielding and digital interference suppression methods, single-trial somatosensory evoked responses could be detected. Our results advance the deployment of OPM-based on-scalp MEG in lighter magnetic shields.Peer reviewe