On-scalp MEG using high-Tc SQUIDs: Measuring brain activity with superconducting magnetometers
Doktorsavhandling, 2019

This thesis describes work done towards realizing on-scalp magnetoencephalography (MEG) based on high critical temperature (high-Tc) superconducting quantum interference device (SQUID) sensors. MEG is a non-invasive neuroimaging modality that records the magnetic fields produced by neural currents with good spatial and high temporal resolution. However, state-of-the-art MEG is limited by the use of liquid helium-cooled sensors (T ~ 4 K). The amount of thermal insulation between the sensors and the subject's head that is required to achieve the extreme temperature difference (~300 K), typically realized in the form of superinsulation foil and ~2 centimeters of vacuum, limits measurable signals. Replacing the sensors with high-Tc SQUIDs can mitigate this problem. High-Tc SQUIDs operate at much higher temperatures (90 K) allowing significant reduction of the stand-off distance (to ~1 mm). They can furthermore be cooled with liquid nitrogen (77 K), a cheaper, more sustainable alternative to the liquid helium used for cooling in conventional MEG systems.

The work described in this thesis can be divided into three main areas: (I) simulation work for practical implementations of on-scalp systems, (II) development of a 7-channel high-Tc SQUID-based on-scalp MEG system, and (III) on-scalp MEG recordings.

In the first part, spatial information density (SID), a metric to evaluate the performance of simulated MEG sensor arrays, is introduced and - along with total information capacity - used to compare the performance of various simulated full-head on-scalp MEG sensor arrays. Simulations demonstrate the potential of on-scalp MEG, with all on-scalp systems exhibiting higher information capacity than the state-of-the-art. SID further reveals more homogeneous sampling of the brain with flexible systems. A method for localizing magnetometers in on-scalp MEG systems is introduced and tested in simulations. The method uses small, magnetic dipole-like coils to determine the location and orientation of individual sensors, enabling straightforward co-registration in flexible on-scalp MEG systems. The effects of different uncertainties and errors on the accuracy of the method were quantified.

In the second part, design, construction, and performance of a 7-channel on-scalp MEG system is described. The system houses seven densely-packed (2 mm edge-to-edge), head-aligned high-Tc SQUID magnetometers (9.2 mm x 8.6 mm) inside a single, liquid nitrogen-cooled cryostat. With a single filling, the system can be utilized for MEG recordings for >16 h with low noise levels (~0-130 fT). Using synchronized clocks and a direct injection feedback scheme, the system achieves low sensor crosstalk (<0.6%). 

In the third part, on-scalp MEG recordings with the 7-channel system as well as its predecessor, a single-channel system, are presented. The recordings are divided into proof-of-principle and benchmarking experiments. The former consist of well-studied, simple paradigms such as auditory evoked activity and visual alpha. Expected signal components were clearly seen in the on-scalp recordings. The benchmarking studies were done to compare and contrast on-scalp with state-of-the-art MEG. To this end, a number of experimental stimulus paradigms were recorded on human subjects with the high-Tc SQUID-based on-scalp systems as well as a state-of-the-art, commercial full-head MEG system. Results include the expected signal gains that are associated with recording on-scalp as well as new details of the neurophysiological signals. Using the previously described on-scalp MEG co-registration method enabled source localization with high agreement to the full-head recording (the distance between dipoles localized with the two systems was 4.2 mm).

high-Tc SQUID

neuroimaging

magnetoencephalography

localization

on-scalp MEG

sensor array

magnetometer

Kollektron (A423), MC2, Kemivägen 9, Göteborg
Opponent: Associate Professor Stefania Della Penna, Institute of Advanced Biomedical Technologies and Department of Neuroscience, Imaging and Clinical Sciences, G. D’Annunzio University, Chieti, Italy

Författare

Christoph Pfeiffer

Chalmers, Mikroteknologi och nanovetenskap (MC2), Kvantkomponentfysik

Evaluation of realistic layouts for next generation on-scalp MEG: spatial information density maps

Scientific Reports,; Vol. 7(2017)p. Article no 6974 -

Artikel i vetenskaplig tidskrift

Localizing on-scalp MEG sensors using an array of magnetic dipole coils

PLoS ONE,; Vol. 13(2018)

Artikel i vetenskaplig tidskrift

Pfeiffer, C., et al. A 7-channel high-Tc SQUID-based on-scalp MEG system (IEEE Trans. Biomed. Eng., accepted for publication)

Pfeiffer, C., et al. Sensor localization using magnetic dipole-like coils: A method for highly accurate co-registration in on-scalp MEG (Neuroimage, under review)

Similarities and differences between on-scalp and conventional in-helmet magnetoencephalography recordings

PLoS ONE,; Vol. 12(2017)p. Article no e0178602 -

Artikel i vetenskaplig tidskrift

Andersen, L.M., et al. On-scalp MEG SQUIDs are sensitive to early somatosensory activity unseen by conventional MEG

Feedback solutions for low crosstalk in dense arrays of high-T-c SQUIDs for on-scalp MEG

Superconductor Science and Technology,; Vol. 30(2017)p. art. nr 054006-

Artikel i vetenskaplig tidskrift

This thesis is about the development of a new magnetoencephalography (MEG) system for functional neuroimaging. Unlike magnetic resonance imaging, which provides a detailed 3D image of the brain’s structure, MEG images the where and when of brain activity.  Clinically, MEG can be used to localize important or impaired areas in the brain. For example, it is used to guide life-saving epilepsy surgeries. As the name indicates, MEG measures brain activity by recording magnetic fields, specifically, the weak magnetic fields that are generated by the currents that appear when neurons in the brain fire. These magnetic fields are extremely weak. Measuring them requires very sensitive sensors and extensive magnetic shielding. State-of-the-art MEG systems use highly sensitive superconducting sensors that operate at extremely low temperatures (-269 °C). A major limiting factor of the technology is the thick thermal insulation (on the order of 2 cm) that is required to operate the sensors near a person’s head.
 
This thesis covers an alternative approach, where we use a newer type of sensor that operates at higher temperature (-196 °C). Thus, the thermal insulation can be reduced to approximately 1 mm, so that the sensors can operate significantly closer to the head. We found that by coming closer, one can, in theory, obtain more information from the brain. As a first step towards a full system (covering the whole head) with this approach, we developed a small multichannel system employing seven sensors. With this 7-channel system, we demonstrated the feasibility of the approach in several recordings on humans. In addition to higher signals compared to the state-of-the-art system, we found several unexpected signal components that may reflect previously unseen brain activity. As such, the system in particular, and the approach in general, show great promise for improved functional neuroimaging and may lead to new discoveries in neuroscience and better treatments.

Nanoscale superconducting devices for a closer look at brain activity (NeuroSQUID)

Knut och Alice Wallenbergs Stiftelse, 2015-01-01 -- 2019-12-31.

Styrkeområden

Nanovetenskap och nanoteknik (2010-2017)

Ämneskategorier

Medicinteknik

Neurovetenskaper

Annan fysik

Elektroteknik och elektronik

Signalbehandling

Infrastruktur

Nanotekniklaboratoriet

ISBN

978-91-7905-173-0

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4640

Utgivare

Chalmers tekniska högskola

Kollektron (A423), MC2, Kemivägen 9, Göteborg

Opponent: Associate Professor Stefania Della Penna, Institute of Advanced Biomedical Technologies and Department of Neuroscience, Imaging and Clinical Sciences, G. D’Annunzio University, Chieti, Italy

Mer information

Senast uppdaterat

2019-09-27