Development of high-Tc SQUID magnetometers for on-scalp MEG
This thesis describes the development of high critical temperature superconducting quantum interference device (high-Tc SQUID) magnetometers based on bicrystal grain boundary and nanowire junctions for the potential use in on-scalp magnetoencephalography (MEG), which is a new generation MEG technique with reduced sensor-to-subject standoff distances.
MEG is a method of mapping neural dynamics in the human brain by recording the magnetic fields produced by neural currents. Its passive and non-contact nature allows doctors and neuroscientists to safely and effectively carry out clinical diagnoses and scientific research on the human brain. State-of-the-art MEG systems utilize low-Tc SQUID sensors with sensitivities of 1--5 fT/√Hz down to 1 Hz to measure the extremely tiny biomagnetic fields (~100 fT) from the brain. However, low-Tc SQUIDs require liquid helium cooling to reach their operating temperature (< 10 K). The complicated cryogenics limit the sensor-to-subject distance to 20 mm at best.
On-scalp MEG, where sensors are placed with close proximity (few millimeters) to the scalp of the subject, can be realized with the aid of helium-free MEG sensor technologies. In this thesis, we designed, fabricated and characterized high-Tc SQUID magnetometers made from YBa2Cu3O7-x (YBCO) that can operate with liquid nitrogen cooling (77 K) based on bicrystal grain boundary or nanowire junctions. Single-layer bicrystal devices with a directly connected pickup loop were demonstrated to have a magnetic flux noise of 5 µΦ0/√Hz with an effective area of 0.24 mm2, giving a magnetic field sensitivity of 40 fT/√Hz at 77 K. For nanowire-based devices, a two-level coupling approach was implemented, where the flip-chip SQUID is connected to a washer-type pickup loop with the inner hole size matching that of a flux transformer input coil. This improved the effective area of nanowire-based SQUID magnetometers to 0.46 mm2. Combining with the magnetic flux noise of 55 µΦ0/√Hz for this type of devices, the best magnetic field sensitivity obtained was 240 fT/√Hz at 77 K. A simulation method was developed and demonstrated to give an accurate evaluation of the effective area and inductances in the design of SQUID magnetometers. Using this method, nanowire-based SQUID magnetometers with thick washers were predicted to give an improved effective area of 2.2 mm2.
A single-channel high-Tc MEG system housing the 40 fT/√Hz bicrystal grain boundary SQUID magnetometer was used to benchmark against low-Tc SQUIDs in a state-of-the-art MEG system (Elekta Neuromag® TRIUX, courtesy of NatMEG) based on recordings on a head phantom. It was shown that the expected amplitude gain of magnetic field signals associated with the on-scalp sensors (reduced standoff distances to ~3 mm) can be obtained while the single-channel signal-to-noise ratio was still lower than its low-Tc counterpart. Also a systematic benchmarking procedure that is objective, fast, and feasible for application to various on-scalp MEG sensing technologies was established. The functionality of this procedure was proved with MEG recordings of auditory and somatosensory evoked fields (AEFs and SEFs, respectively) on one human subject.
Bicrystal grain boundary