Development of high-Tc SQUID magnetometers for on-scalp MEG

Doctoral thesis, 2017

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.