High-Tc SQUIDs for magnetophysiology - development of a magnetometer system and measurements of evoked fields from hippocampal neurons in vitro

Doctoral thesis, 2006

This thesis concerns development of a system based on a high-transition-temperature superconducting quantum interference device (high-Tc SQUID) magnetometer. The system has been designed and used for magnetic field recordings from brain slices, i.e. magnetophysiology. Magnetophysiology is a complementary method to electrophysiology, which is routinely used today. One aim of magnetophysiology is to aid in the assessment of the full nature of the signals in magnetoencephalography (MEG), which is a method for multi-channel whole-head recordings of magnetic fields from the brain. Dipole models of post-synaptic currents have been used to determine the magnetic fields above hippocampal slices. These simulations indicate that a high-Tc SQUID system with a sensor-to-sample separation as small as 65 µm would have superior signal-to-noise ratio compared with the low-Tc SQUID systems previously used elsewhere. A directly-coupled magnetometer with a side length of 2~mm has been fabricated from a YBa2Cu3O7-δ thin film. The large input washer was composed of strips of 5 µm width in order to prevent flux-trapping. The magnetic field noise of the magnetometer was 0.75 pT/Hz1/2 and the effective area was 0.071 mm2. A successful method has been developed for the ∼20 min transportation of the slices to the measurement lab. Evoked magnetic fields from transverse hippocampal slices from rats have been recorded. The magnetic fields were ∼5 pT and showed a high correlation with the excitatory post-synaptic potential measured in close connection to the magnetic field recordings. With further development of the system, it may be possible to verify a higher signal-to-noise ratio than experienced in low-Tc SQUID systems. This would enable fewer averages, resulting in shorter experiments. Magnetic field studies of less dense brain structures than the hippocampus could be possible.