Ultra-sensitive SQUID systems for applications in biomagnetism and ultra-low field MRI

Abstract

We present the use of our ultra-sensitive SQUID system in the field of biomagnetism and ultra-low field (ULF) MRI. A current sensor configuration is used where a pickup coil is inductively coupled to the SQUID. A 1st-order axial gradiometer system, operated in a liquid He dewar with negligible noise, achieves a measured coupled energy sensitivity $\epsilon_{c}$ of 40 $h$ and a white noise below 200 aT $\mathrm{Hz}^{-1/2}$. As an example of its use in biomagnetism, we discuss single trial magnetoencephalography measurements of high frequency bursts at 600 Hz from the somatosensory cortex which are related to synchronized spiking activity of individual neurons. We also deploy this system for ultra-low field MRI where it is operated inside an MRI coil system with several fast-switchable field and gradient coils. This necessitates the use of a current limiter in the input circuit and a 2nd-order axial gradiometer leading to an increased noise of 380 aT $\mathrm{Hz}^{-1/2}$. Here, we demonstrated full tensor current density imaging of impressed currents in phantoms. For further improvement of the noise, the fabrication process for the nanometer-sized Josephson junctions based on the HfTi self-shunted junction technology has been extended to a SIS process with AlOx as the insulating layer. We achieved noise levels of 330 $\mathrm{n}\Phi_{0}\mathrm{Hz}^{-1/2}$ and 550 $\mathrm{n}\Phi_{0}\mathrm{Hz}^{-1/2}$, corresponding to energy sensitivities of 5 $h$ and 20 $h$ for uncoupled and coupled SQUIDs, respectively.