Unshielded SQUID Sensors for Ultra-Low-Field Magnetic Resonance Imaging

We have implemented a new field-tolerant low-critical-temperature superconducting quantum interference device current sensor with submicrometer Josephson junctions. The small junction area enables the fabrication of devices with lower sensor noise and higher tolerance against magnetic fields. An application benefitting from these properties is ultra-low-field magnetic resonance imaging (ULF MRI). In the currently ongoing BREAKBEN project, new technology is being pushed forward by combining ULF MRI with magnetoencephalography in a single instrument. We describe the sensor architecture aimed for this purpose and justify the guidelines behind the design. Experimental characterization reveals that, with the aid of reasonable amount of heating, unshielded sensor operation after high magnetic pulses at least up to 150 mT appears to be possible