Brain MRI at ultra-low field: new multifunction instrument

While conventional MRI uses increasingly high magnetic fields, another approach has emerged, where the signal is detected in an ultra-low field (ULF) on the order of Earth's field. Despite similarities, ULF-MRI differs from high-field MRI in interesting ways. At ULF, the magnetic pulses can be applied silently and with an open-geometry coil system, and the signal can be modeled from theory with high accuracy. The unique possibilities of ULF-MRI also include the combination of MRI and magnetoencephalography (MEG), the measurement of the weak magnetic fields generated by brain activity. ULF-MRI and associated methods such as current-density imaging (CDI) improve the localization of brain activity.

Superconducting quantum-interference devices (SQUIDs) configured as pulse-tolerant sensors for ULF-MRI can also be used for MEG. Compared to our first hybrid prototype, our current-generation system uses more robust techniques and new software. Our dedicated amplifiers allow switching off even B₀ even during an imaging sequence, enabling the encoding of 3D information for current-density imaging (CDI). We demonstrate the state of the art of ULF-MRI and describe Dynamical Coupling for Additional dimensioNs (DynaCAN) – designing pulse waveforms to couple to a dynamical system with memory.