Gradient-excitation encoding combined with frequency and phase encodings for three-dimensional ultra-low-field MRI

Ultra-low-field magnetic resonance imaging (ULF MRI) in microtesla fields is a new technology with features unseen in tesla-range MRI. Instead of induction coils as sensors, superconducting quantum interference device (SQUID) sensors are used, providing a frequency-independent signal-to-noise ratio (SNR). Owing to its tolerance for large relative imaging-field inhomogeneities, electromagnet shimming is not necessary. ULF MRI can also be combined with magnetoencephalography (MEG) to image the brain with close to millimetre-millisecond resolution. In this paper, the hybrid MEG-MRI device developed at Aalto University will be presented, as well as a 3D imaging scheme combining gradient-excitation encoding with frequency and phase and encodings. It is noteworthy that, regarding the presented gradient-excitation encoding in ULF MRI, the kilohertz-range Larmor frequencies allow MR signals to propagate unattenuated through tissue, which is not the case in tesla-range MRI with Larmor frequencies even above 100 MHz. Thus, the presented encoding method is especially compatible with ULF MRI, where the use of three different encoding mechanisms for three-dimensional imaging is possible. The feasibility of image reconstruction with the gradient-excitation-encoding method is demonstrated by simulations.