ABSTRACT
Direct imaging of impressed dc currents inside the head can provide valuable conductivity information, possibly improving electro-magnetic neuroimaging. Ultra-low field magnetic resonance imaging (ULF MRI) at µT Larmor fields can be utilized for current density imaging (CDI). Here, a measurable impact of the magnetic field BJ, generated by the impressed current density J, on the MR signal is probed using specialized sequences. In contrast to high-field MRI, the full tensor of BJ can be derived without rotation of the subject in the scanner, due to a larger flexibility in the sequence design. We present an ULF MRI setup based on a superconducting quantum interference device (SQUID), which is operating at a noise level of 380â¯aTâ¯Hz-1/2 and capable of switching all imaging fields within a pulse sequence. Thereby, the system enables zero-field encoding, where the full tensor of BJ is probed in the absence of other magnetic fields. 3D CDI is demonstrated on phantoms with different geometries carrying currents of approximately 2â¯mA corresponding to current densities between 0.45 and 8â¯A/m2. By comparison to an in vivo acquired head image, we provide insights to necessary improvements in signal-to-noise ratio.
