Demonstration of full tensor current density imaging using ultra-low field MRI

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 B_J, 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 B_J 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 380 aT 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 B_J is probed in the absence of other magnetic fields. 3D CDI is demonstrated on phantoms with different geometries carrying currents of approximately 2 mA corresponding to current densities between 0.45 and 8 A/m². By comparison to an in vivo acquired head image, we provide insights to necessary improvements in signal-to-noise ratio.