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 $\mu$T 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 nterference 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/m$^{2}$. By comparison to an in vivo acquired head image, we provide insights to necessary improvements in signal-to-noise ratio.