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

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 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/m 2 . By comparison to an in vivo acquired head image, we provide insights to necessary improvements in signal-to-noise ratio.

Evaluating the Performance of Ultra-Low-Field MRI for in-vivo 3D Current Density Imaging of the Human Head

“…To understand the effects of noise, we recap the sequence and reconstruction method designed by Vesanen et al [6]. More detailed information on the experimental implementation, including the sequence diagram, can be gleaned from Hömmen et al [9].…”

“…In particular, if also the main magnetic field B 0 can be switched on and off during the pulse sequence, it is possible to measure full-tensor information of the effects of B J (r), providing a way to directly estimate J(r) [6,7]. The field switching can be achieved [8,9] in ultra-low-field (ULF) MRI, where the main field is not produced by a persistent superconducting magnet as in conventional highfield MRI. Zero-field-encoded current density imaging (CDI) using superconducting quantum interference device (SQUID)based ULF MRI was first proposed by Vesanen et al [6].…”

“…Zero-field-encoded current density imaging (CDI) using superconducting quantum interference device (SQUID)based ULF MRI was first proposed by Vesanen et al [6]. It has recently been demonstrated in phantom measurements and is most promising regarding in-vivo implementation [9]. Since current impressed in vivo in the human head is limited by safety regulations to the low-mA range [10,11] and only a small fraction of the current passes the relatively highly-resistive skull [12,13], a sufficient signal-to-noise ratio (SNR) may be difficult to reach.…”

“…However, the ultimate sensitivity combining the lowest noise and the highest polarizing field in a single setup has not been demonstrated. Hömmen et al used an ultra-sensitive single-channel SQUID system with a noise level of 380 aT/ √ Hz for the demonstration of CDI [9]. This noise performance was about 10-20 times better than in commercially available SQUID systems, but the polarizing field of 17 mT was comparatively low.…”

“…In addition, two existing ULF-MRI setups are examined more closely regarding their performance in a CDI application. The first is the single-channel setup of PTB, Berlin, described by Hömmen et al [9], which is now equipped with an updated polarization setup specially designed for the shape of the human head. The second setup is a whole-head multi-channel system, a successor of the one described by Vesanen et al [16], located at Aalto University, Helsinki.…”