Ultra-low-noise amplifier for ultra-low-field MRI main field and gradients

In ultra-low-field (ULF) MRI, applied fields on the order of 100 μT are present during the measurement with one or more SQUID sensors. The current noise in the coils that produce the fields must be extremely small in order not to add noise to the measurement, especially when measured using magnetometer pickup coils. In addition, to allow other than the most basic pulse sequences, the applied fields must often be ramped up and down at millisecond time scales, requiring relatively high voltages and a sufficient bandwidth. Since commercial power amplifiers are far from satisfying these requirements, we designed and implemented a dedicated ULF MRI current amplifier. We demonstrate that, using our amplifier, it is possible to make ULF MRI measurements without increased noise even with magnetometer pickup coils that are oriented to directly measure the magnetic field component along the applied field. Using tailored amplifiers for pulsing gradients and uniform fields opens many opportunities in developing and implementing advanced ULF MRI pulse sequences.