Figure 1 shows a schematic of the low field MRI system configuration. To suppress the Earth’s field a mu metal shield is used.
A tetra coil is used to produce the imaging field B0, ranging from 50 µT to 2 mT. A Maxwell pair produces the diagonal gradient field, two sets of planar gradient coils produce off-diagonal fields and an excitation coil provides oscillating B1 pulses to manipulate the polarization of the sample. The loops of the B1 coil should be perpendicular to the other coils to reduce the mutual inductance between the coils.An aluminum shield surrounding the entire system reduces environmental magnetic field noise.
The spins are excited by an oscillating field B1, which we call the excitation field. The coil is wound around two frames in a Helmholtz arrangement. An open geometry of the coils allows flexibility for setting up the experiment. As the amplifiers does not need to be active during measurement, physical relays can be included to decouple them during signal acquisition (see fig. 2).
Three pairs of gradient producing coils can be implemented for spatial decoding of the signal. All three gradients are bi-planar designs and can produce similarly sized gradients, thus allowing an interchangeably use during an MRI sequence. The gradient coils are pulsed and can be relayed out (like the B1 coil) during signal acquisition.First a system is constructed for comparison with already existing setups, which uses Faraday coils for the signal detection4. Theory predicts, that the signal to noise ratio should increase by a factor of 5 to 10 using a SQUID based sensor for detection5. After optimizing the system the goal of the project will be to develop biocompatible hyperpolarized contrast agents with parahydrogen3, which can be used for first in vivo studies.
Literatur: Matlashov et al.