Pre-polarized MRI in a zero readout magnetic field and radiofrequency selective excitation in zero-field NMR

Pre-polarized MRI in a zero readout magnetic field would be advantageous for a number of reasons, mainly because of the elimination of magnetic field inhomogeneity and the effects of magnetic susceptibility gradients produced by the applied magnetic field. Here, I propose a pulse sequence and image reconstruction methodology that takes fully into account the concomitant components of the gradient magnetic field that would necessarily play a role in pre-polarized MRI in a zero readout magnetic field, and that are usually neglected in conventional MRI. The approach presented here applies not only to PMRI in a zero readout field but any MRI experiment where the gradient is on the same order of magnitude or greater than the constant applied magnetic field. For example, in high-resolution MRI, very large gradients are applied to obtain resolutions that are usually on the order of micrometers. Because of the large magnetic field gradients that are applied, high-resolution MRI performed at low-magnetic fields would result in large concomitant components of the gradient. Whether the applied readout magnet is very small or zero or whether very high-strength gradient magnetic fields are applied, a new approach to MRI is required that fully accounts for the concomitant components of the gradient magnetic field.; Zero-field NMR was developed to overcome the instrinsic broadening of the resonance line in high-fields due to the dependence of interactions such as dipolar coupling on the orientation of the molecule with respect to the applied magnetic field in high-field, solid-state NMR. One major limitation, however, of zero-field NMR is the inability, so far, to selectively excite spins based on the frequency of the spins in the zero-field NMR spectrum. Selective excitation is important to selectively excite and detect NMR spectra from selected spins in order to simplify complex zero-field spectra which can result from just a few coupled spins in zero-field NMR. Selective excitation of spins is also applied to selective decoupling using multiple-pulse sequences or continuous-wave radiofrequency irradiation. Here, I suggest a method for radiofrequency selective excitation of the spins based on the NMR frequency of the spins in the zero-field spectrum. I derive the resonance conditions for radiofrequency selective excitation in a purely J-coupled and purely dipolar coupled spin system and show simulations of the effect of selective excitation using the applied radiofrequency field. The applied rf-pulse selectively rotates spin pairs based on the J-coupling or dipolar coupling frequency of the spins in purely coupled spin systems.