Polarization encoding as a novel approach to MRI.

In magnetic resonance imaging (MRI), there have been three basic techniques to encode the spatial origin of the signals, i.e. Fourier and radio frequency encoding and the use of sensitivity information of sensor arrays. In this paper, we introduce a new encoding method, which we call polarization encoding. The method utilizes sets of polarizing fields with various spatial profiles; it is tailored for MRI at ultra-low fields (ULF MRI). In ULF MRI, signals from a prepolarized sample are typically detected using an array of SQUID (superconducting quantum interference device) sensors at microtesla fields. The prepolarization is achieved with a field of the order of 10-100mT preceding the signal acquisition. In polarization encoding, the prepolarizing field is varied in a way to gain additional information about the sample. The method may also prove useful for modalities that in the absence of any precession aim to image the DC magnetization profile of a sample.