Auxiliary phase encoding in multi spin-echo sequences: application to rapid current density imaging.

Multi spin-echo sequences such as single-shot RARE are very sensitive to the initial phase of the transverse magnetization, and they can preserve only the transverse magnetization component which is aligned with the axis of the refocusing pulse rotation. Therefore, two separate single-shot RARE experiments with phases of refocusing pulses 90 degrees apart have to be run and their complex images summed to obtain an error-free phase map of the initial transverse magnetization. This is particularly useful when auxiliary phase encoding is integrated in the preparation period of the RARE sequence, such as when encoding flow, displacement, susceptibility, pH or temperature. In this paper, the two-shot RARE approach is verified first theoretically and then experimentally by demonstrating its application to rapid current density imaging (CDI). The sequence consists of the preparation period which triggers electric pulses in the sample followed by the RARE acquisition period. Electric currents through the sample induce a magnetic field change in the direction of the static magnetic field and a phase change of the initial magnetization proportional to it. To calculate one component of current density two orthogonal components of magnetic field change must be measured. In general, for 2D non-symmetrical samples, this can be done by rotating the sample to a perpendicular orientation. The proposed CDI method allows much for faster magnetic field change mapping than the standard spin-echo based CDI.