Spatial phase encoding exploiting the Bloch-Siegert shift effect

The present work introduces an alternative to the conventional B0-gradient spatial phase encoding technique. By applying far off-resonant radiofrequency (RF) pulses, a spatially dependent phase shift is introduced to the on-resonant transverse magnetization. This so-called Bloch-Siegert (BS) phase shift has been recently used for B1(+)-mapping. The current work presents the theoretical background for the BS spatial encoding technique (BS-SET) using RF-gradients.

Since the BS-gradient leads to nonlinear encoding, an adapted reconstruction method was developed to obtain undistorted images. To replace conventional phase encoding gradients, BS-SET was implemented in a two-dimensional (2D) spin echo sequence on a 0.5 T portable MR scanner.

A 2D spin echo (SE) measurement imaged along a single dimension using the BS-SET was compared to a conventional SE 2D measurement. The proposed reconstruction method yielded undistorted images.

BS-gradients were demonstrated as a feasible option for spatial phase encoding. Furthermore, undistorted BS-SET images could be obtained using the proposed reconstruction method.