Spatial phase encoding exploiting the Bloch–Siegert shift effect

The present work introduces an alternative to the conventional $$B_{0}$$ -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 $$B_{1}^{ + }$$ -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.