Phase Constrained Encoding (PACE): A Technique for MRI in Large Static Field Inhomogeneities

In spin echo imaging, magnetization is assigned to a location defined by its frequency of rotation. In the presence of a static magnetic field inhomogeneity, however, this location does not correspond to the true location of the magnetization. This paper describes a magnetic resonance imaging technique called phase constrained encoding (PACE) that assigns magnetization to its true location through the use of a spin echotrain and alternating readout gradients. Small artifactual sidebands occur in the point spread function but can be minimized or eliminated using higher gradient strengths, more echoes, and/or additional acquisitions. Implementation of a simple version of this technique confirms simulations.