A three-dimensional variable-density spiral spatial-spectral RF pulse with rotated gradients.

Three-dimensional spatial-spectral radiofrequency pulses using a stack-of-spirals trajectory can achieve two-dimensional spatial localization and one-dimensional spectral selection simultaneously. These pulses are useful, for example, in reduced field-of-view applications that also require frequency specificity such as lipid imaging. A limitation of the pulse design is that the length of the spiral trajectory is fixed by the frequency separation of lipid and water. This restricts the highest possible excitation resolution of the spatial profile over a given field of excitation. In this work, we examine the use of periodically rotated variable-density spirals to increase the spatial excitation resolution without changing the frequency selectivity. Variable-density spirals are used to undersample the high spatial frequencies such that higher excitation resolutions can be obtained with a small expense in increased aliasing of the slice profile. The periodic rotation of the spiral trajectories reduces the impact of the undersampling by distributing the aliasing in the frequency domain. The technique is demonstrated with simulations, phantom studies, and imaging human leg muscle at 3 T. It was found in the human study that the spatial excitation resolution could be improved from 6 x 6 to 8 x 8 (matrix size over a fixed field of view) while decreasing aliasing by approximately 40-60%.