Three-dimensional MRI with an undersampled spherical shells trajectory.

The shells trajectory is a 3D data acquisition method with improved efficiency compared to Cartesian sampling. It is a true center-out trajectory that does not repeatedly resample the center of k-space, and also offers advantages for motion correction. This work demonstrates that k-space undersampling can be combined with the shells trajectory to further accelerate the acquisition. The undersampling was implemented by removing selected interleaves from shells with larger radii. Because only the outer portion of k-space was undersampled, the artifacts introduced were of low energy and high spatial frequency. The undersampling rate was determined by a Kaiser window with a variable shape parameter beta. Various undersampling schemes with different beta values were examined. Phantom and volunteer studies demonstrate that when up to a twofold acceleration is achieved, only minor artifacts are introduced by undersampling the shells trajectory. For a fixed acquisition time, the improved efficiency can be used to increase spatial resolution.