2D arbitrary shape-selective excitation summed spectroscopy (ASSESS)

Conventional single-voxel localization for MR spectroscopy (MRS) is restricted to selecting only rectangular-shaped regions of interest (ROIs). The complexity of tissue shapes of interest and the desire to maximize the signal-to-noise ratio (SNR) while minimizing partial-volume effects require more sophisticated localization techniques. A group of spatially selective RF pulses are proposed in this work for the measurement of spectra from regions of arbitrary shape based on using a radial trajectory in k-space. Utilizing a single k-line per excitation results in a broad spectroscopic bandwidth. However, spatial localization accuracy is compromised for nutation angles > 10 degrees because of the small-tip-angle approximation of the Bloch equations. By interleaving multiple radial k-lines per excitation with nonselective refocusing pulses, one can achieve accurate localization for nutation angles up to 90 degrees while simultaneously maintaining the spectral bandwidth. The technique is described and compared with existing localization methods, and in vivo results are demonstrated.