Chemical-shift artifact reduction in hadamard-encoded MR spectroscopic imaging at high (3T and 7T) magnetic fields

Proton MR spectroscopic imaging (MRSI) at higher magnetic fields (B ₀) suffers metabolite localization errors from different chemical-shift displacements (CSDs) if spatially-selective excitation is used. This phenomenon is exacerbated by the decreasing radiofrequency (RF) field strength, B₁, at higher B₀s, precluding its suppression with stronger gradients. To address this, two new methods are proposed: 1) segmenting the volume-of-interest (VOI) into several slabs, allowing proportionally stronger slice-select gradients; and 2) sequentially cascading rather than superposing the components of the Hadamard selective pulses used for reasons of better point-spread function (PSF) to localize the few slices within each slab. This can reduce the peak B₁ to that of a single slice. Combining these approaches permits us to increase the selective gradient four- to eightfold per given B₁, to 12 or 18mT/m for 4- or 2-cm VOIs. This "brute force" approach reduces the CSD to under 0.05 cm/ppm at 7T, or less than half that at 3T.