Three‐dimensional hadamard‐encoded proton spectroscopic imaging in the human brain using time‐cascaded pulses at 3 tesla

Purpose To reduce the specific‐absorption‐rate (SAR) and chemical shift displacement (CSD) of three‐dimensional (3D) Hadamard spectroscopic imaging (HSI) and maintain its point spread function (PSF) benefits. Methods A 3D hybrid of 2D longitudinal, 1D transverse HSI (L‐HSI, T‐HSI) sequence is introduced and demonstrated in a phantom and the human brain at 3 Tesla (T). Instead of superimposing each of the selective Hadamard radiofrequency (RF) pulses with its N single‐slice components, they are cascaded in time, allowing N ‐fold stronger gradients, reducing the CSD. A spatially refocusing 180° RF pulse following the T‐HSI encoding block provides variable, arbitrary echo time (TE) to eliminate undesirable short T 2 species' signals, e.g., lipids. Results The sequence yields 10–15% better signal‐to‐noise ratio (SNR) and 8–16% less signal bleed than 3D chemical shift imaging of equal repetition time, spatial resolution and grid size. The 13 ± 6, 22 ± 7, 24 ± 8, and 31 ± 14 in vivo SNRs for myo ‐inositol, choline, creatine, and N ‐acetylaspartate were obtained in 21 min from 1 cm 3 voxels at TE ≈ 20 ms. Maximum CSD was 0.3 mm/ppm in each direction. Conclusion The new hybrid HSI sequence offers a better localized PSF at reduced CSD and SAR at 3T. The short and variable TE permits acquisition of short T 2 and J ‐coupled metabolites with higher SNR. Magn Reson Med 72:923–933, 2014. © 2013 Wiley Periodicals, Inc.