Spin-echo MRS in humans at high field: LASER localisation using FOCI pulses

Abstract

Significant improvements in spin-echo MRS are possible when voxel localisation is performed using high bandwidth frequency offset corrected inversion (FOCI) pulses as opposed to more conventional lower bandwidth pulses. The reduced chemical shift displacement errors result in a spectrum that more accurately reflects the actual metabolite distribution within any region of interest that is selected graphically on a series of scout images, and can lead to improved metabolite detection in the case of homonuclear J-coupled spins. At 4.7 T, FOCI pulses with a 20 kHz bandwidth result in extremely sharp and uniform selection profiles, and negligible contamination from outside of the voxel of interest, for all signals in the 1H spectral range that is normally studied. A ‘FOCI’ adiabatic half-passage is observed to provide good excitation over the 1H spectral range. Single shot performance with echo-time (TE) ⩾ 48 ms is reported using a four-port drive birdcage head coil. GAMMA simulations show that, for many detectable metabolites at 4.7 T, LASER localisation using FOCI pulses with TE = 48 ms results in 1H anti-phase spectral components that are the same order as would be obtained from a symmetric PRESS sequence with TE = 32 ms. Timing schemes are proposed to enable good measurement of lactate with very little signal loss arising from chemical shift displacement errors at TE = 144 and 288 ms.