Quantitative evaluation of the lactate signal loss and its spatial dependence in PRESS localized H-1 NMR spectroscopy

Localized H-1 NMR spectroscopy using the 90 degrees - t(1) - 180 degrees - t(1) + t(2) - 180 degrees - t(2) - Acq. PRESS sequence can lead to a signal loss for the lactate doublet compared with signals from uncoupled nuclei w hich is dependent on the choice of tl and t2. The most striking signal loss of up to 78% of the total signal occurs with the symmetrical PRESS sequenc e (t(1) = t(2)) at an echo time of 2/J (similar or equal to 290 ms). Calcul ations have shown that this signal loss is related to the pulse angle distr ibutions produced by the two refocusing pulses which leads to the creation of single quantum polarization transfer (PT) as well as to not directly obs ervable states (NDOS) of the lactate AX(3) spin system: zero- and multiple- quantum coherences, and longitudinal spin orders. In addition, the chemical shift dependent voxel displacement (VOD) leads to further signal loss. By calculating the density operator for various of the echo times TE = n/J, n = 1, 2, 3,..., we calculated quantitatively the contributions of these effe cts to the signal loss as well as their spatial distribution. A maximum sig nal loss of 75% can be expected from theory for the symmetrical PRESS seque nce and TE = 2/J for Hamming filtered sinc pulses, whereby 47% are due to t he creation of NDOS and up to 28% arise from PT, Taking also the VOD effect into account (2 mT/m slice selection gradients, 20-mm slices) leads to 54% signal loss from NDOS and up to 24% from PT, leading to a maximum signal l oss of 78%. Using RE-BURP pulses with their more rectangular pulse angle di stributions reduces the maximum signal loss to 44%. Experiments at 1.5 T us ing a lactate solution demonstrated a maximum lactate signal loss for sinc pulses of 82% (52% NDOS, 30% PT) at TE = 290 ms using the symmetrical PRESS sequence. The great signal loss and its spatial distribution is of importa nce for investigations using asymmetrical PRESS sequence at TE=2/J. (C) 200 1 Academic Press.