The performance of volume selection sequences for in vivo NMR spectroscopy: implications for quantitative MRS

Previous work has demonstrated that deficiencies in volume selection sequences used in magnetic resonance spectroscopy may compromise the quality of the spectra obtained. In this paper, further studies on the ISIS and PRESS sequences are presented. Under conditions of partial saturation, ISIS can exhibit serious contamination with extraneous signal, particularly when a small volume of interest (VOI) is selected. ISIS protocols should therefore use VOIs that are large relative to the target volume, and repetition times that are as long as practicable. In PRESS, contamination is found to be minimised by using a VOI that is small relative to the target volume, and to be independent of repetition time. PRESS performance is also independent of echo time, except when very short echo times are used. These results are consistent with previously published work on ISIS and PRESS, and it is now possible to establish generic features of these sequences and to understand the implications for quantitative spectroscopy. T1-weighting of contamination in ISIS can compromise both relative and absolute quantification techniques in several respects. Contamination in PRESS is largely independent of relaxation times and would be easier to model and correct for in the context of quantitative spectroscopy.