Offset-dependent partial saturation in binomial solvent suppression sequences.

Abstract

In vivo spectroscopic experiments are commonly done under partially T1 saturated conditions and saturation factors (observed signal intensity divided by fully relaxed intensity) used for quantitative analysis of absolute and relative metabolite concentrations. The conventional calculation of the saturation factor assumes uniform excitation across the spectrum. This assumption is violated when selective-excitation solvent peak suppression techniques, such as binomial sequences, are used. In this case, the degree of partial saturation, and hence the saturation factor, is dependent on the resonance offset. This has implications for the frequency offset of maximum excitation, solvent suppression effectiveness, absolute and relative quantitative measurements, and progressive saturation T1 measurements. In this paper, the jump and return (JR) and jump and return echo (JRE) sequences are examined in detail with regard to these implications. It is shown that offset-dependent partial saturation depends on the sequence used and can become significant. A saturation factor which is a function of offset can be used to correct for this effect and allow for proper quantitative interpretation of experimental results under partially saturated conditions.