Detection of homonuclear decoupled in vivo proton NMR spectra using constant time chemical shift encoding: CT-PRESS.

A new pulse sequence, termed CT-PRESS, is presented, which allows the detection of in vivo 1H NMR spectra with effective homonuclear decoupling. A PRESS sequence with a short echo-time TE, used for spatial localization, is supplemented by an additional 180 degrees pulse. The temporal position of this 180 degree pulse is shifted within a series of experiments, while the time interval between signal excitation and detection is kept constant. CT-PRESS is a two-dimensional (2D) spectroscopic experiment as far as data acquisition and processing are concerned, although only diagonal signals are generated in the 2D spectrum. However, since the principle of constant time chemical shift encoding is used in the t1 domain, effective homonuclear decoupling is obtained by projecting the 2D spectrum onto the corresponding f1 axis. Thus, good spectral resolution and high signal-to-noise ratio are obtained. The main advantage, as compared to localized 2D J-resolved MRS, is that optimized experiments can be performed for coupled resonances of interest by choosing the sequence parameters dependent on the type of multiplets, the J-coupling constants and T2. Major fields of application will be parametric studies on coupled resonances, (e.g., T1, diffusion behavior or magnetization transfer) and/or the detection of spatial and temporal changes of metabolites with coupled spin systes.