  Magnetic resonance spectroscopy (MRS) allows the analysis of biochemical
processes non invasively and in vivo. Still, its application in clinical
diagnostics is rare. Routine MRS is limited to spatial, chemical and temporal
resolutions of cubic centimetres, mM and minutes. In fact, the signal of many
metabolites is strong enough for detection, but the resonances significantly
overlap, exacerbating identification and quantification. In addition, the
signals of water and lipids are much stronger and dominate the entire spectrum.
To suppress the background and isolate selected signals, usually, relaxation
times, J-coupling and chemical shifts are used. Here, we propose methods to
isolate the signals of selected molecular groups within endogenous metabolites
by using long-lived spin states (LLS). We exemplify the method by preparing the
LLSs of coupled protons in the endogenous molecules N-acetyl-L-aspartic acid
(NAA). First, we store polarization in long-lived, double spin states and then
apply saturation pulses and double quantum filters to suppress background
signals. We show that LLS can be used to selectively prepare and measure the
signals of chosen metabolites or drugs in the presence of water, inhomogeneous
field and highly concentrated fatty solutions. The pH measurement presented
here is one of the possible applications.
