In Vivo Detection of 15N-Coupled Protons in Rat Brain by ISIS Localization and Multiple-Quantum Editing

Abstract

Three-dimensional image-selected in vivo spectroscopy (ISIS) was combined with phase-cycled 1H–15N heteronuclear multiple-quantum coherence (HMQC) transfer NMR for localized selective observation of protons J-coupled to 15N in phantoms and in vivo. The ISIS–HMQC sequence, supplemented by jump–return water suppression, permitted localized selective observation of 2–5 μmol of [15Nindole]tryptophan, a precursor of the neurotransmitter serotonin, through the 15N-coupled proton in 20–40 min of acquisition in vitro at 4.7 T. In vivo, the amide proton of [5-15N]glutamine was selectively observed in the brain of spontaneously breathing 15NH4+-infused rats, using a volume probe with homogeneous 1H and 15N fields. Signal recovery after three-dimensional localization was 72–82% in phantoms and 59 ± 4% in vivo. The result demonstrates that localized selective observation of 15N-coupled protons, with complete cancellation of all other protons except water, can be achieved in spontaneously breathing animals by the ISIS–HMQC sequence. This sequence performs both volume selection and heteronuclear editing through an addition/subtraction scheme and predicts the highest intrinsic sensitivity for detection of 15N-coupled protons in the selected volume. The advantages and limitations of this method for in vivo application are compared to those of other localized editing techniques currently in use for non-exchanging protons.