Proton‐decoupled, overhauser‐enhanced, spatially localized carbon‐13 spectroscopy in humans

Abstract

Spatially localized, natural abundance, carbon (¹³C) NMR spectroscopy has been combined with proton (¹H) decoupling and nuclear Overhauser enhancement to improve ^I3C sensitivity up to five-fold in the human leg, liver, and heart. Broadband-decoupled ¹³C spectra were acquired in 1 s to 17 min with a conventional 1.5-T imaging/ spectroscopy system, an auxiliary ¹H decoupler, an air-cooled dual-coil coplanar surface probe, and both depth-resolved surface coil spectroscopy (DRESS) and one-dimensional phase-encoding gradient NMR pulse sequences. The surface coil probe comprised circular and figure-eight-shaped coils to eliminate problems with mutual coupling of coils at high decoupling power levels applied during ^I3C reception. Peak decoupler RF power deposition in tissue was computed numerically from electromagnetic theory assuming a semi-infinite plane of uniform biological conductor. Peak values at the surface were calculated at 4 to 6 W/kg in any gram of tissue for each watt of decoupler power input excluding all coil and cable losses, warning of potential local RF heating problems in these and related experiments. The average power deposition was about 9 mW/kg per watt input, which should present no systemic hazard. At 3 W input, human ¹³C spectra were decoupled to a depth of about 5 cm while some Overhauser enhancement was sustained up to about 3 cm depth, without ill effect. The observation of glycogen in localized natural abundance ¹³C spectra of heart and muscle suggests that metabolites in the citric acid cycle should be observable noninvasively using ¹³C-labeled substrates. © 1989 Academic Press, Inc.