Broadband proton decoupled natural abundance 13C NMR spectroscopy of humans at 1.5 T

Abstract

The feasibility of broadband proton decoupled in vivo¹³C NMR spectroscopy of humans at 1.5 T was explored. A dual surface coil set‐up was used, comprising a circular ¹³C coil and a butterfly ¹H decoupling coil placed at one third of its width away from the body. A calibration procedure was introduced to evaluate the specific absorption rate (SAR) in any gram of tissue for the inhomogeneous decoupling field generated by a surface coil. For the WALTZ‐4 sequence it was demonstrated that broadband decoupled spectra of both subcutaneous adipose and underlying muscle or liver tissue could be obtained at 1.5 T without exceeding recommended maximum SAR values. Broadband decoupling caused an additional resolution enhancement ascribed to the removal of (¹H–¹³C) long range couplings. Broadband proton decoupled spectra of subcutaneous adipose tissue were obtained in less than 10 min showing highly resolved and intense signals of fully relaxed carbon spin systems of triacylglycerols. Broadband proton decoupled ¹³C NMR spectra of calf muscle showed several resonances for metabolites resolved from triacylglycerol signals (e.g. C₁–C₅ of glycogen, C₄ of histidine, aromatic and carbonyl carbons of aminoacids and N linked carbons of ethanolamine, choline and creatine). With an acquisition time of 20–30 min, the C₁ glycogen signal was observed with a root mean square signal‐to‐noise ratio of about 15. Not only the glycogen C₁ signal but also its C₂–C₆ signals could be monitored in dynamic studies. Finally broadband proton decoupled ¹³C spectra were obtained with signals from liver tissue (notably the carbons of glycogen). The localization of liver tissue from surrounding muscle tissue could be verified on the basis of the intensities of resonances typical of muscle tissue compounds. The signal strength of the liver glycogen C₁ signal was 2 to 3 times that of the glycogen C₁ signal of muscle after the same number of scans.