Toward structure and metabolism of glycogen C1-C6 in humans at 7 T by localized 13C MRS using low-power bilevel broadband 1H decoupling.

This work aims to develop and implement a pulse-acquire sequence for three-dimensional (3D) single-voxel localized ¹³C MRS in humans at 7 T, in conjunction with bilevel broadband ¹H decoupling, and to test its feasibility in vitro and in vivo in human calf muscle with emphasis on the detection of glycogen C₁-C₆. A localization scheme suitable for measuring fast-relaxing ¹³C signals in humans at 7 T was developed and implemented using the outer volume suppression (OVS) and one-dimensional image selected in vivo spectroscopy (ISIS-1D) schemes, similar to that which was previously reported in humans at 4 T. The 3D ¹³C localization scheme was followed by uniform ¹³C adiabatic excitation, all complemented with an option for bilevel broadband ¹H decoupling to improve both ¹³C sensitivity and spectral resolution at 7 T. The performance of the pulse-acquire sequence was investigated in vitro on phantoms and in vivo in the human calf muscle of three healthy volunteers, while measuring glycogen C₁-C₆. In addition, T₁ and T₂ of glycogen C₁-C₆ were measured in vitro at 7 T, as well as T₁ of glycogen C₁ in vivo. The glycerol C₂ and C_1,3 lipid resonances were efficiently suppressed in vitro at 7 T using the OVS and ISIS-1D schemes, allowing distinct detection of glycogen C₂-C₆. While some glycerol remained in calf muscle in vivo, the intense lipid at 130 ppm was efficiently suppressed. The ¹³C sensitivity and spectral resolution of glycogen C₁-C₆ in vitro and glycogen C₁ in vivo were improved at 7 T using bilevel broadband ¹H decoupling. The T₁ and T₂ of glycogen C₁-C₆ in vitro at 7 T were consistent compared with those at 8.5 T, while the T₁ of glycogen C₁ in vivo at 7 T resulted similar to that in vitro. Localized ¹³C MRS is feasible in human calf muscle in vivo at 7 T, and this will allow further extension of this method for ¹³C MRS measurements such as in the brain.