Broadband heteronuclear Hartmann-Hahn sequences with short cycle times

Sensitivity-enhanced13C MR spectroscopy of the human brain at 3 Tesla

A new coil design for sensitivity-enhanced 13C MR spectroscopy (MRS) of the human brain is presented. The design includes a quadrature transmit/receive head coil optimized for 13C MR sensitivity. Loss-less blocking circuits inside the coil conductors allow this coil to be used inside a homogeneous circularly polarized 1H B1 field for 1H decoupled 13C MRS. A quadrature 1H birdcage coil optimized for minimal local RF heating makes broadband 1H decoupling in the entire human brain possible at 3 Tesla while remaining well within international safety guidelines for RF absorption. Apart from a substantial increase in sensitivity compared to conventional small linear coils, the quadrature 13C coil combined with the quadrature 1H birdcage coil allows efficient cross polarization (CP) in the brain, resulting in an additional 3.5-fold sensitivity improvement compared to direct 13C measurements without nuclear Overhauser enhancement (NOE) or polarization transfer. Combined with the gain in power efficiency, this setup allows broadband 1H to 13C CP over large areas of the brain. Clear 13C resonances from glutamate (Glu), glutamine (Gln), aspartate (Asp), lactate (Lac), and gamma-aminobutyrate (GABA) carbon spins in the human brain demonstrate the quality of 13C MR spectra obtained in vivo with this coil setup.

Spin state selectivity and heteronuclear Hartmann-Hahn transfer

Spin state selectivity can be obtained if inphase and antiphase magnetization can be transformed into each other. Heteronuclear Hartmann-Hahn transfer is usually used for the transfer of inphase magnetization. For a two spin system, however, a building block can be constructed that transfers inphase into antiphase magnetization and vice versa. The article gives a detailed description of the building block as well as experiments with elements similar to spin state selective coherence transfer (S(3)CT) and spin state selective excitation (S(3)E). The possibility of 'sensitivity enhancement' is pointed out and an application in the double selective measurement of coupling constants using a combination of CW-cross-polarization and the S(3)CT-type approach is demonstrated.