Windowed stochastic proton decoupling for in vivo 13C magnetic resonance spectroscopy with reduced RF power deposition

Proton decoupled in vivo 13C magnetic resonance specetrscopy (MRS) is a useful noninvasive method for studying animal and human brain metabolism and neurotransmitter cycling. With the enhanced 13C signal due to infusion of exogenous 13C-labeled substrates such as glucose, the turnover of important metabolites in brain can be quantitatively measured (1–8). One of the major difficulties associated with 13C MRS has been the need to decouple large 1H-13C scalar couplings (1JCH = 125–145 Hz) for alkyl carbons of major brain metabolites. Tissue heating due to RF losses associated with electrically conducting samples places a restriction on RF power deposition. For effective proton decoupling, the radio frequency field strength of decoupling pulses (γB2) has to be much greater than 1JCH. Because chemical shift dispersion is proportional to static magnetic field strength (γB0), the proton decoupling bandwidth andγB2 increase linearly with γB0. Therefore, RF power required for proton decoupling increases as a function of (γB0)2, a formidable challenge for performing 13C MRS at high magnetic field strength. To obtain necessary RF efficiency with acceptable RF power deposition, most human brain 13C MRS studies have used the coherent cyclic WALTZ sequence for proton decoupling at 1.5–4 Tesla (2–4, 8) at decoupling power levels just under the SAR threshold set by the FDA. Coherent cyclic decoupling sequences such as MLEV and WALTZ achieve broadband decoupling of protons so that the CH splittings are eliminated from 13C spectra over the entire range of proton chemical shifts. As such, they have long superseded stochastic decoupling (9) in modern NMR spectroscopy. At very low γB2, however, coherent decoupling schemes break down quickly (10) because a single cycle of WALTZ-4 may occupy a significant portion of the FID sampling time. A new strategy for in vivo 13C MRS was developed recently (10). Instead of infusing the commonly used [1-13C]glucose which primarily labels the alkyl carbons of amino acids we infused [2-13C] or [2,5-13C2]glucose and detected the 13C signals of glutamate, glutamine, γ-aminobutyric acid (GABA), aspartate, and N-acetylaspartate (NAA) in the carboxylic/amide spectral region. The carboxylic/amide carbons are only coupled to protons via weak long-range 1H-13C couplings. It was found that these carbons could be effectively decoupled using stochastic decoupling schemes with very low RF power. In sharp contrast to the alkyl spectral region, no interfering lipid signals were found in the carboxylic/amide spectral region for brain studies. Because decoupling power increases as a function of (γB0)2 even stochastic decoupling may become problematic at very high magnetic field strength. For brain 13C MRS studies using the carboxylic/amide spectral region, the protons to be decoupled are clustered in two specific regions of the spectrum: the alkyl protons resonating at 1.91 (GABA H3)-3.90 (aspartate H2) ppm and the amide protons resonating at 6.83 (Gln HZ)-7.60 (Gln HE) ppm. Using white or pseudo white stochastic decoupling schemes considerable RF power is wasted in the empty spectral window spanning the 3.90–6.83 ppm region because the spectral density function (i.e., the Fourier transform of the autocorrelation function δ) of ideal white noise is independent of frequency. We realize that a large reduction in RF power deposition (≈50%) for stochastic decoupling is achievable if the large empty spectral window could be excluded from the effective band of stochastic decoupling sequences. In this work, we propose a strategy for reducing RF power deposition of stochastic proton decoupling. The proposed method is based on the Rayleigh’s theorem (11), which states that the total power in a signal is invariant with respect to Fourier transform. That is, the power calculated in the time domain and the frequency domain is the same. Since proton spins are decoupled by frequency components at and near their resonance frequency we multiplied the frequency spectrum of the stochastic decoupling pulse by an ideal double bandpass filter with rectangular response functions that eliminates frequency components outside the 1.91–3.90 ppm and 6.83–7.60 ppm regions. The effect of the proposed method on the performance of the stochastic decoupling sequence was demonstrated experimentally for localized in vivo 13C MRS of rat brain at 11.7 Tesla.