High Field In vivo¹³C Magnetic Resonance Spectroscopy of Brain by Random Radiofrequency Heteronuclear Decoupling and...

In vivo¹³C magnetic resonance spectroscopy (MRS) is a unique and effective tool for studying dynamic human brain metabolism and the cycling of neurotransmitters. One of the major technical challenges for in vivo¹³C-MRS is the high radio frequency (RF) power necessary for heteronuclear decoupling. In the common practice of in vivo¹³C-MRS, alkanyl carbons are detected in the spectra range of 10-65 ppm. The amplitude of decoupling pulses has to be significantly greater than the large one-bond ¹H-¹³C scalar coupling (¹J_CH = 125-145 Hz). Two main proton decoupling methods have been developed: broadband stochastic decoupling and coherent composite or adiabatic pulse decoupling (e.g., WALTZ); the latter is widely used because of its efficiency and superb performance under inhomogeneous B₁ field. Because the RF power required for proton decoupling increases quadratically with field strength, in vivo¹³C-MRS using coherent decoupling is often limited to lowmagnetic fields [<=4 Tesla (T)] to keep the local and averaged specific absorption rate (SAR) under the safety guidelines established by the International Electrotechnical Commission (IEC) and the US Food and Drug Administration (FDA). Alternately, carboxylic/amide carbons are coupled to protons via weak long-range ¹H-¹³C scalar couplings, which can be decoupled using low RF power broadband stochastic decoupling. Recently, the carboxylic/amide ¹³C-MRS technique using low power random RF heteronuclear decoupling was safely applied to human brain studies at 7T. Here, we review the two major decoupling methods and the carboxylic/amide ¹³C-MRS with low power decoupling strategy. Further decreases in RF power deposition by frequency-domain windowing and time-domain random under-sampling are also discussed. Low RF power decoupling opens the possibility of performing in vivo¹³C experiments of human brain at very high magnetic fields (such as 11.7T), where signal-to-noise ratio as well as spatial and temporal spectral resolution are more favorable than lower fields.