Like in vivo MRS, fMRS can probe different nuclei, such as hydrogen (H) and carbon (C). The H nucleus is the most sensitive and is most commonly used to measure metabolite concentrations and concentration dynamics, whereas C is best suited for characterizing fluxes and pathways of brain metabolism. The natural abundance of C in the brain is only about 1%; therefore, C fMRS studies usually involve the isotope enrichment via infusion or ingestion.
In the literature C fMRS is commonly referred to as functional C MRS or just C MRS.