Methods And Human Subjects

13C Magnetic Resonance Spectroscopy: In principle, the infusion or ingestion of highly enriched 13C precursors and metabolites into a patient with essentially zero (actually 1.1%) 13C MR signal background, should dramatically enhance our ability to image metabolites, determine metabolic flux rates and follow metabolic markers for the purpose of clinical diagnosis and research3. The first multi-purpose human MRI scanners provided all of the equipment necessary for this purpose in 19834, but the spectacular success of anatomical MRI resulted in 'simplification' to the exclusion of all nuclei other than proton. Broad-band clinical MR scanners essential for clinical 13C MRS and MRI are more expensive to manufacture and hence confined to specialist Centers. 13C MRS significantly outscores 1H MRS (3 - 15 metabolites), 31P MRS (5 - 7 metabolites), 23Na, 7Li or 19F (1 - 5 metabolites each) in the number of human metabolites - upwards of 50 - which can be monitored.

Figure 2. Modeling requires the simplest scheme which can explain the C observations: The example given is for derivation of the in vivo rate of NAA-synthesis.

Nevertheless, as you would expect, for the physicians and radiologists involved in clinical MRS the actual numbers of studies (PUBMED: 1985-2005 Published MRS studies !H = 4000; 31P = 400; 13C = 40) reflect the seemingly insuperable difficulties of installing and applying proton-decoupled 13C MRS to their patients. We have adapted a routine 1.5 Tesla clinical MRI scanner to permit rapid 13C MRS of the brain in patients5 and developed several clinical infusion protocols acceptable to FDA (IND 56,510) for use in infants, children and adults6. Using a simplified oral or intravenous infusion protocol, patients and controls received 0.23G. kg-1 of 1-13C glucose and were followed by serial proton-decoupled 13C MRS for up to 180 minutes7. The experimental details have been published.

13C Data analysis: As the 13C pool expands it displaces the 12C pool; the extent to which this has occurred is termed fractional enrichment. A single 13C MRS acquisition can be used to determine fractional enrichment of the given 13C metabolite pool at a given time; more usually, multiple 13C MRS acquisitions establish the rate(s) of enrichment of the critical metabolic pools. These new data are expressed as flux-rates. Several hundred peaks appear over the course of such studies so that data analysis has been automated to permit accurate identification of 13C resonances, quantification of 13C metabolites with reference to an internal standard, myoinositol, which is known not to become enriched during the 13C glucose infusion protocol6, and derivation of fractional enrichment over time for each metabolite of interest (JAUYANG8).


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