What can be measured with 13C MRS?
With natural abundance 13C MRS the C1 peak of glycogen can be easily observed, and the chemical concentration can be determined from an external glycogen standard. Other peaks, from C=C bonds of triglycerides, CH2 and CH3 resonances of freefatty acids, and glycerol are more complex, representing several different atoms that cannot be easily resolved. Therefore, determining chemical concentrations is very difficult. The problem with 13C MRS is that the 13C atom represents only 1.1% of the total atoms; hence, signaltonoise issues become a problem. For example, at 4.7T a useful 13C spectrum from the human gastrocnemius requires about 5min to collect, compared with a 31P spectrum, which can be collected in a few seconds. By administering a 13C labeled substrate (often 113C glucose administered intravenously) the low signal generated by natural abundance 13C MRS enriched from 1.1% abundance to near 100% abundance, thereby dramatically increasing the signal. This technique can be used to tra
