Depth selective quantification of phosphorus metabolites in human calf muscle

We have used phase modulated rotating frame imaging, a depth selective NMR technique, in conjunction with two methods of calibration, in order to determine the feasibility of quantifying phosphorus compounds in selected regions of a biological sample. In one method, we related peak areas obtained from a sample to the peak area of an external reference located within the receiver coil, previously calibrated against a standard. In the second method we measured peak areas from a standard separately and related them directly to the area of the sample peak. We examined the calf muscles of male volunteers, positioning the coil to obtain signal mainly from the gastrocnemius in the shallow slices and soleus in the deep slices. The concentrations of Pi, phosphocreatine and ATP (in mmol/kg tissue) were 3.0 +/- 0.5, 18 +/- 2 and 4.8 +/- 0.2 at 2 cm depth (mean +/- SD; n = 5). These results are similar to published biopsy data. A comparison with published values obtained from other NMR quantification methods suggests that, while it is possible to obtain consistent results within one laboratory, it is not currently possible to obtain similar results between laboratories. This is evident from the fact that different laboratories report different phosphocreatine/ATP ratios, indicating that the techniques are limited by variations in the processes of data collection and peak area measurement.