Origin of the P MR signal at 5.3 ppm in patients with critical limb ischemia.

An unknown intense signal (P ) with a mean chemical shift of 5.3 ppm was observed in P MR spectra from the calf muscles of patients with the diabetic foot syndrome. The aim of the study was to identify the origin of this signal and its potential as a biomarker of muscle injury. Calf muscles of 68 diabetic patients (66.3 ± 8.6 years; body mass index = 28.2 ± 4.3 kg/m ) and 12 age-matched healthy controls were examined by (dynamic) P MRS (3 T system, P/ H coil). Phantoms (glucose-1-phosphate, P and PCr) were measured at pH values of 7.05 and 7.51. At rest, P signals with intensities higher than 50% of the P intensity were observed in 10 of the 68 examined diabetic subjects. We tested two hypothetical origins of the P signal: (1) phosphorus from phosphoesters and (2) phosphorus from extra- and intracellular alkaline phosphate pools. 2,3-diphosphoglycerate and glucose-1-phosphate are the only phosphoesters with signals in the chemical shift region close to 5.3 ppm. Both compounds can be excluded: 2,3-diphosphoglycerate due to the missing second signal component at 6.31 ppm; glucose-1-phosphate because its chemical shifts are about 0.2 ppm downfield from the P signal (4.9 ppm). If the P signal is from phosphate, it represents a pH value of 7.54 ± 0.05. Therefore, it could correspond to signals of P in mitochondria. However, patients with critical limb ischemia have rather few mitochondria and so the P signal probably originates from interstitia. Our data suggest that the increased P signal observed in patients with the diabetic foot syndrome is a biomarker of severe muscular damage.