Phosphatidylcholine contributes to in vivo ³¹P MRS signal from the human liver

To demonstrate the overlap of the hepatic and bile phosphorus (³¹P) magnetic resonance (MR) spectra and provide evidence of phosphatidylcholine (PtdC) contribution to the in vivo hepatic ³¹P MRS phosphodiester (PDE) signal, suggested in previous reports to be phosphoenolpyruvate (PEP). Phantom measurements to assess the chemical shifts of PEP and PtdC signals were performed at 7 T. A retrospective analysis of hepatic 3D ³¹P MR spectroscopic imaging (MRSI) data from 18 and five volunteers at 3 T and 7 T, respectively, was performed. Axial images were inspected for the presence of gallbladder, and PDE signals in representative spectra were quantified. Phantom experiments demonstrated the strong pH-dependence of the PEP chemical shift and proved the overlap of PtdC and PEP (∝2 ppm relative to phosphocreatine) at hepatic pH. Gallbladder was covered in seven of 23 in vivo 3D-MRSI datasets. The PDE_gall/γ-ATP_liver ratio was 4.8-fold higher (p = 0.001) in the gallbladder (PDE_gall/γ-ATP_liver = 3.61 ± 0.79) than in the liver (PDE_liver/γ-ATP_liver = 0.75 ± 0.15). In vivo 7 T ³¹P MRSI allowed good separation of PDE components. The gallbladder is a strong source of contamination in adjacent ³¹P MR hepatic spectra due to biliary phosphatidylcholine. In vivo ³¹P MR hepatic signal at 2.06 ppm may represent both phosphatidylcholine and phosphoenolpyruvate, with a higher phosphatidylcholine contribution due to its higher concentration. (orig.)