Hepatic disease affects over 10% of people in Western populations^{1, 2}. Phosphomonoester (PME) and phosphodiester (PDE) concentrations are emerging biomarkers indicating the presence of non-alcoholic fatty liver disease³ or liver cirrhosis⁴. The region of the liver phosphorus magnetic resonance spectroscopy (³¹P-MRS) spectrum containing PME and PDE has several other resonances which make accurate quantitation difficult. Increasing field strength from 3T to 7T improves spectral resolution for in vivo ³¹P-MRS⁵, which should allow for improved quantitation. As yet, only inorganic phosphate (P_i) concentrations have been reported at 7T⁶. In this study, we use our 16-element receive array coil⁷ to report PME and PDE concentrations from the livers of 10 healthy volunteers.

Each voxel can be corrected to yield an absolute concentration⁸:

$$[M] = \frac{S_MF_M\nu_R\eta_M[R]}{S_RF_R\nu_M\eta_R}$$

in which S is the signal per voxel, F is the saturation correction factor, ν is the filling factor, and $$$\eta$$$ is the sensitivity correction of the tissue or reference sample in the voxel. The subscripts stand for reference R and metabolite M.

When using an endogenous reference (i.e. concentration of adenosine triphosphate (ATP) = 2.5mM), $$$\eta_M = \eta_R$$$ and $$$\nu_M =\nu_R$$$ reducing Eq.1 to:

$$[M] = \frac{S_MF_M}{S_RF_R}\times2.5\tt{mM} $$

10 fasted, healthy volunteers (6M/4F, 26.7 ± 4.8 years, 72.4 ± 10.3 kg, recruited ethically) were scanned in a Magnetom 7T (Siemens) using a 16-channel ³¹P receive array coil (Rapid Biomedical, Germany)⁹.

A 3D UTE-CSI pulse sequence, with 1s TR, excitation bandwidth covering all metabolites from uridine diphosphoglycerate to phosphocholine (-12 to 10 ppm), acquisition weighting with 10 averages at k=0, and one B1-insensitive train to obliterate signal (BISTRO) saturation band to suppress overlying skeletal muscle¹⁰, was used to acquire a 16x16x8 matrix of spectra over a 27 x 24 x 20 cm³ field of view covering the liver in a total of 28 min.

Spectra from each receive element were combined with whitened singular value decomposition (WSVD) combination¹¹. The combined spectra were analysed following our established protocol⁷; in brief: peaks were fitted with linewidth-constrained AMARES¹², fiducial markers were used to localize each coil element, and saturation correction factors were computed using the Biot-Savart law and literature metabolite T₁ values¹³. Concentrations were then calculated for this work using Eq. 2, assuming a 2.5mM concentration of ATP¹⁴.

A typical liver spectrum is shown in Figure 1. The individual PME and PDE peaks were quantified separately and then summed together for comparison with the literature. The concentrations for individual subjects are plotted in Figure 2. The average concentrations from our study are presented alongside comparators from the literature in Table 1 and Figure 3.

The assumption of a 2.5mM ATP concentration is reasonable in the fasted, healthy human liver14, 15, and is consistent with exogenous reference studies^16-18 (see Fig. 3). PME is significantly smaller in this study than in several previous studies^{16, 18, 19} (at a p=0.05 level for Student’s t-test, see Fig 3). However, the difference for PDE is more significant in all cases (at a p=0.0001 level, 3.7-7.1mM difference).

At low field strength, it is difficult to distinguish the phosphoenolpyruvate (PEP) / phosphatidylcholine (PtdCh) peak from the PDE peak because the peaks strongly overlap⁵. At 3T it is only possible with proton decoupling²⁰ (see Fig. 4). Another contributing factor is an underlying resonance in the PDE region due to endoplasmic reticulum. This broadens from 210Hz at 1.9T to 7000Hz at 7T and becomes indistinguishable from baseline noise²¹. In a direct comparison between 1.5T and 3T, there was a small reduction in the underlying broad peak and a small positive difference between PME/PDE ratios²⁰. This effect is likely to have become still more prominent at 7T, as the underlying peak has broadened enough to be below the noise threshold for detection.

There are two possible sources of additional ATP, which would reduce the apparent PDE concentration: skeletal muscle and blood. The average phosphocreatine (PCr) contamination was 0.66 ± 0.12mM. Given the 4:1 ratio of PCr:ATP in skeletal muscle, the average ATP contamination is less than 0.17mM. Correcting for this would give a PDE concentration of 4.43mM. The 2,3-diphosphoglycerate (2,3-DPG) to ATP ratio is about 9:1²² in blood, so any contamination would increase the 2,3-DPG signal, overlapping PDE, more than ATP and the apparent PDE concentration would be increased.

The resolution of PDE into separate peaks at 7T allows more accurate quantitation and presents an opportunity to determine which metabolites are the true biomarkers for hepatic diseases. We anticipate that this will improve the predictive accuracy of the method compared to lower field strengths.