Absolute Quantification of Phosphor-Containing Metabolites in the Liver Using ³¹ P MRSI and Hepatic Lipid Volume Correction at 7T Suggests No Dependence on Body Mass Index or Age

Background: Hepatic disorders are often associated with changes in the concentration of phosphorus-31 (³¹ P) metabolites. Absolute quantification offers a way to assess those metabolites directly but introduces obstacles, especially at higher field strengths (B₀ ≥ 7T).

Purpose: To introduce a feasible method for in vivo absolute quantification of hepatic ³¹ P metabolites and assess its clinical value by probing differences related to volunteers' age and body mass index (BMI).

Study type: Prospective cohort.

Subjects/phantoms: Four healthy volunteers included in the reproducibility study and 19 healthy subjects arranged into three subgroups according to BMI and age. Phantoms containing ³¹ P solution for correction and validation.

Field strength/sequence: Phase-encoded 3D pulse-acquire chemical shift imaging for ³¹ P and single-volume ¹ H spectroscopy to assess the hepatocellular lipid content at 7T.

Assessment: A phantom replacement method was used. Spectra located in the liver with sufficient signal-to-noise ratio and no contamination from muscle tissue, were used to calculate following metabolite concentrations: adenosine triphosphates (γ- and α-ATP); glycerophosphocholine (GPC); glycerophosphoethanolamine (GPE); inorganic phosphate (P_i ); phosphocholine (PC); phosphoethanolamine (PE); uridine diphosphate-glucose (UDPG); nicotinamide adenine dinucleotide-phosphate (NADH); and phosphatidylcholine (PtdC). Correction for hepatic lipid volume fraction (HLVF) was performed.

Statistical tests: Differences assessed by analysis of variance with Bonferroni correction for multiple comparison and with a Student's t-test when appropriate.

Results: The concentrations for the young lean group corrected for HLVF were 2.56 ± 0.10 mM for γ-ATP (mean ± standard deviation), α-ATP: 2.42 ± 0.15 mM, GPC: 3.31 ± 0.27 mM, GPE: 3.38 ± 0.87 mM, P_i : 1.42 ± 0.20 mM, PC: 1.47 ± 0.24 mM, PE: 1.61 ± 0.20 mM, UDPG: 0.74 ± 0.17 mM, NADH: 1.21 ± 0.38 mM, and PtdC: 0.43 ± 0.10 mM. Differences found in ATP levels between lean and overweight volunteers vanished after HLVF correction.

Data conclusion: Exploiting the excellent spectral resolution at 7T and using the phantom replacement method, we were able to quantify up to 10 ³¹ P-containing hepatic metabolites. The combination of ³¹ P magnetic resonance spectroscopy imaging data acquisition and HLVF correction was not able to show a possible dependence of ³¹ P metabolite concentrations on BMI or age, in the small healthy population used in this study.

Level of evidence: 2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;49:597-607.