31P-MRS saturation transfer for assessing human hepatic ATP synthesis at clinical field strength

Abstract

Background: ³¹P-magnetic resonance spectroscopy (MRS) saturation transfer (ST) allows for noninvasive investigation of liver energy metabolism by assessing flux rates of adenosine triphosphate (ATP) synthesis. However, this technique has rarely been applied at clinical field strengths because of long examination times and contamination from muscle tissue. Our aim was to establish a new method to robustly assess ATP synthesis using a clinical scanner.

Methods: A prospective single-center study was performed (January 2023-August 2024) within the German Diabetes Study. We established a suitable ³¹P-MRS ST protocol, tested it in vitro and in vivo and assessed its reproducibility. We assessed the hepatic apparent spin-lattice relaxation time of inorganic phosphate ( $T_{1,Pi}^{\text{'}}$ ), equilibrium forward rate constant ( $k_f$ ), and forward ATP synthesis rate ( $F_{ATP}$ ) in nine control volunteers (CON) (six females) and eight patients (five females) with type 1 diabetes (T1D) and compared differences by ANOVA.

Results: Reproducibility assessment in nine CON, aged 27 ± 4 years (mean ± standard deviation), yielded coefficients of variation for repeated measurements of 7.1% and 21.3% for $T_{1,Pi}^{\text{'}}$ and $k_f$ , respectively. Group comparison revealed higher hepatic $k_f$ (0.34 ± 0.03 s^-1 versus 0.16 ± 0.03 s^-1; p = 0.001) and $F_{ATP}$ (35.3 ± 3.5 mM/min versus 16.4 ± 3.5 mM/min; p = 0.002) in CON than in T1D, aged 42 ± 15 years, respectively.

Conclusion: This ³¹P-MRS ST method allowed for robust assessment of hepatic ATP synthesis at clinical field strength and was sensitive enough to detect differences between CON and T1D volunteers.

Relevance statement: Noninvasive methods to investigate hepatic energy metabolism are urgently needed to evaluate liver health while preventing unnecessary biopsies. For broad clinical applicability, the robustness shown by the proposed method at clinical field strength is crucial.

Trial registration: ClinicalTrials.gov: NCT01055093-Prospective study on diabetes mellitus and its complications in newly diagnosed adult patients (GDC), NCT01055093, Registered: 01/22/2010, https://clinicaltrials.gov/study/NCT01055093?term=NCT01055093&rank=1#study-overview .

Key points: The proposed magnetic resonance spectroscopy method calculates hepatic ATP synthesis rates at clinical field strength. The protocol shows acceptable reproducibility and spectra without contamination from muscle. The method can detect differences between participants with type 1 diabetes and controls.

Keywords: Adenosine triphosphate; Diabetes mellitus (type 1); Energy metabolism; Liver; Magnetic resonance spectroscopy.

Fig. 1

Flowchart of the study participants. $k_f$, Forward rate constant; MRS, Magnetic resonance spectroscopy

Fig. 2

Amplitude modulation of the delays alternating with nutations for tailored excitation (DANTE) saturation pulse scheme. a, b In vitro results of the calculated normalized magnetization of γ-adenosine triphosphate (γ-ATP) with varying subpulse flip angle (β) at repetition time (TR) of 0.7 s for the scheme: (i) M5D9 (5 saturation bands each 9 Hz apart); and (ii) M3D12 (3 saturation bands each 12 Hz apart). c, d In vivo results of the same DANTE pulse train schemes for two control volunteers. The dashed line symbolizes a γ-ATP saturation of 5%. Q, Spillover; Vol, Volunteer

Fig. 3

a, b Results of the saturation recovery experiment for determination of the apparent spin-lattice relaxation time of P_i ($T_{1,Pi}^{\prime }$) during active saturation of γ-adenosine triphosphate (γ-ATP) for two control individuals. $T_{1,Pi}^{\prime }$ was determined by fitting five data points (dotted line) and by the 3TR method using only data points at 0.7, 1.7, and 2.7 s (solid line). P_i, Inorganic phosphate; TR, Repetition time

Fig. 4

Determination of the apparent spin-lattice relaxation time of P_i ($T_{1,Pi}^{\prime }$) by a saturation recovery experiment in a control volunteer. Active saturation on the γ-adenosine triphosphate (γ-ATP) resonance was applied at three different TRs (0.7, 1.7, and 2.7 s) and $T_{1,Pi}^{\prime }$ was obtained by fitting the inorganic phosphate (P_i) amplitudes according to the displayed formula. The blue curve represents the relaxation behavior for $T_{1,Pi}^{\prime }$ = 520 ms. Spectra are apodized by a 10 Hz Gaussian filter. ATP, Adenosine triphosphate; PDE, Phosphodiesters; PME, Phosphomonoesters; TR, Repetition time

Fig. 5

Hepatic ³¹P-magnetic resonance spectroscopy saturation transfer (ST) spectra showing the ST effect on inorganic phosphate (P_i). The black arrow indicates the frequency where the delays alternating with nutations for tailored excitation saturation pulse scheme was applied (directly on γ-ATP), and the resulting spectrum is shown as a solid black line. The spectrum of a second experiment performed with the same saturation pulse scheme applied at the mirrored frequency is shown in blue dotted line. The effect of the γ-ATP saturation on the exchanging P_i, is displayed as ΔM. Spectra are apodized by a 5 Hz Gaussian filter. ΔM percentage difference in P_i amplitude between saturating and control irradiation. ATP, Adenosine triphosphate; PDE, Phosphodiesters; PME, Phosphomonoesters

Fig. 6

Results of the comparison of subgroups of control volunteers and type 1 diabetes (T1D) hepatic saturation transfer experiments. Comparison is shown for: (a) the apparent spin-lattice relaxation time of P_i ($T_{1,Pi}^{\prime }$); (b) the percentage difference in P_i amplitude between saturating and control irradiation (ΔM); (c) the forward rate constant ($k_f$); and (d) the forward ATP synthesis rate ($F_{ATP}$). All graphs show mean ± standard error of the mean