(31)P-MRS of healthy human brain: ATP synthesis, metabolite concentrations, pH, and T1 relaxation times

Abstract

The conventional method for measuring brain ATP synthesis is (31)P saturation transfer (ST), a technique typically dependent on prolonged pre-saturation with γ-ATP. In this study, ATP synthesis rate in resting human brain is evaluated using EBIT (exchange kinetics by band inversion transfer), a technique based on slow recovery of γ-ATP magnetization in the absence of B1 field following co-inversion of PCr and ATP resonances with a short adiabatic pulse. The unidirectional rate constant for the Pi → γ-ATP reaction is 0.21 ± 0.04 s(-1) and the ATP synthesis rate is 9.9 ± 2.1 mmol min(-1) kg(-1) in human brain (n = 12 subjects), consistent with the results by ST. Therefore, EBIT could be a useful alternative to ST in studying brain energy metabolism in normal physiology and under pathological conditions. In addition to ATP synthesis, all detectable (31)P signals are analyzed to determine the brain concentration of phosphorus metabolites, including UDPG at around 10 ppm, a previously reported resonance in liver tissues and now confirmed in human brain. Inversion recovery measurements indicate that UDPG, like its diphosphate analogue NAD, has apparent T1 shorter than that of monophosphates (Pi, PMEs, and PDEs) but longer than that of triphosphate ATP, highlighting the significance of the (31)P-(31)P dipolar mechanism in T1 relaxation of polyphosphates. Another interesting finding is the observation of approximately 40% shorter T1 for intracellular Pi relative to extracellular Pi, attributed to the modulation by the intracellular phosphoryl exchange reaction Pi ↔ γ-ATP. The sufficiently separated intra- and extracellular Pi signals also permit the distinction of pH between intra- and extracellular environments (pH 7.0 versus pH 7.4). In summary, quantitative (31)P MRS in combination with ATP synthesis, pH, and T1 relaxation measurements may offer a promising tool to detect biochemical alterations at early stages of brain dysfunctions and diseases.

Keywords: 31P MRS; ATP; T1 relaxation time; brain metabolism; chemical exchange; inversion transfer; magnetization transfer; pH.

FIG. 1

A 7T ³¹P MR spectrum, group-averaged for 12 subjects, acquired from resting human brain using a spatially nonselective single-pulse sequence with long TR of 30 sec, before (a) and after (b) baseline correction, and the result of ³¹P peak fitting (c). The fitting residual (bottom red trace) in (c) represents the spectral subtraction (b – c). Abbreviation: PE, phosphoethenolamine; PC, phosphocholine, GPE, glycerophosphoethanolamine; GPC, glycerophosphocholine; Piⁱⁿ and Pi^ex, intra- and extracellular inorganic phosphate; PCr, phosphocreatine; ATP, adenosine triphosphate; NAD, nicotinamide adenine dinucleotide; UDPG, uridine diphosphate glucose.

FIG. 2

Brain concentration of P-metabolites (a) and Mg²⁺ (b), and intra- and extracellular pH (c), averaged for n = 12 subjects. Data quantification was based on the ³¹P peak fitting shown in Figure 1c, with P-metabolites concentrations evaluated by the integral of ³¹P peaks, [Mg²⁺] by chemical shift difference between α- and β-ATP, and pH by the chemical shift difference between Pi and PCr.

FIG. 3

A typical ³¹P MR spectral series acquired from resting human brain at 7 T using a non-selective inversion-recovery sequence at TR 30 sec, NA = 6 and varying inversion delay time t. The insets showed the regional spectra enlarged for clear view of ³¹P signals of low-intensity in the chemical shift regions of 4.5 – 5.5 ppm (left) and −6.5 – −10.5 ppm (right); the spectra were colored coded from blue to red to indicate the gradual increase of the inversion delay time t. For comparison, the last trace of the spectral series represents the fully relaxed brain ³¹P MR spectrum acquired at TR 30 sec without applying the inversion pulse.

FIG. 4

(a) Plots of normalized ³¹P magnetization against the inversion delay time t for the evaluation of apparent T₁ relaxation time of brain metabolites. The data were from the signal intensity measurements in the inversion-recovery ³¹P spectra shown in Figure 3, and the solid curves represent the fitting based a mono-exponential process. Note the rapid relaxation of ATP, NAD and UDPG (P-metabolites with two or more coupling phosphate groups) as compared to PME, PDE, Pi and PCr (P-metabolites with a single phosphate group). (b) Excitation profile of the ³¹P spectrum showing the dependence of signal intensity on transmitter offset frequency. The excitation profile was used for correction of metabolite concentration based on signal intensity.

FIG. 5

A typical ³¹P MR spectral series acquired with EBIT sequence using a band inversion pulse selectively inverting PCr and ATP spins, and followed by a varying delay time t All data are shown with identical y-scale. The insets show the regional spectra in the chemical shift region of 2 – 8 ppm, enlarged for a clear view of magnetization transfer effect at intracellular inorganic phosphate (Piⁱⁿ) from γ-ATP due to inversion delay (green trace: inversion delay time t = 3 ms; red trace: t = 3.2 sec; and black trace: fully relaxed spectrum). For comparison, the last trace of the spectral series represents the fully relaxed brain ³¹P MR spectrum acquired at TR 30 sec without applying the inversion pulse.

FIG. 6

(a) Comparison of intra- and extracellular Pi signal changes in response to increase in the inversion recovery delay time t. All data are shown with identical y-scale, and taken from the EBIT spectra shown in Figure 5. (b) Normalized Z-magnetizations versus delay time t for extracellular Pi (top) and intracellular Pi and γ-ATP (bottom, averaged for 12 subjects). The solid curves in the plots represent the data fitting using a bi-exponential equation (Eq. [7]) for Pi and a mono-exponential equation for γ-ATP.