Exchange kinetics by inversion transfer: integrated analysis of the phosphorus metabolite kinetic exchanges in resting human skeletal muscle at 7 T

Abstract

Purpose: To develop an inversion pulse-based, chemical exchange saturation transfer-like method for detection of (31) P magnetization exchanges among all nuclear magnetic resonance visible metabolites suitable for providing an integrated kinetic analysis of phosphorus exchange reactions in vivo.

Methods: The exchange kinetics by inversion transfer (EKIT) sequence includes application of a frequency-selective inversion pulse arrayed over the range of relevant (31) P frequencies, followed by a constant delay and a hard readout pulse. A series of EKIT spectra, each given by a plot of Z-magnetization for each metabolite of interest versus frequency of the inversion pulse, can be generated from this single data set.

Results: EKIT spectra reflect chemical exchange due to known biochemical reactions, cross-relaxation effects, and relayed magnetization transfers due to both processes. The rate constants derived from EKIT data collected on resting human skeletal muscle were: ATP synthesis via ATP synthase (0.050 ± 0.016 s(-1) ), ATP synthesis via creatine kinase (0.264 ± 0.023 s(-1) ), and cross-relaxation between neighboring spin pairs within ATP (0.164 ± 0.022 s(-1) ).

Conclusion: EKIT provides a simple, alternative method to detect chemical exchange, cross relaxation, and relayed magnetization transfer effects in human skeletal muscle at 7 T.

Keywords: T1 relaxation time; chemical exchange; magnetization transfer; nuclear Overhauser effects; skeletal muscle.

FIG. 1

EKIT pulse sequence and ³¹P magnetization exchange pathways. a: The EKIT pulse sequence consists of a low-power, frequency-sweep inversion pulse, followed by a constant delay period, t_d, and a hard readout pulse. b: ³¹P magnetization exchange system in skeletal muscle, showing the various exchange pathways that are possible in skeletal muscle, including phosphorous chemical exchange reactions indicated by vertical arrows, and Nuclear Overhauser Effect (NOE) indicated by horizontal arrows. ATP and ADP are marked by rectangular boxes. A five-pool model including Pi, PCr and ATP but not ADP, was adopted in this work. Abbreviations: AK, adenylate kinase; CK, creatine kinase.

FIG. 2

EKIT spectrum of human calf skeletal muscle. a: A typical T2w MR image acquired from human calf using a partial volume coil. Abbreviations: Sol, soleus; GM, gastrocnemius medial; GL, gastrocnemius lateral. b: The EKIT spectrum illustrated here reflects the intensity of PCr signal plotted as a function of frequency of the inversion pulse (150 different frequencies) acquired from calf of a 25 yr healthy female with a BMI = 22.2 kg/m². Other ³¹P MRS acquisition parameters: TR 5 second, t_d 1 second, scan time 12.5 min. Note that PCr magnetization is attenuated after inversion of either γ ATP (white arrow) or β ATP (insert). c: A conventional ³¹P MR spectrum acquired showing the full chemical shift region of calf muscle ³¹P resonances, with the chemical shift aligned with that of sweeping inversion pulse used in the EKIT spectrum b.

FIG. 3

3D and 2D display of EKIT spectra of resting human calf muscle. a: 3D EKIT spectrum plotted by arraying the entire ³¹P MR spectra acquired using a series of offset inversion frequencies (PCr as reference at 0 ppm), with an enlarged region (inset) showing EKIT exchange peaks among the ATP spins (exchange effects marked by arrows with solid head, on-resonance inversion signals marked by arrows with thin lines). b: 2D contour plot showing ATP cross-relaxation (NOE) generated from subtraction spectra with off-resonance inversion as baseline. Note that the NAD peak which lies along the diagonal of this plot shows no exchange effect with ATP in resting skeletal muscle.

FIG. 4

EKIT spectra of resting human calf muscle. a: A conventional ³¹P MR spectrum and (b-f) EKIT spectra with observation resonance at b: Pi, c: PCr, d: γ-ATP, e: α-ATP, and f: β-ATP acquired from resting human calf muscle at t_d = 1 s. These spectra represent averages from 7 subjects. In some spectra, small exchange peaks were enlarged 3-fold.

FIG. 5

EKIT spectra and data fitting. EKIT spectra for observation of a: Pi, b: PCr, c: γ-ATP, d: α-ATP, and e: β-ATP at inversion delays of 0, 1, 2, and 3 seconds. The EKIT data were acquired from resting human calf muscle averaged for 7 subjects. The major chemical exchange reactions are indicated by solid black arrows while the effects of relayed MT are marked by the hollow arrows. The smaller EKIT exchange peaks were vertically enlarged (x4). The solid curves represent the least-squares fitting of all data to a five-pool kinetic exchange model shown in FIG. 1b.

FIG. 6

Plots of normalized EKIT peak intensity versus delay time t_d for several exchange processes. a: plots of M_z/M_z ^max after an on-resonance inversion pulse for several metabolites, b: for chemical exchanges mediated by CK and ATPase, c: for NOE within ATP, and d: for relayed MT effects. These data are identical to those shown in FIG. 5 with the solid lines showing the best fit of the data to the exchange model.