Investig Magn Reson Imaging.  2016 Mar;20(1):53-60. 10.13104/imri.2016.20.1.53.

Alternating Acquisition Technique for Quantification of in vitro Hyperpolarized [1-13C] Pyruvate Metabolism

• Yang S
• Lee J
• Joe E
• Lee H
• Song HT
• Kim DH

• KMID: 2161370
• DOI: http://doi.org/10.13104/imri.2016.20.1.53

Abstract

PURPOSE
To develop a technique for quantifying the 13C-metabolites by performing frequency-selective hyperpolarized 13C magnetic resonance spectroscopy (MRS) in vitro which combines simple spectrally-selective excitation with spectrally interleaved acquisition.
METHODS
Numerical simulations were performed with varying noise level and K(p) values to compare the quantification accuracies of the proposed and the conventional methods. For in vitro experiments, a spectrally-selective excitation scheme was enabled by narrow-band radiofrequency (RF) excitation pulse implemented into a free-induction decay chemical shift imaging (FIDCSI) sequence. Experiments with LDH / NADH enzyme mixture were performed to validate the effectiveness of the proposed acquisition method. Also, a modified two-site exchange model was formulated for metabolism kinetics quantification with the proposed method.
RESULTS
From the simulation results, significant increase of the lactate peak signal to noise ratio (PSNR) was observed. Also, the quantified K(p) value from the dynamic curves were more accurate in the case of the proposed acquisition method compared to the conventional non-selective excitation scheme. In vitro experiment results were in good agreement with the simulation results, also displaying increased PSNR for lactate. Fitting results using the modified two-site exchange model also showed expected results in agreement with the simulations.
CONCLUSION
A method for accurate quantification of hyperpolarized pyruvate and the downstream product focused on in vitro experiment was described. By using a narrow-band RF excitation pulse with alternating acquisition, different resonances were selectively excited with a different flip angle for increased PSNR while the hyperpolarized magnetization of the substrate can be minimally perturbed with a low flip angle. Baseline signals from neighboring resonances can be effectively suppressed to accurately quantify the metabolism kinetics.

Keyword

Hyperpolarized; 13C; Metabolic imaging; in vitro; Spectroscopy

MeSH Terms

Kinetics
Lactic Acid
Magnetic Resonance Imaging
Magnetic Resonance Spectroscopy
Metabolism*
NAD
Noise
Pyruvic Acid*
Signal-To-Noise Ratio
Spectrum Analysis
Lactic Acid
NAD
Pyruvic Acid

Figure

• 

  Fig. 1 Simplified pulse sequence diagram for the proposed alternating acquisition mode. Narrow-band frequency-selective excitation RF pulses were implemented into a conventional free-induction decay chemical shift imaging (FID-CSI) sequence to allow spectrally-selective excitation and acquisition of lactate and pyruvate between every TR in an interleaved manner.

• 

  Fig. 2 Simulation results comparing the conventional, non-selective acquisition (a) with the proposed method (b). Time-averaged spectra (top row) show that signal peaks from lactate and pyruvate can be completely separated while also increasing the lactate peak SNR (PSNR) proportional to the flip angle used in the proposed interleaved acquisition scheme. Bottom row displays peak signal dynamics of lactate and pyruvate at every TR.

• 

  Fig. 3 Comparison of error maps showing estimation error from calculating the apparent conversion rate constant values using the conventional (a) and the proposed method (b). Error values are significantly reduced (maximum error at ~23.0% with the proposed method, down from 40.0% error using the conventional method) over all noise and Kp value ranges.

• 

  Fig. 4 In vitro experiment results comparing the averaged acquired spectra and signal dynamics using the conventional (a) and the proposed method (b). Signals from pyruvate and lactate are completely separated in the case of the proposed method, with increased lactate PSNR.

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