Synopsis

We present an indirect ¹H-[¹³C] MRS detection method (selective proton-observed, carbon edited (selPOCE) that can be used in the frontal lobe of human brain, allows for separation of overlapping glutamate (Glu) and glutamine (Gln) peaks and provides well-defined spatial localization of the ¹³C-signal. The method was successfully implemented at 4 Tesla and allowed for the selective detection of ¹³C-labeling in Glu and Gln in a timely manner (15 min), indicating that the selPOCE can be used for quantitative studies of brain energy metabolism and neurotransmission in a relatively small volume localized in the human frontal lobe.

Purpose

¹³C MRS in the frontal lobe of human brain during infusion of ¹³C-labeled substrates requires acquisition methods characterized by a low specific absorption rate (SAR). Avoiding high radiofrequency (RF) power deposition is critical for frontal lobe MRS applications because of the close proximity of the eyes. So far ¹³C MRS studies of frontal brain have relied on pulse-acquire-based sequences and ¹³C-labeling strategies that enrich the carboxylic carbons of brain metabolites (1,2). Because signals of natural abundance lipids from the scalp do not overlap with the peaks of interest detection of carboxylic carbons can be achieved without localization, thereby reducing the number of required RF pulses. Here we present an alternative indirect ¹H-[¹³C] MRS detection method that can be used in the frontal lobe, allows for separation of overlapping glutamate (Glu) and glutamine (Gln) peaks, yet also provides well-defined spatial localization of the ¹³C-signal.

Methods

MRS. All MRS experiments were carried out on a 4 Tesla whole-body magnet interfaced to a Bruker Avance III HD spectrometer, running on Paravision 6 (Bruker Instruments, Billerica, MA, USA). The RF coil setup consisted of two circular surface coils positioned 10 mm apart in parallel planes: an outer ¹³C transmit only coil (∅ 135 mm) and an inner ¹H transmit/receive coil (∅ 95 mm). MRS data acquisition relied on a selective proton-observed, carbon edited (selPOCE) pulse sequence. The selPOCE method is a combination between POCE based on stimulated echoes (3) and 2D HSQC (4). The conventional two-step POCE experiment is extended with two additional experiments that modulate the phase of two target signals (e.g. H4-Glu and H4-Gln), by utilizing the chemical shift difference on the ¹³C channel (e.g. 107 Hz between C4-Glu and C4-Gln at 4 Tesla). A similar approach was previously described for a HMQC-based variant on rat brain (5).

[1-¹³C]-Glucose infusion. A primed, variable infusion of [1-¹³C]-glucose (1.1M, 99% enriched, Cambridge Isotopes, Cambridge, MA, USA) was performed to rapidly reach plasma glucose concentrations of 10 mM. During the duration of the infusion (90 to 120 min), the plasma glucose level was allowed to gradually drop to ~7 mM.

Results

SAR. RF power output was measured at the RF amplifier and corrected for hardware losses. Average effective RF power output was 0.51 W for the ¹H channel and 0.31 W for the ¹³C channel. Using the B₁⁺ profiles of the RF coils for an estimation of the head volume affected by the B₁⁺ field, we estimate to be significantly below the FDA guideline for average SAR (3W/kg).

MRS. The selectivity of the pulse sequence for specific resonances is illustrated by data (Fig. 1) acquired in a phantom containing a mix of unlabeled and ¹³C-labeled acetate and lactate. The conventional POCE experiment (Fig. 1E and F) separates ¹³C-labeled from unlabeled resonances. The additional two scans of the selPOCE experiment modulate the phase of ¹H signals, thereby allowing [3-¹³C]-lactate and [2-¹³C]-acetate to be separated based on their ¹³C chemical shift difference (Fig. 1G and H). Fig. 2 shows how this principle can be applied for the detection of [4-¹³C]-Glu-H4 and [4-¹³C]-Gln-H4, for which the difference in ¹H chemical shift is only 0.11ppm (18.8 Hz at 4 Tesla). The 107 Hz difference on the ¹³C axis, allows robust separation of Glu and Gln H4 resonances. The faster in vivo T₂* relaxation make the Glu and Gln overlap even more problematic as shown in the ¹H MR spectrum acquired in the frontal lobe of a healthy volunteer (Fig. 4A). SelPOCE spectra acquired from a voxel (45 mL, Fig. 3) located in the frontal lobe during infusion of [1-¹³C]-glucose are shown in Fig. 4. Similar to the results obtained in the Glu-Gln phantom, combining individual in vivo selPOCE MR spectra generates difference spectra of all ¹³C-bound protons (Fig. 4B), as well as individual MR spectra of [4-¹³C]-Glu-H4 (Fig 4D) and [4-¹³C]-Gln-H4 (Fig 4E), respectively.

Discussion & Conclusion

The presented data illustrate the feasibility of a localized, ¹H MRS-based ¹³C detection method that can be used to detect ¹³C-labeling of brain metabolites in the frontal lobe of human brain. The method was successfully implemented at 4 Tesla and allowed for the selective detection of ¹³C-labeling in Glu and Gln in a timely manner (15 min). In conclusion, selPOCE can be used for quantitative studies of brain energy metabolism and neurotransmission in a relatively small volume localized in the human frontal lobe.

Acknowledgements

References

1. Li S, Zhang Y, Wang S, Yang J, Ferraris Araneta M, Farris A, Johnson C, Fox S, Innis R, Shen J. In vivo 13C magnetic resonance spectroscopy of human brain on a clinical 3 T scanner using [2-13C]glucose infusion and low-power stochastic decoupling. Magn. Reson. Med. 2009;62:565–573. doi: 10.1002/mrm.22044.

2. Li S, Zhang Y, Wang S, Araneta MF, Johnson CS, Xiang Y, Innis RB, Shen J. 13C MRS of occipital and frontal lobes at 3 T using a volume coil for stochastic proton decoupling. NMR Biomed. 2010;23:977–985. doi: 10.1002/nbm.1524.

3. Pfeuffer J, Tkác I, Choi I-Y, Merkle H, Ugurbil K, Garwood M, Gruetter R. Localized in vivo 1H NMR detection of neurotransmitter labeling in rat brain during infusion of [1-13C] D-glucose. Magn. Reson. Med. 1999;41:1077–1083. doi: 10.1002/(SICI)1522-2594(199906)41:6<1077::AID-MRM1>3.0.CO;2-#.

4. de Graaf RA, De Feyter HM, Rothman DL. High-sensitivity, broadband-decoupled 13C MR spectroscopy in humans at 7T using two-dimensional heteronuclear single-quantum coherence. Magn. Reson. Med. 2014:n/a-n/a. doi: 10.1002/mrm.25470.

5. Henry P-G, Roussel R, Vaufrey F, Dautry C, Bloch G. Semiselective POCE NMR spectroscopy. Magn. Reson. Med. 2000;44:395–400. doi: 10.1002/1522-2594(200009)44:3<395::AID-MRM9>3.0.CO;2-5.