Multiband Spectral-Spatial RF Excitation for Hyperpolarized [2‑13C]Dihydroxyacetone 13C-MR Metabolism Studies

Irene Marco-Rius¹, Peng Cao¹, Cornelius von Morze¹, Matthew Merritt², Karlos X Moreno³, Gene-Yuan Chang⁴, Michael A Ohliger¹, David Pearce⁴, John Kurhanewicz¹, Peder EZ Larson¹, and Daniel B Vigneron¹

¹Department of Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, CA, United States, ²Department of Biochemistry and Molecular Biology, University of Florida, Gainesville, FL, United States, ³Department of Chemistry, Engineering, Pre-Pharmacy, and Physics, South Texas College, Weslaco, TX, United States, ⁴Department of Medicine, Division of Nephrology, University of California San Francisco, San Francisco, CA, United States

Synopsis

¹³C-MR spectra of hyperpolarized [2-¹³C]dihydroxyacetone (DHAc), a new agent for imaging gluconeogenesis, was acquired using specialized acquisition methods in the rat liver and kidney in vivo. Because the resonances originating from the metabolism of [2-¹³C]DHAc have a large frequency distribution, we designed a novel spectral-spatial (SPSP), multi-band excitation pulse that corrects for chemical shift misregistration, resulting in accurate spatial-spectral selectivity. The metabolic products phosphoenolpyruvate (PEP) and glycerol 3-phosphate (G3P) were detected, evidencing metabolism of the hyperpolarized substrate towards the glycolytic pathway and activity of the enzyme glycerol 3-phosphate dehydrogenase.

Target audience

Researchers interested in hyperpolarized ¹³C metabolic imaging and spectroscopy.

Purpose

Recently, [2‑¹³C]dihydroxyacetone (DHAc) has emerged as a promising molecular imaging agent for investigation of gluconeogenesis¹. In perfused livers, [2‑¹³C]DHAc rapidly metabolized into glycerol 3-phosphate (G3P) through dihydroxyacetone phosphate (DHAP) and entered the gluconeogenic pathway at the glyceraldehyde 3-phosphate (GA3P) position¹. Subsequently, intermediates characteristic of glycolysis and gluconeogenesis, such as glucose, phosphoenolpyruvate (PEP), pyruvate and lactate were detected. Since the chemical shift range of [2‑¹³C]DHAC and its metabolic products spans 144 ppm (69-213ppm, over 4.6kHz at 3T), a MR scan with spatial selectivity as well as a broadband spectral coverage is needed to optimally detect hyperpolarized (HP) [2‑¹³C]DHAc and probe the distribution of metabolic products in different organs. This study was designed to compare the distributions of glycolytic and gluconeogenic products in liver and kidney of fasted rats, using HP [2‑¹³C]DHAc MR.

Methods

A specialized RF excitation pulse was designed for independent flip angle control over five spectral-spatial (SPSP) excitation bands corrected for chemical shift misregistration effects, leveraging the sparse nature of the spectra to achieve the desired broadband excitation as well as the independent flip angle control over multiple frequency bands. The RF pulse was designed using an in-house software package in Matlab (TheMathworks Inc., Natick, MA). Technical details of this software have been reported previously^2,3. Figure 1 shows a 15.2ms RF pulse designed for a 1-cm slab acquired in a 3T scanner, with the following resonances and their corresponding bandwidths and flip angles (FA): 213.0ppm ± 2ppm (DHAc), 0º; 151.0ppm ± 1ppm (PEP), 67.5º; 96.1ppm ± 1ppm (DHAc hydrate), 0º; 88.0ppm ± 1ppm (additional resonance), 45º; and 73.0ppm ± 3ppm (G3P), 45º. The SPSP RF pulse was tested and applied in a 3T clinical MRI system equipped with 50 mT/m, 200 mT/m/ms gradients and a broadband RF amplifier to investigate HP [2-¹³C]DHAc metabolism in five 24h-fasted Sprague Dawley rats. Dynamic ¹³C-MRS was acquired using an optimal flip angle scheme to excite sequentially an axial 1cm-slab across the liver and an axial 1cm-slab across the kidneys (acquisition parameters: TR=3 s; FA=20-30º on the metabolic products, 2º on the DHAc hydrate, and 0.3º on the substrate; receiver bandwidth=10 kHz; number of points=2 k; acquisition started 15 s after the beginning of the substrate injection). An axial 2cm-thick MRS image was also acquired using the same SPSP RF pulse (acquisition parameters: TE/TR=10/150 ms; matrix=8x8; FOV=8x8 cm²; FA=10º; temporal resolution=3 s/frame; random walk k-space trajectory; 3.8-fold undersampling and compressed sensing reconstruction).

Results and Discussion

Tests on ¹³C-phantom solutions showed that the multi-band SPSP RF pulses provided the desired tailored excitation, meeting requirements for both spatial and spectral selectivity (Figure 2). Experiments with HP [2-¹³C]DHAc in rat liver and kidney in vivo showed that the five-band SPSP RF pulse allowed for acquisition of all the resonances of interest simultaneously without chemical shift misregistration (Figure 3). Therefore, spectral acquisition could be started during injection of the hyperpolarized agent without saturating DHAc in other slabs during delivery due to misregistration. The ¹³C-spectra obtained with the SPSP RF pulse acquisition in vivo produced a high-SNR G3P peak, clearly indicating that there was 19 times more G3P in the liver than in the kidney (Table 1, Figure 3c). This is not a surprising result, since liver is the main regulator of triglyceride production. PEP was also observable, albeit with lower SNR. This study has also demonstrated the potential of these SPSP RF pulses to perform MR spectroscopic imaging. As an example, MRSI data localizing the G3P production within the liver is displayed in Figure 4, and it could be extended to 3D MRSI. This feature would allow spatial localization of the metabolic signal from within small voxels, while preserving the spectral selectivity of the resonances of interest.

Conclusion

A specialized multiband SPSP RF pulse covering a spectral range over 144 ppm enabled in vivo characterization of HP [2‑¹³C]DHAc metabolism simultaneously in rat liver and kidney. This SPSP design is flexible and with minor modifications it could be accommodated to fit the spectral profile to study other metabolites with wide spectral distribution, e.g. [2-¹³C]pyruvate and its products.

Acknowledgements

We thank Dr. Robert Bok for experimental help. This work was supported by an intramural UCSF radiology department seed grant, and NIH grants P41EB013598, P41EB015908, R21EB016197, R37HL34557 and R01EB016741. CVM was supported by NIH K01DK099451.

References

1. Moreno KX, et al. Real-time detection of hepatic gluconeogenic and glycogenolytic states using hyperpolarized [2-¹³C]dihydroxyacetone. J Biol Chem. 2014;289:35859–35867.

2. Larson PEZ, et al. Multiband excitation pulses for hyperpolarized ¹³C dynamic chemical shift imaging. J Magn Reson. 2008;194(1):121–127.

3. Kerr AB, et al. Multiband spectral-spatial design for high-field and hyperpolarized C-13 applications. In: Proc Intl Soc Mag Reson Med. Vol. 16. Toronto; 2008. p. 226.

Figures

Figure 1. Five-band spectral-spatial RF pulse. (a) RF and gradient waveforms. (b) Spectral profile at the center of the slab, and (c) spatial profile. As a reference, urea was chosen as the 0-Hz offset, eventhough it was not one of the frequency bands controlled during the design of the experiment.

Figure 2. (a) Sketch of the 2-cm-slab positions on the three syringes containing ¹³C-urea solution used as a phantom. (b) Signal intensity measurements of the SPSP RF pulse performance. FA=20º calibrated at the transmitter frequency -2895 Hz. Shaded area is the simulated signal in the slab through the phantom.

Figure 3. (a) Liver and kidney ¹³C-MRS of hyperpolarized [2-¹³C]DHAc using the SPSP RF pulse. (b) Dynamic curves of the metabolic products. (c) Ratio of the first five time points of PEP and G3P from the liver over the kidney (mean±std, n=5). (d) Anatomic reference of the 1-cm slabs.

Table 1. Integrated HP [2-¹³C]DHAc metabolite ratios (liver over kidney). Data were normalized to the DHAc hydrate integral of each organ prior to ratio calculation. The mean result is shown also in Figure 3c.

Figure 4. Axial MRSI of the liver acquired 12 s after the start of the injection of HP [2‑¹³C]DHAc in a fasted rat using the SPSP RF pulse. The liver contour is delineated in yellow, highlighting that metabolism of DHAc occurs mainly inside this organ.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)

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