Introduction

Hyperpolarized ¹³C MR imaging and spectroscopy have been used to investigate changes in metabolism in the heart related to various diseases and physiological perturbations in pre-clinical models and has shown tremendous clinical potential¹. The feasibility of acquiring hyperpolarized ¹³C images from human hearts following injections of HP [1-¹³C]pyruvate solution has been recently demonstrated². Following the rapid multi-slice ¹³C imaging data acquisition, ¹³C spectroscopic data were acquired from the whole heart to detect any residual hyperpolarized magnetization. The spectroscopic data obtained from this add-on acquisition provided complementary information that helps to interpret the imaging results and to design acquisition methods in future studies.

Methods

Subjects: Healthy subjects (N=5) were recruited and gave written informed consent under a protocol approved by the institutional Research Ethics Board and approved by Health Canada as a Clinical Trial Application. An oral carbohydrate load (35g of Gatorade^TM powder in water, containing 34g of sucrose and dextrose) was administered approximately 1 hour before the pyruvate injection. Hardware and sample preparation: Each subject was positioned supine and feet first within a ¹³C volume transmit coil system (GE Healthcare) installed on a GE MR750 3.0 Tesla MRI scanner (GE Healthcare). The ¹³C receiver coil system consisted of two separate paddles each containing four receiver elements. One paddle was positioned on the anterior chest wall over the heart, with the other paddle under the upper left back. Hyperpolarized [1-¹³C] pyruvate solution was prepared using a GE SPINLab^TM system equipped with the Quality Control (QC) module. After sample dissolution, the QC parameters were evaluated by the study pharmacist to ensure the parameters were within specifications. Upon release, the dose syringe was rapidly loaded onto a Spectris Solaris power injector (Medrad) and a 0.43 cc/kg dose of the ~250mM pyruvate solution was injected at 5mL/s followed by a 25mL saline flush at 5mL/s. ¹³C MRS acquisitions: The ¹³C imaging data acquisition was initiated at the end of the saline flush and required 18 cardiac cycles to complete (14 - 21s depending on heart rate). Following the ¹³C imaging^2,3, the residual hyperpolarized magnetization was used to acquire MR spectroscopic data from the whole heart. For the first subject, a slice-selective pulse was used (nominal flip angle = 30°, 10 cm axial slab covering the heart) and the acquisition was not cardiac gated (TR = 3s). For the four subsequent subjects, a 200µs non-selective RF pulse was used (nominal flip angle = 18°) and the acquisition was cardiac gated such that the TRs were approximately 3s (3 to 4 inter-beat intervals, depending on heart rate). The shorter RF pulse was intended to excite the ¹³CO₂ resonance along with the other metabolites.

Results and Discussion

For the 5 subjects, the average starting time of the ¹³C MRS acquisition was 56s after the start of hyperpolarized ¹³C pyruvate injection. Despite the long delay time and the consumption of most of the pre-polarized magnetization by the preceding multi-slice ¹³C imaging acquisition, ¹³C pyruvate, lactate, alanine and bicarbonate were observed in all subjects. ¹³CO₂ was also detected in two of the datasets acquired using the non-selective pulse from the first 4-5 spectra (Fig. 1) and intracellular pH values estimated from these data sets were 7.1 and 7.3. The lactate and bicarbonate to pyruvate ratios are shown in Fig. 2. For the 4 datasets acquired using the non-selective pulse (subjects 2-5), the metabolite / pyruvate ratios were similar over the time period that the peaks had adequate SNR (> 10). Subject 1 had significantly higher metabolite / pyruvate ratios, likely because the slice selective pulse excited less of the pyruvate in the cardiac chambers compared with the non-selective pulse. The average linewidth from the 5 datasets for ¹³C pyruvate, lactate, and bicarbonate were 20.1Hz, 25.0Hz, and 19.2Hz, respectively. The broad linewidths were likely due to the data being acquired from the whole heart, and the larger linewidth for ¹³C lactate may also be associated with the observation from the imaging. The bicarbonate-to-pyruvate ratios remained relatively stable during the time points with sufficient SNR for quantification, ranging from 20 to 70 seconds. In contrast, the lactate-to-pyruvate ratio increased over a similar time frame for all subjects (Fig. 2).

Conclusion

¹³C MRS data acquired from the whole heart in normal human subjects after injection of hyperpolarized [1-¹³C]pyruvate and multi-slice ¹³C imaging provided additional information not available in the imaging data. The inter-subject consistency in the observed metabolite ratios from these spectra demonstrated the potential for changes in cardiac metabolism due to disease to be interrogated with simple whole-heart spectroscopy.

Acknowledgements

The authors are grateful to Tracey Rideout, Sergio DeFigueiredo and Stephanie Vidotto for pharmacy technician support for this study, to Julie Green for coordinating the study, and to Ruby Endre and Garry Detzler for MR technician support.

References

1. Schroeder MA et al. Circ. 2011;124:1580-94. 2. Cunningham CH et al. Circ Res. 2016;Epub. 3. Lau AZ et al. MRM. 2010;64:1323-31.