Multi-band echo-planar spectroscopic imaging of hyperpolarized 13 C probes in a compact preclinical PET/MR scanner

Abstract

Purpose: Hyperpolarized (HP) ¹³ C MRI has enabled real-time imaging of specific enzyme-catalyzed metabolic reactions, but advanced pulse sequences are necessary to capture the dynamic, localized metabolic information. Herein we describe the design, implementation, and testing of a rapid and efficient HP ¹³ C pulse sequence strategy on a cryogen-free simultaneous positron emission tomography/MR molecular imaging platform with compact footprint.

Methods: We developed an echo planar spectroscopic imaging pulse sequence incorporating multi-band spectral-spatial radiofrequency (SSRF) pulses for spatially coregistered excitation of ¹³ C metabolites with differential individual flip angles. Excitation profiles were measured in phantoms, and the SSRF-echo planar spectroscopic imaging sequence was tested in rats in vivo and compared to conventional echo planar spectroscopic imaging. The new sequence was applied for 2D dynamic metabolic imaging of HP [1-¹³ C]pyruvate and its molecular analog [1-¹³ C] $\alpha$ -ketobutyrate at a spatial resolution of 5 mm × 5 mm × 20 mm and temporal resolution of 4 s. We also obtained simultaneous ¹⁸ F-fluorodeoxyglucose positron emission tomography data for comparison with HP [1-¹³ C]pyruvate data acquired during the same scan session.

Results: Measured SSRF excitation profiles corresponded well to Bloch simulations. Multi-band SSRF excitation facilitated efficient sampling of the multi-spectral kinetics of [1-¹³ C]pyruvate and [1-¹³ C] $\alpha$ - $\mathrm{ketobutyrate}$ . Whereas high pyruvate to lactate conversion was observed in liver, corresponding reduction of $\alpha$ -ketobutyrate to [1-¹³ C] $\alpha$ -hydroxybutyrate ( $\alpha$ HB) was largely restricted to the kidneys and heart, consistent with the known expression pattern of lactate dehydrogenase B.

Conclusion: Advanced ¹³ C SSRF imaging approaches are feasible on our compact positron emission tomography/MR platform, maximizing the potential of HP ¹³ C technology and facilitating direct comparison with positron emission tomography.

Keywords: dynamic nuclear polarization; metabolic imaging; spectroscopy.