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

PURPOSE

Hyperpolarized (HP) 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 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 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- C]pyruvate and its molecular analog [1- C] -ketobutyrate at a spatial resolution of 5 mm × 5 mm × 20 mm and temporal resolution of 4 s. We also obtained simultaneous F-fluorodeoxyglucose positron emission tomography data for comparison with HP [1- 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- C]pyruvate and [1- C] - . Whereas high pyruvate to lactate conversion was observed in liver, corresponding reduction of -ketobutyrate to [1- C] -hydroxybutyrate ( HB) was largely restricted to the kidneys and heart, consistent with the known expression pattern of lactate dehydrogenase B.

CONCLUSION

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