Multi‐band echo‐planar spectroscopic imaging of hyperpolarized ¹³C 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] -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 multispectral kinetics of [1-13 C]pyruvate and [1-13 C] -ketobutyrate. Whereas high pyruvate to lactate conversion was observed in liver, corresponding reduction of -ketobutyrate to [1-13 C] -hydroxybutyrate ( 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

Detection of early-stage NASH using non-invasive hyperpolarized 13C metabolic imaging

“…At the time of the HP experiment, each HP sample was dissolved with 18 mL superheated water and neutralized using NaOH/Tris buffer. 2.4 mL contrast agent was injected into the rat tail vein over 10 s. HP 13 C images through rat abdomen were acquired using a previously described flyback echo planar spectroscopic imaging (EPSI) pulse sequence 53 , with spatial resolution of 5 mm × 5 mm × 20 mm and temporal resolution of 4 s. Multi-band spectral-spatial RF excitation was used to apply a smaller flip angle of 5 on the primary substrate [1- 13 C] KB and 12 on [1- 13 C] HB, conserving the magnetization of the primary substrate for subsequent metabolic conversion 54 . Metabolite data was quantified by regional integration and summation over time to produce metabolite AUC ratios.…”