Accurate flip-angle calibration for 13C MRI

Abstract

13C imaging and spectroscopy in the presence of injected labeled compounds can vastly extend the capability of MRI to perform metabolic imaging. The details of imaging in the presence of injected compounds, however, pose new technological challenges. Pulse sequences, in general, rely on precise flip-angle (FA) calibration to create high signal-to-noise ratio (SNR), artifact-free images. Signal quantification also requires precise knowledge of the excitation FA. In MRI scans that rely on signal acquisitions from injected compounds, however, such FA calibration is challenged by low natural-abundance 13C signal levels before injection, and by time-varying signal following injection. A method to precisely set the FA at the 13C frequency based on FA calibration at the 23Na frequency is presented here. A practical implementation of a coil (a dual-tuned, 23Na/ 13C low-pass birdcage coil) suitable for this calibration in vivo is also documented. Accurate FA calibration is demonstrated at the 13C frequency for in vivo rat experiments using this approach. © 2007 Wiley-Liss, Inc.