A broadband phased-array system for direct phosphorus and sodium metabolic MRI on a clinical scanner

Despite their proven gains in signal‐to‐noise ratio and field‐of‐view for routine clinical MRI, phased‐array detection systems are currently unavailable for nuclei other than protons (1H). A broadband phased‐array system was designed and built to convert the 1H transmitter signal to the non‐1H frequency for excitation and to convert non‐1H phased‐array MRI signals to the 1H frequency for presentation to the narrowband 1H receivers of a clinical whole‐body 1.5 T MRI system. With this system, the scanner operates at the 1H frequency, whereas phased‐array MRI occurs at the frequency of the other nucleus. Pulse sequences were developed for direct phased‐array sodium (23Na) and phosphorus (31P) MRI of high‐energy phosphates using chemical selective imaging, thereby avoiding the complex processing and reconstruction required for phased‐array magnetic resonance spectroscopy data. Flexible 4‐channel 31P and 23Na phased‐arrays were built and the entire system tested in phantom and human studies. The array produced a signal‐to‐noise ratio improvement of 20% relative to the best‐positioned single coil, but gains of 300–400% were realized in many voxels located outside the effective field‐of‐view of the single coil. Cardiac phosphorus and sodium MRI were obtained in 6–13 min with 16 and 0.5 mL resolution, respectively. Lower resolution human cardiac 23Na MRI were obtained in as little as 4 sec. The system provides a practical approach to realizing the advantages of phased‐arrays for nuclei other than 1H, and imaging metabolites directly. Magn Reson Med 43:269–277, 2000. © 2000 Wiley‐Liss, Inc.