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 phasedarray 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. (C) 2000 Wiley-Liss, Inc.