A variable field strength system for hyperpolarized noble gas MR imaging of rodent lungs

Hyperpolarized Noble Gas (HNG), He-3 (helium) or Xe-129 (xenon), MR imaging has become a promising approach for visualizing lung anatomy and function. It has been theoretically predicted that low field strengths (0.05-0.2 T) may provide optimal signal-to-noise ratio (SNR) and spatial resolution for clinical HNG MR imaging. These optimum field strengths correspond to frequencies between 1.62-6.5 MHz and 0.59-2.35 MHz for He-3 and Xe-129 respectively. The optimum field strength depends on the size and geometry of the sample and radiofrequency (RF) coil as well as the field dependence of the HNG MR properties in the lung (e.g., relaxation times, susceptibility effects). In particular, little is known about the field dependence of the apparent transverse relaxation time, T-2*, which is expected to strongly influence HNG signal. In this paper, a broadband (0.1-100 MHz) variable field strength MR imaging system for rodents is described. A custom-built resistive magnet was constructed and shimmed to provide the necessary homogeneity for imaging. RF coils and transmit/receive switches for different frequencies were also developed. Preliminary proton (H-1) and hyperpolarized Xe-129 images of test objects are presented, including a desiccated lung phantom. In Vivo Xe-129 signals from rat lungs were acquired at 73.5 mT and T-2* was estimated to be approximately 80 +/- 8 ms, in good agreement with previously reported values. The MR system developed should be useful for imaging rodent lungs at different field strengths to verify the expected SNR and spatial resolution possible using HNG imaging and to investigate long range diffusion and oxygen-induced transverse relaxation. (C) 2008 Wiley Periodicals, Inc.