Enhanced Xe T relaxation in whole blood and in the presence of SPIONs at low magnetic field strengths.

Purpose: To compare the effect of superparamagnetic iron oxide nanoparticles (SPIONs) on the T of Xe and H and to measure the relaxation of Xe in blood at low and high magnetic field strengths.

Methods: Xe and H T relaxometry was performed at low- and high-field strengths in samples containing different SPION concentrations, while imaging was used to compare the contrast obtainable in these two field regimes. In vivo experiments at variable field strengths were performed to determine the depolarization of Xe in blood and the feasibility of in vivo dissolved-phase spectroscopy and imaging at low field.

Results: The SPION relaxivity was substantially greater at low field for H, increasing from 0.92 ± 0.06 mM s at 11.7T to 31.5 ± 1.8 mM s at 0.6 mT, and for Xe, which increased from 0.13 ± 0.03 mM s at 11.7T to 7.32 ± 0.71 mM s at 2.1 mT. The additional MR signal loss increased from 0.7% at 9.4T to 20.6 ± 4.2% at 0.6 mT for H and from -0.7 ± 3.4% at 9.4T to 12.7 ± 3.5% at 2.1 mT for Xe. Blood was found to depolarize Xe below 3T in a manner inversely proportional to the field strength. In vitro studies at 2.1 mT suggest Xe relaxation times below 5 s in blood dilutions as low as 0.4% volume.

Conclusion: SPIONs longitudinal relaxivity increases at low field both for H and Xe. The depolarization of xenon in blood, which is found to increase below 3T, effectively prevents in vivo dissolved-phase spectroscopy and imaging at low-field strengths.