Enhanced 129Xe T 1 relaxation in whole blood and in the presence of SPIONs at low magnetic field strengths
Nicholas Bryden, Sebastian W. Atalla, Michele Kelley, Leah R. Holmes, Rosa T. Branca
Abstract
Purpose
To compare the effect of superparamagnetic iron oxide nanoparticles (SPIONs) on the T 1 of 129Xe and 1H and to measure the relaxation of 129Xe in blood at low and high magnetic field strengths.
Methods
129Xe and 1H T 1 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 129Xe 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 1H, increasing from 0.92 ± 0.06 mM s−1 at 11.7T to 31.5 ± 1.8 mM s−1 at 0.6 mT, and for 129Xe, which increased from 0.13 ± 0.03 mM s−1 at 11.7T to 7.32 ± 0.71 mM s−1 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 1H and from −0.7 ± 3.4% at 9.4T to 12.7 ± 3.5% at 2.1 mT for 129Xe. Blood was found to depolarize 129Xe below 3T in a manner inversely proportional to the field strength. In vitro studies at 2.1 mT suggest 129Xe relaxation times below 5 s in blood dilutions as low as 0.4% volume.
Conclusion
SPIONs longitudinal relaxivity increases at low field both for 1H and 129Xe. 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.