NMR of Laser-Polarized Xe in Blood Foam

Laser-polarized Xe dissolved in a foam preparation of fresh in blood found 4.5 { 1 s in red blood cells and 9 { 2 s in human blood was investigated. The NMR signal of Xe dissolved plasma (18) . This experiment used thermally polarized Xe in blood was enhanced by creating a foam in which the dissolved and therefore required many hours of signal averaging. PoXe exchanged with a large reservoir of gaseous laser-polarized tential systematic problems with such long measurement Xe. The dissolved Xe T1 in this system was found to be signifitimes include sedimentation and degradation of the blood. cantly shorter in oxygenated blood than in deoxygenated blood. In their recent demonstration of an injectable polarized The T1 of Xe dissolved in oxygenated blood foam was found to Xe agent, Bifone et al. (19) found a 5 s Xe T1 in blood be approximately 21 ({5) s, and in deoxygenated blood foam to using an open-air sample tube, which may have affected the be greater than 40 s. To understand the oxygenation trend, T1 value obtained. In the reported in vivo rodent investigations measurements were also made on plasma and hemoglobin foam of polarized Xe dissolved in blood and other tissues preparations. The measurement technique using a foam gas–liquid exchange interface may also be useful for studying foam coars(12, 13, 15) , only effective polarization lifetimes were deening and other liquid physical properties. q 1997 Academic Press termined because of the Xe exchange with the lung gas space and other tissues. In contrast, the measurement reported here exploits the Xe gas–blood phase exchange in a closed in vitro system. The surface area for exchange was INTRODUCTION delibrately increased by shaking the blood–gas sample to create a foam. Because the gas phase Xe has a longer T1 Of the stable noble gas isotopes, only He and Xe have and greater density than the blood phase Xe, the gas phase spin-2 nuclei which make them suitable for MRI. Optical served as a source of Xe magnetization. Thus, the foam pumping techniques (1–3) allow the spin polarization of system increased the amplitude and lengthened the observed these noble-gas nuclei far above thermal levels. Such large lifetime of the NMR signal from Xe dissolved in the blood. nuclear polarizations greatly enhance the NMR sensitivity The Xe exchange between the gas and blood phases was of Xe and He, enabling a variety of applications in the accounted for in the analysis of the observed decaying Xe physical sciences (4, 5) , as well as in biomedical NMR NMR signals. Because of the dependence of the exchange (6, 7) . Only Xe is practical for tissue MRI because its on the surface area, the Xe signal may also be an indicator solubility in tissue is much larger than that of He (8) . of foam-structure evolution such as coarsening or drainage. Laser-polarized He (9–11) and Xe (12–14) magnetic resonance images of animal and human lung gas spaces have THEORY been reported. In vivo Xe tissue resonances have been observed in live mice (13) , live rats (12, 15) , and most recently in the human brain (14) . The rodent resonances had Let the Xe magnetization in the gas phase be labeled G , and that dissolved in the blood phase B . Here and decay times of Ç10–50 seconds. A chemical-shift image of inhaled, laser-polarized Xe in a live rat brain has also been throughout this report, we use the term ‘‘magnetization’’ to describe the ensemble magnetic moment—an extensive obtained (16) . For laser-polarized Xe NMR to be a useful tool for variable. The Xe NMR resonance frequencies of these phases are observed to be approximately 200 ppm apart studies of perfused tissue, the Xe spin polarization (or longitudinal) relaxation time, T1 , in arterial blood should be which makes both separately measurable. The total number of xenon atoms is conserved in a closed system. The greater than the time it takes for inhaled Xe to be transported to a particular tissue (typically 5–10 s in humans). Xe magnetization in each phase can change because of (a ) exchange between the phases, (b ) RF interrogation, or The dependence of the concentration of polarized Xe in various tissues on the T1 in blood has been modeled in Ref. (c ) T1 relaxation of the spin polarization back to thermal equilibrium. (17) . A previous in vitro measurement of the T1 of Xe