Arvind K. Venkatesh

MRI of the lung gas-space at very low-field using hyperpolarized noble gases

In hyperpolarized (HP) noble-gas magnetic resonance imaging, large nuclear spin polarizations, about 100,000 times that ordinarily obtainable at thermal equilibrium, are created in 3He and 129Xe. The enhanced signal that results can be employed in high-resolution MRI studies of void spaces such as in the lungs. In HP gas MRI the signal-to-noise ratio (SNR) depends only weakly on the static magnetic field (B(0)), making very low-field (VLF) MRI possible; indeed, it is possible to contemplate portable MRI using light-weight solenoids or permanent magnets. This article reports the first in vivo VLF MR images of the lungs in humans and in rats, obtained at a field of only 15 millitesla (150 Gauss).

Evaluation of carrier agents for hyperpolarized xenon MRI

Several biocompatible carrier agents, in which xenon is highly soluble and has a long T1, were tested, and injected in living rats. These included saline, Intralipid suspension, perfluorocarbon emulsion and 129Xe gas‐filled liposomes. The T1 of 129Xe in these compounds ranged from 47 to 116 s. Vascular injection of these carrier agents was tolerated well, encouraging their use for further experiments in live animals. In vivo spectra, obtained from gas‐filled liposomes and perfluorocarbon solutions, suggest that these carrier agents have potential for use in angiography and perfusion imaging. Copyright

Hyperpolarized (129)Xe T (1) in oxygenated and deoxygenated blood

The viability of the new technique of hyperpolarized 129Xe MRI (HypX‐MRI) for imaging organs other than the lungs depends on whether the spin–lattice relaxation time, T1, of 129Xe is sufficiently long in the blood. In previous experiments by the authors, the T1 was found to be strongly dependent upon the oxygenation of the blood, with T1 increasing from about 3 s in deoxygenated samples to about 10 s in oxygenated samples. Contrarily, Tseng et al. (J. Magn. Reson. 1997; 126: 79–86) reported extremely long T1 values deduced from an indirect experiment in which hyperpolarized 129Xe was used to create a ‘blood‐foam’. They found that oxygenation decreased T1. Pivotal to their experiment is the continual and rapid exchange of hyperpolarized 129Xe between the gas phase (within blood‐foam bubbles) and the dissolved phase (in the skin of the bubbles); this necessitated a complicated analysis to extract the T1 of 129Xe in blood. In the present study, the experimental design minimizes gas exchange after the initial bolus of hyperpolarized 129Xe has been bubbled through the sample. This study confirms that oxygenation increases the T1 of 129Xe in blood, from about 4 s in freshly drawn venous blood, to about 13 s in blood oxygenated to arterial levels, and also shifts the red blood cell resonance to higher frequency. Copyright

Physiological response of rats to delivery of helium and xenon: implications for hyperpolarized noble gas imaging

The physiological effects of various hyperpolarized helium and xenon MRI‐compatible breathing protocols were investigated in 17 Sprague–Dawley rats, by continuous monitoring of blood oxygen saturation, heart rate, EKG, temperature and endotracheal pressure. The protocols included alternating breaths of pure noble gas and oxygen, continuous breaths of pure noble gas, breath‐holds of pure noble gas for varying durations, and helium breath‐holds preceded by two helium rinses. Alternate‐breath protocols up to 128 breaths caused a decrease in oxygen saturation level of less than 5% for either helium or xenon, whereas 16 continuous‐breaths caused a 31.5% ± 2.3% decrease in oxygen saturation for helium and a 30.7% ± 1.3% decrease for xenon. Breath‐hold protocols up to 25 s did not cause the oxygen saturation to fall below 90% for either of the noble gases. Oxygen saturation values below 90% are considered pathological. At 30 s of breath‐hold, the blood oxygen saturation dropped precipitously to 82% ± 0.6% for helium, and to 76.5% ± 7.4% for xenon. Breath‐holds longer than 10 s preceded by pre‐rinses caused oxygen saturation to drop below 90%. These findings demonstrate the need for standardized noble gas inhalation procedures that have been carefully tested, and for continuous physiological monitoring to ensure the safety of the subject. We find short breath‐hold and alternate‐breath protocols to be safe procedures for use in hyperpolarized noble gas MRI experiments. Copyright