Hyperpolarized129Xe MR Imaging of the Oral Cavity

Abstract

The demonstration of MR images using hyperpolarized cylindrical 25 cm Pyrex polarization cell of 2 cm diameter Xe has introduced a potential new contrast modality for and 7 cm length was coated with octadecyltrichlorosilane imaging (1, 2) . Recently, others obtained hyperpolarized (OTS) (11) to inhibit the loss of Xe hyperpolarization He images of the guinea pig lung (3, 4) and the human due to collisions of Xe atoms with the container walls. The lung (5) . There is considerable interest in extending the cell was fitted with high-vacuum O-ring valves and filled technique to physiological human imaging. As a step toward with 3 atm of natural xenon, 0.2 atm of nitrogen, and a small this, we present the first hyperpolarized xenon images ob(Cmg) quantity of solid rubidium. The polarization cell was tained from a human subject: an image of the human oral maintained at 857C using a resistively heated oven to achieve cavity. an optimum rubidium vapor density of 10 /cm. The polarAlthough Xe has a spin-2 nucleus and is NMR detectization cell was placed in the fringing field of our MRI magnet and illuminated for approximately 30 minutes with able, its relatively low tissue concentration (10 times less high-power (C20 W), circularly polarized light tuned to than water protons) , coupled with its low sensitivity, renders the rubidium D1 transition (between the 5S1/2 and 5 P1/2 conventional imaging approaches useless. The novel techelectronic states) at 795 nm from a multistripe GaA1As nique of noble-gas hyperpolarization, by collisional spin exdiode laser array (FWHME 1.5 nm) manufactured by Optochange with optically pumped rubidium vapor, yields up to power, Inc. The nitrogen buffer gas collisionally deexcited a hundred thousandfold enhancement in detectability (6– rubidium atoms in the excited 5P1/2 state, preventing radia9) . Hyperpolarized xenon MRI (HypX-MRI) offers a new tion trapping. range of exploitable responses and contrast, and promises temporal and spatial resolutions comparable to conventional Hyperpolarization of Xe is achieved through spin-exchange collisions with optically pumped Rb vapor. AbsorpMRI employing H2O. For imagining organs with air spaces, such as the lungs tion of the circularly polarized laser light produces a high electron spin polarization in the rubidium atoms through and sinuses, He, with its greater gyromagnetic moment, yields an MR signal that is 7.6 times larger than that of depopulation optical pumping. Subsequent gas collisions between the rubidium and Xe transfer some of this polarizaXe for a given polarization and equal number of spins, but the extremely low solubility of helium in the blood tion to the Xe nuclei. After 30 min of optical pumping and spin exchange, the Xe nuclear polarization was (10 ) precludes its use for imaging of tissue. Xenon has a solubility in blood an order of magnitude greater than that roughly 8%, resulting in an MR signal enhancement of about 20,000 times that of thermal equilibrium. The polarization of helium (10 ) . It is lipophilic, as its anesthetic properties would suggest, and achieves high concentrations in lipidcell was then removed from the oven and transported to the opening of the MRI magnet. The cell was rapidly cooled in rich tissues (10 ) . For the first tests of HypX-MRI in a human subject, we ice water and the hyperpolarized Xe was condensed from the polarization cell into an evacuated 25 cm cylindrical imaged the oral cavity of a subject inhaling hyperpolarized xenon. The oral cavity can be imaged with the relatively Pyrex condensation cell that was fitted with a high-vacuum O-ring valve and submerged in liquid nitrogen. T1 is nearly small quantities of hyperpolarized xenon that we are currently able to produce and serves the purposes of a first three hours for frozen Xe at C80 K in a magnetic field above 500 G (12) . The condensation cell was warmed to demonstration. When larger amounts of hyperpolarized xenon become available, imaging the lung gas-space, lung tisroom temperature to return the Xe to a gaseous state. The condensation/sublimation procedure resulted in about 10% sue, and various distal organs should be possible. Natural-abundance xenon gas (26% Xe) of research loss of the Xe hyperpolarization. The subjects then pressed the opening of the xenon-filled cell to their lips while lying grade purity (99.995%) was acquired from Isotec, Inc. A