MRI of laser-polarized liquid xenon

We demonstrated magnetic resonance imaging of laser-polarized liquid xenon (see Fig. 1) and imaged exchange between the liquid and vapor phases (see Figs. 2 and 3). Xenon is unique among the noble gases, remaining liquid near room temperature: its liquid-vapor critical point is at ~ 290 K and 58 atm, and its solid-liquid-vapor triple point is at ~ 161 K and 0.8 atm. Xenon is also nearly inert chemically, but because of the xenon atom's large electric polarizability the liquid is an excellent solvent-particularly for aliphatic materials. Thus liquid xenon approaches the behavior of an ideal, inert, NMR-detectable solvent. The exceptionally large magnetization density of laser-polarized liquid xenon should allow MRI with micron-scale spatial resolution, the highest resolution achieved with MRI in any system. Applications may include: imaging of density equilibration and convective flow near xenon's liquid-vapor critical point; enhancement of the nuclear spin polarization of molecules dissolved in liquid xenon (e.g., to aid NMR molecular spectroscopy and quantum computing); and mapping of the dynamics of two-phase (liquid-gas) flows-convection, interfacial shear, etc.

FIGURE 1. (a) Magnetic resonance image of a laser polarized liquid xenon drop at 166 K in the corner of a tilted Pyrex cell. Image resolution is 195 x 195 microns (limited by 7 G/cm magnetic field gradients provided by the MRI instrument). (b) Complimentary image of laser polarized xenon vapor above the liquid drop. Image resolution is 860 x 860 microns. Note the physically displaced "ghost" image of the liquid drop due to rf-excited vapor atoms condensing into the liquid phase before signal acquisition. The bottom of the gas image distorts slightly due to the magnetic susceptibility difference between the gas and liquid, as well as the large dipolar magnetic field created by the polarized liquid xenon drop. The xenon images were acquired using fast low-flip-angle-excitation gradient echo sequences with chemical-shift but non-slice-selective rf pulses. The rf pulse flip angle was nominally 12 degrees. For the liquid image, a data matrix of 128 x 64 points was acquired, the field of view (FOV) was 25 x 12.5 mm, the echo time (TE) was 15.5 ms, and the scan repetition time (TR) was 150 ms. For the vapor image, the data matrix was 128 x 128, FOV = 110 x 110 mm, TE = 10.5 ms, and TR = 150 ms. All images were obtained without signal averaging.

FIGURE 2. Magnetic resonance images of the evaporation of laser polarized liquid xenon into the vapor. First, the magnetization of the xenon vapor was destroyed by application of multiple selective RF and magnetic field gradient pulses. (a­d) Xenon vapor images subsequent to the selective destruction of the vapor magnetization, showing xenon spins that have evaporated from the liquid and diffused into the vapor. Images were obtained with the same technique and parameters as for Fig. 1(b), except for a nominal RF excitation flip angle of 8 degrees and a TR of 25 ms, giving a total acquisition time of 7 sec for each image.

FIGURE 3. Magnetic resonance images of the condensation of laser polarized xenon vapor into the liquid. First, the magnetization of the xenon liquid was destroyed by application of multiple selective RF and magnetic field gradient pulses. (a­d) Xenon liquid images subsequent to the selective destruction of the liquid magnetization, showing xenon spins that have condensed from the vapor and diffused into the liquid. Images were obtained with the same technique and parameters as for Fig. 1(a), except for a nominal RF excitation flip angle of 8 degrees, a TE of 16.5 ms, and a TR of 100 ms, giving a total acquisition time of 14 sec for each image.

Note: These investigations were performed in close collaboration with Prof. David Cory of MIT and Dr. Daniel Williamson of the MR Division at the Brigham and Women's Hospital.

 

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