B. Saam

MRI of the lungs using hyperpolarized noble gases

The nuclear spin polarization of the noble gas isotopes 3He and 129Xe can be increased using optical pumping methods by four to five orders of magnitude. This extraordinary gain in polarization translates directly into a gain in signal strength for MRI. The new technology of hyperpolarized (HP) gas MRI holds enormous potential for enhancing sensitivity and contrast in pulmonary imaging. This review outlines the physics underlying the optical pumping process, imaging strategies coping with the nonequilibrium polarization, and effects of the alveolar microstructure on relaxation and diffusion of the noble gases. It presents recent progress in HP gas MRI and applications ranging from MR microscopy of airspaces to imaging pulmonary function in patients and suggests potential directions for future developments. Magn Reson Med 47:1029–1051, 2002.

MR imaging of diffusion of 3He gas in healthy and diseased lungs

Hyperpolarized 3He gas MRI was used to form maps of the effective diffusivity of gas in human lungs. Images of diffusion as well as spin density are presented from a study of 11 healthy volunteers and 5 patients with severe emphysema. The effective rate of diffusion, De, of the gas is reduced by the alveolar walls; tissue destruction in emphysema is hypothesized to result in larger De. Indeed, the mean value of De in the emphysematous lungs is found here to be about 2.5 times that of healthy lungs, although still smaller than the unrestricted diffusivity of 3He in free air. Histograms of De values across coronal slices are presented. The results are discussed in terms of spatial variations, variations among individuals, healthy and diseased, and variations due to changes in lung volume. Magn Reson Med 44:174–179, 2000.

Rapid Imaging of Hyperpolarized Gas Using EPI

Rapid repetitive MRI of hyperpolarized (HP) gases using echo‐planar imaging (EPI) has been theoretically investigated and experimentally demonstrated for 3He in human lung. A quantitative treatment of signal attenuation and magnetization consumption for the unique circumstance of a rapidly diffusing nonrenewable magnetization source has been performed. Rapid (compared to the human respiratory cycle) and repetitive imaging of the lung gas space with EPI and a single delivered bolus of HP‐3He is feasible using low flip angles, provided the voxels are not too small. A coarse‐grid (32 × 64) EPI pulse sequence has been developed and implemented to image the lungs of healthy volunteers during rebreathing of a HP‐3He/N2 gas mixture. A set of three 10‐mm axial slices was imaged every 0.12 sec for the 36 sec duration of rebreathing, yielding a real‐time visualization of ventilation. Despite some mild artifacts, the images are of good quality and show changes in gas density related to respiratory physiology. Magn Reson Med 42:507–514, 1999.

EDGE ENHANCEMENT OBSERVED WITH HYPERPOLARIZED 3HE

Abstract We have produced one-dimensional (1-D) magnetic resonance images of hyperpolarized 3He in square glass cells and demonstrated edge enhancement of the signal intensity near the parallel impermeable boundaries. The size and position of the edge-enhancement peaks approximately agrees with the theoretical prediction based on solutions of the 1-D Torrey equation with boundary conditions. Additional distortion in our images is due to the long-range dipole fields produced by the polarized spins and by the bulk magnetic susceptibility of the glass. Our experiments, done in an applied field of H0 = 31 G, demonstrate the low-field imaging capacity of hyperpolarized noble gases.

Room-temperature coupling between electrical current and nuclear spins in OLEDs

Organic semiconductors go out for a spin Magnetism is a commonly observed phenomenon in the macroscopic world, but its origins lie in the quirky quantum-mechanical property of electrons and certain nuclei known as spin. Recent research has sought to leverage and expand the role of spin in the operation of electronic devices. Malissa et al. used a highly sensitive spectroscopic technique to probe, and ultimately manipulate, the subtle effects of spin interactions on the current that flows through organic light-emitting diodes (OLEDs) (see the Perspective by Bobbert). They pinpointed coupling between the spins of the current carriers and the hydrogen nuclei in the hydrocarbon-based material making up the device. Science, this issue p. 1487; see also p. 1450 Magnetic resonance spectroscopy enables detection and manipulation of subtle spin interactions in organic semiconductors. [Also see Perspective by Bobbert] The effects of external magnetic fields on the electrical conductivity of organic semiconductors have been attributed to hyperfine coupling of the spins of the charge carriers and hydrogen nuclei. We studied this coupling directly by implementation of pulsed electrically detected nuclear magnetic resonance spectroscopy in organic light-emitting diodes (OLEDs). The data revealed a fingerprint of the isotope (protium or deuterium) involved in the coherent spin precession observed in spin-echo envelope modulation. Furthermore, resonant control of the electric current by nuclear spin orientation was achieved with radiofrequency pulses in a double-resonance scheme, implying current control on energy scales one-millionth the magnitude of the thermal energy.

Spin transfer between laser-polarized 129Xe nuclei and surface protons

We have demonstrated a large polarization transfer from highly polarized gaseous 129Xe to protons in a silicone surface coating. The proton polarization enhancement of ∼ 104–105 over the thermal equilibrium polarization at 0.2 T makes possible the detection of the previously unobservable resonance. We expect that this technique may allow high-resolution NMR to become a viable tool in the study of surfaces.

Collisional 3He and 129Xe frequency shifts in Rb-noble-gas mixtures.

The Fermi-contact interaction that characterizes collisional spin exchange of a noble gas with an alkali-metal vapor also gives rise to NMR and EPR frequency shifts of the noble-gas nucleus and the alkali-metal atom, respectively. We have measured the enhancement factor κ0 that characterizes these shifts for Rb-129Xe to be 493±31, making use of the previously measured value of κ0 for Rb-3He. This result allows accurate 129Xe polarimetry with no need to reference a thermal-equilibrium NMR signal.

Polarization transfer using hyperpolarized, supercritical xenon

Abstract Polarization transfer from hyperpolarized gas to 1 H, 13 C, etc. holds great promise for sensitivity enhancement of solution-state NMR. The route explored here uses hyperpolarized, supercritical xenon as solvent for the organic solute. A method is described for preparation of supercritical xenon solutions with little polarization loss. Detection of Overhauser enhancement of solute proton magnetization by factors of 3 to 7 confirms the intimate contact between 129 Xe and 1 H. Motivated by more efficient polarization transfer expected in solid solutions, we report survival of non-equilibrium solute polarization upon warming from solid to supercritical fluid.

3He spin exchange cells for magnetic resonance imaging

We present a protocol for the consistent fabrication of glass cells to provide hyperpolarized (HP) 3He for pulmonary magnetic resonance imaging. The method for producing HP 3He is spin-exchange optical pumping. The valved cells must hold of order 1 atm⋅L of gas at up to 15 atm pressure. Because characteristic spin-exchange times are several hours, the longitudinal nuclear relaxation time T1 for 3He must be several tens of hours and robust with respect to repeated refilling and repolarization. Collisions with the cell wall are a significant and often dominant cause of relaxation. Consistent control of wall relaxation through cell fabrication procedures has historically proven difficult. With the help of the discovery of an important mechanism for wall relaxation that involves magnetic surface sites in the glass, and with the further confirmation of the importance of Rb metal to long wall-relaxation times, we have developed a successful protocol for fabrication of 3He spin exchange cells from inexpensive an...

Fundamental mechanisms of 3He relaxation on glass

Abstract We present a model of 3 He relaxation on the surface of borosilicate glass which accurately predicts observed relaxation rates and their temperature dependence. Above room temperature 3 He dissolves into Pyrex, where interactions with Fe3+ ions result in a relaxation time of ≈1 ms. Gas exchange across the glass surface of an enclosed vessel leads to T1−1=A/V(3.9±1.4)×10−2 cm/h at room temperature, where A/V is the surface-to-volume ratio. The activation energy for relaxation is 13.7±0.7 kJ/mol and is dominated by the activation energy of 3 He diffusion in glass. This is the first successful confirmation of predicted 3 He relaxation rates in glass vessels.