A technique based on using spin-oriented nuclei of helium-3, which has been developed for nuclear physics experiments, also has impressive capabilities for lung imaging.
Lungs are full of gas, which is normally invisible, making lung ventilation examinations difficult.While X-ray and other techniques can reveal anatomical anomalies, it is difficult to follow directly the actual functioning of the lung.
Some 30 years ago, in the so-called “golden age” of optical pumping, physicists at Mainz University started to develop techniques for polarizing nuclear spins for nuclear physics studies and experiments at CERN’s Isotope Separator Online (ISOLDE). These experiments revealed interesting insights into the behaviour of exotic isotopes.
For helium-3, optical pumping has shown its potential for magnetic resonance tomography when powerful lasers in the near-infrared region were available to produce large quantities of high-grade, spin-polarized gas.
Inhaled helium-3 can be visualized on a magnetic resonance tomogram that gives unprecedentedly detailed images of a breathing lung.
The team that developed the technique has been awarded several prestigious prizes, including the Körber prize for European science and a nomination for the German president’s “Future” prize.
Helium-3: a fascinating tool
Although the helium-3 isotope is extremely rare in nature, it has become more widely available via the beta-decay of synthetic tritium. Since the early 1970s, helium-3 in its superfluid state has become a fascinating laboratory tool for the study of quantum mechanical phenomena that turned out to be an ideal testing ground for fundamental concepts of modern theoretical physics.
In nuclear physics, experiments involving the neutron’s spin are hampered by the fact that a target of free, polarized neutrons is not available.
However, helium-3 is a good approximation for a target of polarized neutrons, because its nuclear spin of 1/2 is due to its unpaired neutron (the two protons have opposing spins).
To force helium-3 nuclear spins to align in one direction, the gas is exposed to a resonant, circularly polarized laser beam directed along the axis of an external magnetic field. By means of the absorption and spontaneous emission of fluorescent light, the spin of the light quanta can be transferred to the atomic electrons that in turn transmit their spin direction to the nucleus via magnetic coupling. Repeated absorption and emission accumulate the nuclei in a specific spin state. The Mainz laser pumping techniques were developed in collaboration with the specialist team of Michele Leduc at the Ecole Normale Superieure in Paris.
Scattering a polarized, high-energy electron beam by the neutron in polarized helium-3 allows the contribution due to the electromagnetic interaction of the probe with the internal charge distribution of the neutron – the effect of interested – to be separated off and even enhanced. First precise values can now provide a test for different theoretical approaches to this fundamental property.
Lung tomography
In 1994 Will Happer’s group at Princeton, in collaboration with magnetic resonance imaging specialists from Duke University, Durham, North Carolina, demonstrated in a seminal paper how highly polarized xenon gas could be used to examine the lungs of a guinea pig.
On learning of this, Mainz physicists W Heil and E W Otten, together with their colleague M Thelen from the department of radiology, saw the potential usefulness of their well established helium-3 physics-laboratory techniques for human lung imaging, and they soon carried out very satisfactory trials, beginning in 1995.
Optical pumping of metastable helium-3 atoms, the method they use, can supply relatively large amounts of gas with relatively high polarization – up to 50%. These magnetic signals are a thousand times as large as those normally encountered in magnetic resonance imaging. Under these conditions, lung imaging becomes straightforward. Patients simply inhale a whiff of gas and the whole procedure is carried out at room temperature.
One obstacle, however, was the storage and transport of the carefully prepared polarized gas, which has to be taken from the laboratory to the clinics. Collisions with the walls of a normal container would quickly destroy the spin orientation. This is overcome by storing polarized gas in glass vessels, the inner surfaces of which are coated with a few monolayers of caesium. In this way, the polarized gas can be stored at pressures of up to 10 bar and kept ready for use for more than 100 h.
Rather than just giving a single lung image while the patients hold their breath, helium-3 imaging can provide ultrafast sequences with a time resolution of less than a tenth of a second – a “movie” of lung ventilation during the breathing cycle.
There is another advantage: helium-3 in contact with paramagnetic oxygen soon loses its polarization. The rate of depolarization is related to the oxygen partial pressure in regions of interest of the lung and, moreover, enables the oxygen uptake in the blood to be measured. For the first time, normal lung functioning can be quantified, and disorders in respiratory distribution can be recognized before any signs or symptoms have become manifest.
The technique is still undergoing trials at Mainz University Hospital and selected European clinics, but helium-3 tomography appears to have a bright future for visualizing and assessing pulmonary ventilation. Only a few accessory tools are needed to perform helium-3 imaging with standard magnetic resonance imaging equipment, so the technique could become widely available within a relatively short time.
