Hyperpolarized helium is opening up new avenues of research in both experimental nuclear physics and diagnostics of the lungs.
ic-automation has developed Helios in collaboration with physicists at the Johannes Gutenberg University in Mainz, Germany. Together with the Institute of Physics, in 2005 it won an award for innovation in medical technology from the German Ministry for Education and Research (BMBF) for its proposed 3He applicator for lung diagnostics.
Helios is the most efficient machine for the production of hyperpolarized helium gas commercially available. It permits polarization levels of over 75% and a delivery rate of over 30 bar*liter of helium per day with an output pressure of several bars.
The prototype of this apparatus has been developed by physicists at the Johannes Gutenberg University in Mainz, Germany, originally for the polarization of neutrons in the field of nuclear physics. In medical applications hyperpolarized helium permits diagnosis of the lungs to be made without the danger of harmful side effects.
Method of operation
Second only to hydrogen helium has the simplest structure of all of the chemical elements. It consists of a doubly positively charged atomic nucleus which is orbited by two negatively charged electrons. The nucleus of normal helium, on the other hand, has four components: two protons, each with a positive charge, and two neutrons which are electrically neutral. Helium-3 (3He) has a neutron missing; the nucleus thus has only three particles. The isotope 3He only very rarely occurs naturally. Its approximate percentage of natural helium is just one millionth (abbreviated to 1 part per million or 1 ppm). Noble gas 3He and its "brother" 4He - of the same atomic type - are both chemically inert. They are not known to have a harmful effect on the human body.
A number of atomic nuclei, 3He among them, have their own angular momentum - or what physicists refer to as "spin". These nuclei can best be described as tiny spherules or globules which rotate about their own axes in circles. In normal conditions the axes of rotation of these "circles" point in all directions. If we are able to persuade these individual "circles" to collectively gravitate in one direction, however, we then speak of polarization, a concept which gives rise to a whole host of new phenomena and practical applications.
The principle of optical pumping was researched in the 1950s at the Ã‰cole Normale in Paris, France, by physicists Kastler and Brossel; Kastler was later awarded a Noble Prize for his findings in 1966. In this method gas atoms are exposed to circularly polarized laser light, i.e. with light which has both a fixed wavelength and a precisely defined sense of rotation. In absorbing this light the atoms adopt its sense of rotation which is in turn transferred to the nuclear spins, causing these to align themselves in a preferred or predominant direction.
Direct optical pumping from the lowest energy state (ground state) is, however, not possible with noble gases. We must thus undertake a number of "detours" before we can achieve the required nuclear spin polarization in atomic ground state. For optical pumping in metastable helium (the long-lived excited state of the 3He atom or 2 3S1 state ;see Colegrove 1963) at the end of the 1980s powerful lasers were developed by Leduc at the Ã‰cole Normale in Paris with which 3He could be polarized quickly and with a high throughput. This is, however, only possible through gas discharge (using a kind of luminous tube) at a very low pressure of ca. 1/1000th of an atmosphere. In this gas discharge process atoms are excited to the required metastable state so that polarized laser light can be absorbed. The orientation of the electronic region of the metastable 3He atoms builds up within the space of microseconds; this is then transferred to the nuclear spin of the ground state atoms practically immediately in collision processes.
Due to the low optical density of the gas long pumping cells have to be constructed so that the provided light can be efficiently absorbed. The length of each pumping cell in the Helios apparatus is 2.4 m (ca. 7.9 ft). The laser system consists of two Yb-doped fiber lasers which emit light in the infrared range (at 1083 nm). This wavelength corresponds to the energy a 3He atom must absorb to be able to participate in the pumping process..
For the application of polarized 3He in the medical imaging of the lungs, for example, a pressure of one or several atmospheres is necessary. The optically pumped gas must thus be compressed to more than a thousand times its original value without polarization being lost - an undertaking which posed an enormous problem just ten years ago. The slightest gas impurity, the tiniest iron filing in the system would destroy the polarization of the nuclear spin in seconds, as nuclear spins react extremely sensitively to foreign atoms and magnetic materials.
The entire apparatus is in a weak homogenous magnetic field (a few gauss). In the optical pumping process of 3He the optical pumping cells (with a volume of ca. 36 liters) are periodically filled with purified gas from a reservoir. A non-magnetic titanium piston compressor compresses the polarized gas to ca. 300 mbar in an interim volume. From here the gas again passes through the compressor stage and is sealed at several atmospheres in a storage cell made of special iron-free glass. The cell containing the polarized 3He gas can be removed once the glass valve has been closed.