Helium-3 Lung MRI
Hyperpolarized helium-3 gas lets magnetic resonance imaging visualize the air spaces of the lungs, which are otherwise invisible to conventional MRI.
Imaging the invisible
Conventional magnetic resonance imaging maps the hydrogen nuclei in water and tissue. The air spaces of the lungs contain almost no water, so they appear as dark voids. To image ventilation directly, a gas that can produce an MRI signal is needed. Hyperpolarized helium-3, inhaled by the patient, fills the airways and alveoli with nuclei that resonate strongly, turning the void into a bright, detailed image of where air actually goes.
Hyperpolarization
At ordinary conditions only a tiny fraction of helium-3 nuclear spins align with a magnetic field, giving a weak signal. Hyperpolarization aligns a very large fraction of the spins using optical pumping, typically spin-exchange with laser-polarized alkali-metal vapor. This boosts the available signal by orders of magnitude, enough that a single breath of the gas produces a clear image in a few seconds before the polarization relaxes away.
What it reveals
Helium-3 lung imaging shows regional ventilation: which parts of the lung fill and empty and which do not. It has been used to study asthma, chronic obstructive pulmonary disease, cystic fibrosis, and the effects of treatment, revealing ventilation defects that spirometry, which measures only whole-lung airflow, cannot localize. Because helium-3 is inert and inhaled briefly, the technique carries no radiation dose.
Supply pressure and alternatives
Helium-3 lung MRI competes for the same scarce isotope as neutron detection and cryogenics. Its research use has been limited partly by supply, and hyperpolarized xenon-129, which is far more abundant, has been developed as an alternative for many studies. Helium-3 still offers advantages in signal strength and diffusion sensitivity where the isotope can be obtained.
- Inhaled hyperpolarized helium-3 makes lung air spaces visible
- Optical pumping aligns a large fraction of nuclear spins
- Reveals regional ventilation defects without radiation dose
- Competes for scarce isotope; xenon-129 is an alternative