Hyperpolarization of 129Xe is accomplished by placing a non-radioactive isotope of the Xenon source gas into a beam of polarized laser light in the presence of very small amounts of an alkali metal. The result is Xenon whose nuclear magnetic spin is highly aligned, but not chemically or biologically different than un-polarized Xenon, a harmless inert gas. Learn more about polarization physics here.
Once the Xenon is hyperpolarized, it can be dispensed in a plastic bag, the amount of polarized gas is verified and then it is administered to the subject already lying down inside the MRI scanner. The patient inhales a small quantity (a few hundred ml) of the gas and undergoes a MRI scan. The MRI scan is typically completed within a 10-20 second breath hold.
Compared to air that is typically within the lung, the hyperpolarized Xenon enhances the MRI signal by a factor of 100,000, making lung structure and regional ventilation exquisitely visible. The image in left-hand panel shows a traditional MRI scan of the lungs; the image in the right-hand panel shows a scan of the patient with hyperpolarized gas MRI. The gas scan localizes large emphysematous bulla and identifies several other, more subtle areas of poor ventilation. 129Xe MRI enables the volume of the ventilated space within the lung to be computed, as well as a large array of other quantitative metrics. Learn more about the ways in which MRI detects hyperpolarized gas here.
Because the HP Xe MRI technique does not use ionizing radiation like CT scans or nuclear scintigraphy scans, the technique can be used repeatedly without harm to the patient, in order to assess the progression of disease over time, or the effectiveness of therapy. This is especially valuable for diseases such as COPD or Cystic Fibrosis, where the disease must be monitored frequently over time and where the disease manifests itself in the smallest of lung airways. Learn more about what diseases can be studied suing HP gas MRI here.
HP Xe MRI technology is unique in its ability to visualize the small airways and pulmonary gas exchange tissues of the lung. By comparison, CT imaging provides excellent spatial resolution, but it visualizes only the first 6 branches of the 23-branch human pulmonary airway tree. By contrast, HP Xe MRI detects xenon that has successfully traversed all 23 branches of human lung to reach the gas exchange tissues.
It is this significantly higher functional resolution provided by HP Xe MRI that is so important to detecting early disease and subtle changes in the progression of disease. For example, a small mucous plug in the 16th generation of airway would be completely undetectable on a CT scan, but would result in a large and noticeable ventilation defect on HP Xe MRI. By imaging lung function, rather than lung structure, HP Xe MRI provides a unique way to assess the pathology of the distal pulmonary spaces where disease originates.