Hyperpolarized 129Xe MRI for Imaging Pulmonary Function

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Non-invasive imaging of pulmonary function is expected to provide critical insights that are needed to spur progress in characterizing and treating chronic pulmonary diseases. The current primary diagnostic measure is pulmonary function testing (PFT), which was introduced in the mid-19th century, ye...

Non-invasive imaging of pulmonary function is expected to provide critical insights that are needed to spur progress in characterizing and treating chronic pulmonary diseases. The current primary diagnostic measure is pulmonary function testing (PFT), which was introduced in the mid-19th century, yet remains the standard of care today. PFTs have the advantage of being non-invasive and widely available, but suffer from poor sensitivity and high variability. Thus, PFTs are ineffective in assessing therapeutic response or disease progression on reasonable time scales, given the frequent heterogeneity of disease and the lung's compensatory mechanisms. It has long been appreciated that improving sensitivity requires assessing the lungs regionally. To this end, methods, such as computed tomography (CT), provide insights into lung structure, but lung function must be inferred. However, of greater concern is the high radiation dose associated with CT, which precludes frequent longitudinal follow-up imaging. Alternatively, regional imaging of both ventilation and perfusion is possible using nuclear medicine techniques such as planar scintigraphy, single photon computed tomography (SPECT), or positron emission tomography (PET). However, as with CT imaging, all these modalities expose the subject to ionizing radiation and cannot be applied serially without a compelling clinical need. Moreover, these nuclear imaging modalities suffer from poor spatial and temporal resolution. The key role for HP 129Xe MRI is that it can enable non-invasive high-resolution imaging of all aspects of pulmonary structure and function. We have recently shown HP 129Xe MRI to visualize pulmonary ventilation with high resolution, as well as the ability to show abnormalities of the alveolar microstructure that are associated with the emphysema phenotype of COPD. We have also demonstrated the fundamentally new capability to directly visualize the uptake of 129Xe into the pulmonary capillary blood and tissues, which can provide an even more complete picture of pulmonary function by supplying regional gas exchange information. Xenon is a noble gas that is not chemically altered by the body. A small fraction of the inhaled Xe is absorbed into the blood stream and has documented anesthetic effects at moderate concentrations. The levels of gas used in this protocol are within the previously derived safe limits for both animals and humans. The stable isotope 129Xe can be hyperpolarized, which is a means to enhance its gross MRI signal by a factor of ∼100,000. Such signal enhancement makes it possible to image the inhaled gas with high spatial and temporal resolution. Moreover, the properties of 129Xe enable images to be acquired with multiple forms of contrast including ventilation, lung microstructure, and regional gas exchange. Because 129Xe MRI uses no ionizing radiation, and only an inhaled gas contrast agent, it has the potential to be used in longitudinal studies to test the effects of therapy or monitor progression of disease noninvasively.

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