Lung MRI using hyperpolarized noble gases
Since arriving at the University of Virginia in 2001, Dr. Miller’s main research focus has been hyperpolarized-gas MRI of the lung, using both helium-3 and xenon-129. Upon inhalation, these non-radioactive contrast agents not only allow us to visualize the ventilated lung airspaces with unprecedented detail and resolution, but offer other forms of contrast as well.
PAO2 Mapping: Because the decay rate of hyperpolarized magnetization is directly proportional to the oxygen concentration in the lung airspaces, T1-weighted imaging of inhaled xenon-129 or helium-3 can be used to measure the partial pressure of oxygen in the alveoli (PAO2). The primary function of the lung is gas exchange, and PAO2 is a quantitative physiological parameter that is directly sensitive to the exchange of oxygen between inhaled air and the bloodstream. Dr. Miller is currently exploring the use of fast spiral pulse sequences to generate three-dimensional, whole-lung PAO2 maps in a single short breath hold, for use in patients with lung disease.
Diffusion-Weighted Imaging: Individual gas particles undergo random Brownian motion as they bounce around inside the lung airspaces. Diffusion-weighted imaging of inhaled hyperpolarized gases measures this random motion, and is sensitive to the degree of restriction imposed by the fine alveolar structure. Research in this area has centered on the development of fast diffusion-weighted pulse sequences that can detect alterations of the lung microstructure caused by emphysema and other diseases.
Respiratory Motion: Grid-tagging of inhaled hyperpolarized gases can be used to track how the different parts of the lung move with inhalation and exhalation. In collaboration with researchers from Radiation Oncology, my group has pioneered the use of this technique to study the way the lung moves during free breathing, in an effort to reduce the radiation exposure of healthy tissue during lung cancer treatment.
RF Coil Development: Imaging these gases requires custom transmit/receive radiofrequency (RF) coils tuned to the helium-3 or xenon-129 resonance. Dr. Miller runs the research division’s RF coil laboratory, where an assortment of custom RF coils have been built for imaging human and animal lungs.
Hyperpolarization Technology: Dr. Miller’s lab recently partnered with Gordon Cates’ research group in the physics department, to build a new helium-3 hyperpolarizer based on hybrid rubidium/potassium spin-exchange optical pumping. This device generates 2.6-liter batches of 50-65% polarized helium-3, and currently supports a variety of human imaging studies in our research group.