Hyperpolarized ¹²⁹Xe MRI In Animal Lungs

Lung diseases are currently a global healthycare concern. According to the data and statistics of WHO (2010), the death toll caused by pulmonary diseases and lung cancer occupied 20% of all deaths every year in China. Pulmonary function tests (PFTs) and computed tomography (CT) are the routine methods for evaluating the structure and function of the lung. However, these diagnostic methods can not offer the quantitative information about the pulmonary function and structure without radiation and invasion. In recent years, hyperpolarized 129Xe, the nuclear spin polarization of which can be enchanced by four to five orders of magnitude than that in the thermal equilibrium via the technique of spin-exchange optical pumping (SEOP), has been demonstrated as an excellent gas contrast agent in imaging the airspace of lung and quantitatively evaluating pulmonary gas exchange function without radiation and invasion. In this study, hyperpolarized 129Xe was used to quantitatively evaluate the pulmonary structure and function in the rat lungs, and the research work includes the experimental setups for hyperpolarized 129Xe MR and MRI, imaging the airspace of the lung using hyperpolarized 129Xe, quantifying the pulmonary structure and function in the rats and imaging the brain using hyperpolarized 129Xe MRI. Significant results are shown as follows:Firstly, a polarizer for producing hyperpolarized 129Xe via the technique of spin exchange optical pumping has been designed and constructed, and the polarizer can enchance the nuclear spin polarization of 129Xe by 50000 folds compared with that in the thermal equilibrium; A gas delivery system for delivering hyperpolarized 129Xe to the lung of rats has been built, and the delivery system could deliver hyperpolarized 129Xe gas to the lung with almost no depolarization according to the experimental requirement accurately and automatically; The dual-tuned birdcage coil for hyperpolarized 129Xe MRI has been designed and built, and the uniform RF field generated by the birdcage coil satisfied the requirement of hyperpolarized 129Xe lung MR and MRI.Secondly, the lung gas MRI was implemented by using hyperpolarized 129Xe via FLASH and ultrashort TE (UTE) pulse sequence, and the influence of constant-flip-angle and the variable-flip-angle excitation RF pulses on the quality of hyperpolarized 129Xe lung MRI was investigated. Hyperpolarized 129Xe lung MRI was utilized to evaluate the influence of X-ray on the lung, and the ventilation defects were found in the local area exposed by X-ray.Thirdly, the pulmonary function and structure were comprehensively quantified by using hyperpolarized 129Xe MR, and the major physiological parameters measured in the radiation-induced lung injury (RILI) rats, including the exchange time constant and septal thickness, showed significant difference compared with that obtained from the healthy ones. The pulse sequence for measuring the pulmonary gas consumption time constant using hyperpolarized 129Xe was designed and implemented, and the time constant measured in the chronic obstructive pulmonary diseases (COPD) induced by smoking exposure increased significantly compared with that in the healthy ones.Finally, the dependence of hyperpolarized 129Xe brain MR signal on the pulmonary oxygen concentration was studied using numerical simulation and in vivo experiments in rats, and the results obtained from these two methods showed a good consistency, additionally, the optimal oxygen concentration for hyperpolarized 129Xe brain MR signal was obtained. Furthermore, hyperpolarized 129Xe brain MRI was achieved in rats.