MRI of Pulmonary Ventilation

Abstract

Gas imaging has opened the new field of direct imaging of pulmonary ventilation by MRI. The use of hyperpolarised 3He gas for MRI of the lung has been pioneered by a number of groups worldwide. Due to the enormous progress in the fields of hyperpolarisation technology, administration of hyperpolarised 3He, MR hardware, and MR pulse sequences significant progress has been made and the translation into the clinical arena has been accomplished. This chapter gives an overview of the technical methods for HP 3He MRI for human lung imaging, focusing on gas polarisation methods, background physics, MRI hardware considerations, MRI pulse sequence considerations, safety considerations for imaging inhaled 3He, and pulse sequence design for probing lung physiology and anatomy. Where possible the methods will be highlighted with reference to the literature and illustrated with clinical examples of images from the author’s home group. For further discussion on the growing clinical applications the reader is referred to Chaps. 8, 9 and 10. Hyperpolarized xenon-129 (129Xe) has enormous potential to provide noninvasive functional information about the lung. In particular, because inhaled xenon follows the same pathway as oxygen, diffusing from alveolar gas spaces to septal tissue and blood, gas exchange parameters can be measured. For example, by measuring the time-dependent septal uptake of 129Xe, information about alveolar surface area, septal thickness and vascular transit times can be obtained. A principal obstacle to the development and application of this technology to humans has been the lack of a polarizer that can provide sufficient quantities of highly polarized 129Xe gas. This problem, however, has recently been solved. To obtain quantitative measures of pulmonary function with 129Xe, two methods have been studied. One is a direct method that measures the magnetic resonance signal from the gas spaces and from the different septal tissue compartments. In principal this is straightforward since the signal from each compartment can be distinguished due to a unique chemical shift frequency. In practice, however, there are limitations because of the very small signal available to measure in the septal tissue. Thus, the direct measurement of 129Xe interphase diffusion in humans has been principally confined to whole lung measurements. To obtain regional maps of 129Xe interphase diffusion, the Xenon Transfer Contrast (XTC) method has been utilized. XTC is an indirect method that measures the attenuation in the gas phase magnetization due to interphase diffusion between the gas phase and septal tissue compartments. Using XTC, regional maps of interphase diffusion in humans has been demonstrated. Fluorine MRI of the lung is an interesting new approach that may have the potential for broader use than MRI based on hyperpolarized gases like He-3 or Xe-129. Although in general the image quality is worse in fluorine MRI than that obtained with hyperpolarized gases, the latter approach has the advantage of very simple requirements: only an MRI system with non-proton imaging capabilities and a dedicated fluorine-19 MRI coil are required. Fluorinated gases do not need complex treatment before use – this makes their application less demanding on the local infrastructure and, potentially, may also reduce costs. However, currently, the most significant drawback of these gases is that they have not yet been approved for human application. The direct visual assessment of the lung parenchyma and imaging of lung ventilation using proton MRI is considerably more difficult than MRI of most other organs due to the very low signal intensity of the lung parenchyma. The low signal intensity is caused by the low average proton density and the short T2* relaxation time of lung tissue. Several methods for proton-MRI-based ventilation measurements have been proposed in order to overcome these difficulties. Currently the most established technique is oxygen-enhanced MRI of the lung, employing inhaled molecular oxygen as a T1-reducing contrast agent, which enhances the signal of the protons in the lung. The clinical application of oxygen-enhanced lung MRI has been assessed in several studies. Main advantages of oxygen- enhanced MRI are the general availability of oxygen and the relative safety of oxygen administration. Potential limitations of oxygen-enhanced lung MRI are the relatively low signal enhancement corresponding to a T1 reduction of about 10 %, and the complex contrast mechanism with contributions due to ventilation, perfusion, and oxygen-diffusion properties of the lung. Newer techniques based on non-enhanced dynamic MR acquisitions appear to be a promising tool for ventilation assessment that may be available in the near future. Other proposed techniques such as imaging after administration of aerosolized gadolinium contrast agents or after infusion of water- in-perfluorocarbon emulsions into the lung require still considerably more research before they might become applicable in clinical MR imaging.