Hyperpolarized Gas Magnetic Resonance Lung Imaging in Children and Young Adults.

The assessment of early pulmonary disease and its severity can be difficult in young children, as procedures such as spirometry cannot be performed on them. Computed tomography provides detailed structural images of the pulmonary parenchyma, but its major drawback is that the patient is exposed to ionizing radiation. In this context, magnetic resonance imaging (MRI) is a promising technique for the evaluation of pediatric lung disease, especially when serial imaging is needed. Traditionally, MRI played a small role in evaluating the pulmonary parenchyma. Because of its low proton density, the lungs display low signal intensity on conventional proton-based MRI. Hyperpolarized (HP) gases are inhaled contrast agents with an excellent safety profile and provide high signal within the lung, allowing for high temporal and spatial resolution imaging of the lung airspaces. Besides morphologic information, HP MR images also offer valuable information about pulmonary physiology. HP gas MRI has already made new contributions to the understanding of pediatric lung diseases and may become a clinically useful tool. In this article, we discuss the HP gas MRI technique, special considerations that need to be made when imaging children, and the role of MRI in 2 of the most common chronic pediatric lung diseases, asthma and cystic fibrosis. We also will discuss how HP gas MRI may be used to evaluate normal lung growth and development and the alterations occurring in chronic lung disease of prematurity and in patients with a congenital diaphragmatic hernia.

Time-resolved and high-resolution MRA in a rabbit model of pulmonary embolism at 7 T: preliminary results.

PURPOSE: Evaluate feasibility of using time-resolved and high-resolution, contrast-enhanced magnetic resonance angiography (MRA) at 7 T for characterization of an animal model of pulmonary embolism. METHODS: MRAs were performed in five rabbits using a 7-T MR scanner. Preceding the MR studies, each rabbit underwent a pulmonary artery catheterization with balloon placement. Two doses of gadodiamide were injected: first during a time-resolved MRA, immediately followed by a high-resolution acquisition. Balloon was then deflated, permitting reperfusion for 5 min. A second dose was then injected and another high-resolution MRA acquired. Measurements of signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and vessel cross-sections down to fourth-order branches were made, among other parameters. RESULTS: Occlusion was detected in all rabbits. Despite a TE of 0.58 ms for the time-resolved MRA, regions of nonuniform enhancement attributed to susceptibility effects at the 7-T field were observed in perfused lung. Mean SNR=7.5+/-3.3 and 134.2+/-46.5 for the lung and aorta, respectively, and mean CNR=126.7+/-46.4 for aorta versus lung were obtained. Diameters of vessels in lung that was never occluded were not statistically different from those in reperfused lung. CONCLUSION: Results show that time-resolved and high-resolution MRA of the lung are feasible at 7 T and provide high SNR, CNR and resolution, but TEs smaller than 0.58 ms are required to avoid susceptibility artifacts in time-resolved MRAs.

Assessment of in vitro vs. in vivo lung structure using hyperpolarized helium-3 diffusion magnetic resonance imaging.

The purpose of this study was to assess the properties of a model system for hyperpolarized He-3 (HHe) diffusion MR imaging created from the lungs of New Zealand white rabbits by drying the lungs while inflated at constant pressure. The dried lungs were prepared by sacrificing the animal, harvesting the lungs en bloc and dehydrating the lungs for several days using dry compressed air. In four rabbits, the apparent diffusion coefficient (ADC) of HHe gas was measured in vivo and, within 1 week, in vitro in the dried lungs. To assess long-term repeatability, in vitro ADC values were measured again 3 months later. Dried lungs from four additional rabbits were imaged twice on the same day to assess the short-term repeatability of ADC measurements, and tissue samples from these lungs were then removed for histology. In vivo and in vitro ADC maps showed similar features and similar distributions of ADC values; mean in vivo and in vitro ADC values differed by less than 12%. The in vitro mean ADC values were highly reproducible, with no more than 5% difference between measurements for the short-term repeatability and less than 17% difference between measurements for the long-term repeatability. Histological samples from the dried lungs demonstrated that the lung structure remained intact. These results suggest that the dried lungs are a useful and inexpensive alternative to human or in vivo animal studies for HHe diffusion MR sequence development, testing and optimization.