Although spirometric results indicate normal lung function, the mean VDI in patients (5.1) found in this study is well above the VDI in healthy subjects (1.6) reported in the literature. A single CPT session induces disparate changes in the distribution and extent of ventilation defects.

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Magnetic resonance imaging of lungs and the investigation of pulmonary pathologies with this technique are limited by low proton spin density, degraded magnetic homogeneity and motion. Inhaled contrast agents (gases or aerosols) can improve the diagnostic value of MRI for lung. Paramagnetic contrast agents such as gadolinium chelates aerosol or dioxygen gas increase the relaxivity of proton in lung parenchyma and can be used to assess the ventilated fraction of the bronchoalveolar space. Similarly, inhalation of non proton-MRI nuclei such as perfluorinated gas or hyperpolarized gases ((3)He or (129)Xe) can provide functional ventilation image. In this review paper, the principles, the practical implementation, the limitations and possible safety issues of these different techniques are summarized. The main pre-clinical and clinical applications of these approaches based on oral contrast agents are reviewed and illustrated with cutting-edge lung MRI studies.

Purpose: To demonstrate ventilation changes in an animal model of methacholine-induced bronchoconstriction using hyperpolarized (HP) helium-3 (He-3) MRI. Materials and Methods:Bronchoconstriction was induced in 11 healthy rats using an intravenous injection of methacholine. The He-3 was laser-polarized using a custom-built system. MRI studies were performed on a 2-Tesla bore magnet. Coronal dynamic ventilation images were obtained using a single inhalation of the laser-polarized He-3 gas before and after methacholine injection. Ventilation image series were processed on a pixelby-pixel basis to generate three regional ventilation parameters: gas flow rate, filling time, and maximum gas volume. Student's paired t-test was used for analysis.Results: Ventilation image series with a temporal resolution of 5 msec were obtained before and after methacholine challenge. Quantitative regional gas dynamic information demonstrated statistically significant differences between the baseline and constricted states. Following methacholine injection, the mean flow values were significantly lower for the right lung (RL) (P ϭ 0.006) and left lung (LL) (P ϭ 0.024), while the mean filling time was found to be greater (RL: P ϭ 0.08, LL: P ϭ 0.021). Gas volume values at maximum inspiration were found to be significantly lower after methacholine (RL: P ϭ 0.002; LL: P ϭ 0.036). Conclusion:He-3 MRI demonstrated and quantified regional ventilation changes in bronchoconstriction conditions in rats. EARLY HYPERPOLARIZED (HP) helium-3 (He-3) MRI studies in humans focused on static imaging of airways and airspaces in an inspiratory breath-hold. However, additional imaging strategies are required to improve the method, especially when obtaining lung function information. Typically, a temporal resolution of a few hundred milliseconds is required in order to monitor the ventilation process. Several approaches that rely on the use of fast imaging sequences during a single gas inspiration have been developed. The feasibility of such techniques, using fast low-angle shot (FLASH), rapid acquisition with relaxation enhancement (RARE), echoplanar imaging (EPI), or spiral acquisition, has already been described (1-6). In a previous work, a radial acquisition strategy combined with the sliding window method (7) was developed. This methodological approach, which is referred to as Sliding Pulmonary Imaging for Respiratory Overview (SPIRO) (8), enables the acquisition of quantitative local gas dynamic information using a single breath of polarized gas. Quantitative ventilation parameters, such as gas arrival time, average inflation rate (AIR), maximum gas volume, and filling time, could be obtained in small animals at the pixel level. In this work we demonstrate ventilation changes in an animal model of methacholine-induced bronchoconstriction using the SPIRO technique. MATERIALS AND METHODS Laser-Polarized He-3 Preparation and AdministrationHP He-3 was prepared using a custom-built polarizer based on a spin-exchange technique (9 -12). He-3 polarization l...

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