Alveolar ventilation/perfusion ratio (V A /Q) is a key parameter in functional imaging of the lung. Herein, regional V A /Q was calculated from regional values of alveolar partial pressure of oxygen (P A O 2 ) measured by hyperpolarized 3 He gas MRI (HP 3 He MRI). Yorkshire pigs (n ‫؍‬ 7, mean weight ‫؍‬ 25 kg) were paralyzed and maintained under isoflurane anesthesia. Animals were placed into a birdcage coil, then transferred to the bore of a 1.5 T MRI unit. Prior to imaging, animals were manually ventilated with room air for 5 min, then a 3 He gas mixture was administered during breathhold and imaging performed. P A O 2 was measured based on the decay rate of 3 He signal. Subjects' blood gas concentrations were measured and these values and Imaging techniques for pulmonary disorders have traditionally focused on anatomic and pathologic descriptions of lung parenchyma, airways, and vasculature. Functional imaging of the lung is a relatively recent development in pulmonary imaging that seeks to image physiologic variables. Recent developments in imaging methods for regional measurements of function have generated considerable interest because of the potential for high-resolution, regional distributions of functional variables to enhance detection of lung pathologies. Additionally, regional measurements of function can provide fundamental insights into the pathophysiologic mechanisms of disease.Several methods have been reported recently for imaging of functional variables: local compliance (1-3), local ventilation (4 -6), local perfusion from arterial spin tagging techniques (7-9) and contrast washout kinetics (5,10,11), and regional determinations of the alveolar ventilation/perfusion ratio (V A /Q) by PET (12-16), CT (17), and MRI-based methods (8,18,19). Regional determinations of V A /Q are of particular interest because of the key role of V A /Q in gas exchange (20), and because V A /Q can be disturbed by pathology that affects either ventilation or perfusion. Local measurements and imaging of V A /Q therefore have the potential to serve as a sensitive marker for lung disease.MR-based functional imaging methods offer a unique combination of potentially high spatial resolution and high temporal resolution. Although proton MRI can be used for lung imaging (21), this approach is generally at a disadvantage in that contrast in signal between tissue and gas phases is low (22). Recently, several investigators have used hyperpolarized (HP) 3 He gas for high-resolution imaging of the airways (23,24), and most recently as a method to assess regional ventilation (4). One complicating factor that all HP 3 He methods share is the oxygen-dependent decay of signal due to spin transfer (25).The Mainz group used this oxygen-dependence of polarized spin decay to measure regional alveolar partial pressure of oxygen (P A O 2 ) in the lung, and suggested that these local measurements of P A O 2 and their correlation with decrease rate could reflect matching of ventilation and perfusion (26 -30). It has long been known that the...

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