Analysis of intrapulmonary O₂concentration by MR imaging of inhaled hyperpolarized helium-3

Inhalation of hyperpolarized ³He allows magnetic resonance imaging (MRI) of ventilated airspaces.³He hyperpolarization decays more rapidly when interacting with paramagnetic O₂. We describe a method for in vivo determination of intrapulmonary O₂ concentrations ([O₂]) based on MRI analysis of the fate of measured amounts of inhaled hyperpolarized³He in imaged regions of the lung. Anesthetized pigs underwent controlled normoventilation in a 1.5-T MRI unit. The inspired O₂ fraction was varied to achieve different end-tidal [O₂] fractions ($F{\text{ET}}_{O_2}$ ). With the use of a specifically designed applicator,³He (100 ml, 35–45% polarized) was administered at a predefined time within single tidal volumes. During subsequent inspiratory apnea, serial two-dimensional images of airways and lungs were acquired. At least once in each animal studied, the radio-frequency excitation used for imaging was doubled at constant$F{\text{ET}}_{O_2}$ . Signal intensity measurements in regions of interest of the animals' lungs (volume range, 54–294 cm³), taken at two different radio-frequency excitations, permitted calculation of [O₂] in these regions of interest. The [O₂] fractions in the regions of interest correlated closely with$F{\text{ET}}_{O_2}$ (R = 0.879;P < 0.0001). O₂-sensitive³He-MRI may allow noninvasive study of regional distribution of ventilation and alveolar$P{O}_2$ in the lung.