Imaging obstructed ventilation with NMR using inert fluorinated gases

We partially obstructed the left bronchi of rats and imaged an inert insoluble gas, SF₆, in the lungs with NMR using a technique that clearly differentiates obstructed and normal ventilation. When the inhaled fraction of O₂ is high, SF₆ concentrates dramatically in regions of the lung with low ventilation-to-perfusion ratios (V̇A/Q̇); therefore, these regions are brighter in an image than where V̇A/Q̇ values are normal or high. A second image, made when the inhaled fraction of O₂ is low, serves as a reference because the SF₆ fraction is nearly uniform, regardless of V̇/Q̇. The quotient of the first and second images displays the low-V̇A/Q̇ regions and is corrected for other causes of brightness variation. The technique may provide sufficient quantification of V̇A/Q̇ to be a useful research tool. The noise in the quotient image is described by the probability density function for the quotient of two normal random variables. When the signal-to- noise ratio of the denominator image is > 10, the signal-to-noise ratio of the quotient image is similar to that of the parent images and decreases with pixel value.