Imaging obstructed ventilation with NMR using inert fluorinated gases

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Abstract
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 O2is high, SF6 concentrates dramatically in regions of the lung with low ventilation-to-perfusion ratios (V˙a/Q˙); therefore, these regions are brighter in an image than whereV˙a/Q˙ 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 ofV˙a/Q˙. The quotient of the first and second images displays the low-V˙a/Q˙ regions and is corrected for other causes of brightness variation. The technique may provide sufficient quantification ofV˙a/Q˙ 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.