Imaging obstructed ventilation with NMR using inert fluorinated gases

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.