We partially obstructed the left bronchi of rats and imaged an inert insolu
ble gas, SF6, in the lungs with NMR using a technique that clearly differen
tiates obstructed and normal ventilation. When the inhaled fraction of O-2
is high, SF6 concentrates dramatically in regions of the lung with low vent
ilation-to-perfusion ratios (Va/Q); therefore, these regions are brighter i
n an image than where VA/Q values are normal or high. A second image, made
when the inhaled fraction of O-2 is low, serves as a reference because the
SF6 fraction is nearly uniform, regardless of VA/Q. The quotient of the fir
st and second images displays the low-VA/Q regions and is corrected for oth
er causes of brightness variation. The technique may provide sufficient qua
ntification of VA/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 denomin
ator image is >10, the signal-to-noise ratio of the quotient image is simil
ar to that of the parent images and decreases with pixel value.