The feasibility of performing 133Xe ventilation imaging following the perfusion study. 
The 99mTC Compton scatter contribution from the perfusion lung study to the subsequent 133Xe ventilation scan was quantitated in 15 consecutive patients undergoing both studies.  Maximum contribution during the wash-in phase occurred during the first 10 seconds; thereafter, 99mTc Compton scatter averaged less than 10% of the total counts accumulated for a 133Xe image.  The contribution during wash-out gradually increased to a maximum of 36% at 4 minutes in a normal subject.  The authors conclude that a properly performed 133Xe ventilation study may immediately follow the 99mTc perfusion lung scan without significant image degradation.