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Figure 5. Normal xenon-133 ventilation study performed in the posterior projection. The study consists of single-breath (wash-in), equilibrium and wash-out phases.

are between 0.5-3 microns in size and are produced by adding the appropriate radiopharmaceutical to a commercially available nebulizer. For a routine ventilation study, 1.11 GBq (30 mCi ) of 99mTc-DTPA in 3 ml of saline is placed within the nebulizer. Oxygen is then forced through the nebulizer at high pressure to produce aerosolized droplets which are inhaled by the patient through a mask or mouth piece. The patient generally breathes from the nebulizer for 3-5 minutes or until 37 MBq (1 mCi) of activity is deposited within the lungs. The distribution of activity within the lungs is proportional to regional ventilation. Multiple image projections can be obtained which correspond with those obtained during subsequent perfusion imaging. Ventilation studies with 99mTc-labeled radioaerosols require minimal patient cooperation, and portable studies or studies in patients on respirators can be performed relatively easily. In general, ventilation is performed prior to perfusion imaging but ventilation imaging with radiolabeled aerosols can also be effectively performed following perfusion imaging. Thus, in patients with a normal or near normal perfusion study, ventilation imaging can be omitted. Disadvantages of 99mTc-labeled radioaerosols relate primarily to central deposition of activity in patients with COPD or airway obstruction and the amount of activity which is wasted within the nebulizer.

Because of the problem with central deposition of 99mTc-labeled radioaerosol in patients with COPD, newer agents have been developed. These include 99mTc-Technegas and 99mTc-Pertechnegas. Both of these agents are formed by burning 99mTc-pertechnetate in a carbon crucible at very high temperatures which produces an ultrafine radiolabeled aerosol (particle size 0.02-0.2 microns). Pertechnegas is purged with 5% oxygen and 95% argon, compared with Technegas which is purged with 100% argon. This relatively minor difference causes profound changes in the

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