Since the mid-1950s, the use of radiopharmaceuticals has made it possible to assess a variety of pulmonary disorders. In 1955, 133Xe was introduced for the study of regional pulmonary ventilation.1 Shortly thereafter, it became possible to evaluate regional pulmonary blood flow using inhaled carbon dioxide containing radioactive 15O2 or intravenous injection of 133Xe dissolved in saline solution.3 In 1964, intravenous injection of 131I-macroaggregated albumin made it feasible to obtain perfusion scans of the lungs.4 Although these techniques rapidly gained wide acceptance as tests of regional abnormalities in ventilation and pulmonary blood flow, the main practical application has been in the diagnostic evaluation of patients with suspected pulmonary embolism (PE). Increasingly, the role of nuclear medicine in respiratory medicine has been expanded to include disorders such as preoperative assessment of lung function, inflammatory lung disease, and lung cancer. The more widespread availability of positron emission tomography (PET) and integrated PET/CT (computed tomography) has provided powerful tools to aid in the diagnosis, staging, and management of patients with lung cancer.
Radiopharmaceuticals and Techniques in Ventilation–Perfusion Lung Scanning
Radiopharmaceuticals commonly utilized in both perfusion and ventilation studies, as well as the techniques employed are discussed in subsequent sections.
Perfusion Agents and Techniques
Clinical application of perfusion lung scanning was first described in 1964, when iodine 131–labeled macroaggregates of albumin was utilized in the evaluation of pulmonary perfusion.4 Currently, the two agents used for pulmonary perfusion imaging are technetium 99m–labeled human albumin microspheres (99mTc HAM) and macroaggregated albumin (99mTc MAA). 99mTc MAA particles range in size from 10 to 150 μm; more than 90% of injected particles measure between 10 and 90 μm. 99mTc HAM particles are relatively uniform in size and range between 35 and 60 μm. However, 99mTc MAA is considered the agent of choice for routine perfusion lung scanning because of its availability, short residence time in the lungs, and relatively low cost.
Radiolabeled particles are injected intravenously while the patient is in the supine position, thereby limiting the effect of gravity on regional pulmonary arterial blood flow. Following the administration of 99mTc MAA, particles mix uniformly with venous blood flowing to the heart; the particles lodge in precapillary arterioles of the lungs, obstructing approximately 0.1% of their total number. The usual administered dose of radioactivity is between 74 and 185 MBq (2–5 mCi) (typically, 200,000 to 500,000 particles are injected during clinical perfusion scan). The usual pediatric-administered activity is 0.5 to 2.0 MBq/kg (20–80 mCi/kg), with a minimum of 7 to 8 MBq (approximately 200 mCi).
The blockage of pulmonary precapillary arterioles by 99mTc MAA is transient; the biologic half-life in the lung ranges between 2 and 8 hours. The physical half-life of 99mTc is 6 hours. The MAA particles ...