[Isotope studies in the detection of pulmonary embolism]

The regional absence of perfusion in the territory situated downstream of a lung embolus can be visualized using a gamma camera and 99m Technetium labelled macroaggregated albumin of albumin microspheres. This is a simple and atraumatic procedure. Following intravenous injection these particles are blocked in the pulmonary capillaries, their distribution in the lungs being proportional to the regional perfusion. A normal perfusion scan virtually excludes pulmonary embolism. Pulmonary embolism typically causes a wedge shaped perfusion defect at one or more sites. However, a regional perfusion defect by itself can not be regarded as a specific sign of pulmonary embolism, since it may be found in many other lung diseases. In most pulmonary diseases matched perfusion and ventilation defects are found. In the case of pulmonary embolism ventilation is usually normal in the ischemic area. To differentiate pulmonary embolism from other lung diseases, a ventilation scan is therefore mandatory in addition to the perfusion scan. For this purpose radioactive inert gases such as 81mKrypton or 133Xenon may be used, or aerosols labelled with 99mTechnetium. Each of these methods has its own advantages and shortcomings. Criteria which may be helpful for the interpretation of combined ventilation/perfusion scans are presented.