Ventilation-perfusion scanning is a radiological procedure which is often used to confirm or exclude the diagnosis of pulmonary embolism. It may also be used to monitor treatment.
The ventilation part of the scan is the inhalation of Krypton 81m, which has a short half life and is a pure gamma emitter. Ventilation is assessed under a gamma camera.
The perfusion part of the scan is achieved by injecting the patient with technetium 99m, which is coupled with macro aggregated albumin (MAA). This molecule has a diameter of 30 to 50 micrometres, and thus sticks in the pulmonary capillaries. Sufficiently few molecules are injected for this not to have a physiological effect. An embolus shows up as a cold area when the patient is placed under a gamma camera. The MAA has a half life of about 10 hours, the technecium about 6 hours; there are no contraindications for this investigation, however in a patient with a right to left shunt the investigation will become a whole body scan, and thus is not useful.
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