Pleural effusion causing unmatched ventilation defects in ventilation and perfusion scanning

In a survey of patients who had chest radiographs and ventilation and perfusion radionuclide scans, 20 patients had pleural thickening or effusion. Defects of ventilation unmatched with defects of perfusion occurred in 13 of the 20. This usually occurred if the fluid was freely mobile and was a result of the procedure adopted for imaging. Upon injection in the supine position, labelled albumin microspheres were trapped in the areas perfused at that time. Since, in this position, the effusion moved posteriorly, the bases of the lungs were better perfused than the dependent areas. On imaging in the erect position when the effusion was subpulmonary, the microspheres remained in the base but the ventilation was reduced, resulting in a mismatch in the basal regions. This appearance could be misinterpreted.