Radioactive Xenon in the Differential Diagnosis of Pulmonary Embolism

Abstract

LUNG SCANNING after the intravenous injection of radioactive particles is a useful procedure in the diagnosis of pulmonary embolism (1). At times the diagnosis cannot be made with certainty on the basis of the scan alone, since other diseases of the lung often result in pulmonary avascularity (2). The occurrence of the characteristic pattern of concave defects at the lateral borders of the lungs without evidence of a corresponding parenchymal lesion on the chest radiograph makes the diagnosis of pulmonary embolism more certain (3), but this is not a constant finding. The differential diagnosis between pulmonary embolism and chronic obstructive pulmonary disease is particularly difficult. For example, in a study of 62 patients with pulmonary emphysema, 95 per cent of the patients had regions of decreased pulmonary arterial blood flow as revealed by lung scanning (4). Such decreases in pulmonary arterial blood flow have been encountered in other pulmonary diseases, but the frequent finding of a normal chest radiograph in obstructive lung disease makes it difficult to distinguish from pulmonary embolism. Therefore, additional tests are required. Radioactive xenon was first used to assess regional ventilation in 1955 by Knipping and his associates (5), who employed external radiation detectors placed over multiple areas of the chest while the patient breathed air containing xenon 133. The radioactive gas method has been improved subsequently (6–8). The purpose of this paper is to describe how the combined use of radioactive xenon together with conventional lung scanning after the injection of radioactive particles aids in the differential diagnosis of pulmonary embolism. The conventional scan is performed to delineate the size, shape, and position of abnormalities of perfusion. The radioactive gas is then used to determine whether the regional vascular defects are associated with abnormal ventilation, which is suggestive of obstructive disease, or are normally ventilated, as observed in patients with no pulmonary embolism. Materials and Methods Xenon 133 is obtained in gaseous form in vials or cylinders from Oak Ridge National Laboratory or from commercial sources as a sterile, pyrogen-free solution in cartridges of 1.8 ml volume, each providing 3 to 4 or 7 to 10 mCi of radioactivity per cartridge. Injection of the aqueous xenon-133 solution into a Stead-Wells spirometer (Fig. 1) results in rapid passage of the radioactivity into the gaseous phase, as a result of the low solubility of the gas in water. Xenon 133 has a half-life of five days and decays to stable cesium 133 by emission of beta particles of 347 keV, maximum energy, a gamma ray of 81 keV, and a K x-ray of 31 keV. The latter two can be measured by external radiation detectors.