XENON XE-133 (Xenon-133) is an isotope of xenon. It is a radionuclide that is inhaled to assess pulmonary function, and to image the lungs. It is also used to image blood flow, particularly in the brain. Xenon Xe 133 diffuses easily, passing through cell membranes and exchanging freely between blood and tissue. It is distributed in the lungs in a manner similar to that of air, thus representing the regions of the lung that are aerated. The gamma photons of xenon Xe 133 can then be employed to obtain counts per minute per lung or region of the lung, or to display their distribution as a scan. Scintigraphs taken during the washout period, as the patient breathes room air, will show any obstruction in the airways as regions of radioactive gas trapping or retention. (In the presence of an abnormal or near normal Tc 99m albumin aggregated perfusion study, a normal ventilation study favors a diagnosis of pulmonary emboli. However, the presence of xenon Xe 133 gas trapping, during washout imaging, in areas of abnormal perfusion, favors a diagnosis of chronic-type obstructive pulmonary disease.)