The ventilation perfusion study is of proven value in the diagnosis of patients with lung disease. The technique would be even more useful if the ventilation study could be performed after the perfusion lung scan. Then only those patients with a perfusion defect would require a ventilation study and the study could be performed in the position which best demonstrates the lesion. This cannot be done satisfactorily with 133Xe. Because of its physical properties, 127Xe is the isotope of choice for ventilation studies. With this gas, the ventilation study can be performed after the perfusion lung scan with no interference from the 99mTc. The resolution of lesions within the lung is better with 127Xe than with 133Xe. Finally, the reduced tissue absorption and high photon yield permit the use of small amounts of 127Xe and therefore reduced cost and a smaller radiation dose to the patient.
