In this a radioactive tracer gas is inhaled and then this radioactive tracer gas is traced(looked) by taking pictures from specialised camera which capture the route and location of tracer gas. These pictures show us the part of lung devoid of tracer gas and then we correlate it with different types of lung pathology. How the test is performed - by using a mouth piece while closing nose with the help of clip, person is asked to inhale the radioactive tracer gas for few minutes and pictures are taken at regular intervals.