Lung fibrosis. Image 3 of 3. Sequence of CT scans of a patient with lung (pulmonary) fibrosis. This is the bottom of the lung with the aortic branches (purple), non-lung tissue (blue), lung tissue (yellow/blue) and air-filled bullae (orange). Lung fibrosis is a condition whereby a lung infection causes inflammation of the tissue around the alveoli (air sacs), producing scar tissue. This scarring leads to breathlessness due to stiffnesss of the lungs. Damaged areas of the alveoli also fill with air, forming bullae or blebs. If they burst, the air released causes pneumothorax (collapsed lung), which can be life-threatening. See M160/062 and 063 for the rest of the sequence.