Abstract: A patient breathing through a mask (34) receives breathing air from a breathing air system (A). A xenon gas supply (12) selectively supplies xenon or other enhancement gases into the breathing air. During each exhalation portion of a respiratory cycle, a portion of the exhaled gas passes through a narrow tube (40) to a xenon concentration detector (44) and a carbon dioxide concentration detector (46). A carbon dioxide concentration comparing circuit (54) compares the carbon dioxide concentration values with characteristics (56) of carbon dioxide concentration during an end-tidal portion of the exhalation respiratory cycle. Xenon concentrations read during the end-tidal portion of the respiratory cycle (62) are utilized to project a blood absorption curve (64). A plurality of CT images are generated by a CT scanner (B) as the blood xenon concentration increases.