Abstract: A patient is disposed within a scan circle or examination region (62) of a CT scanner (B). As the patient starts breathing air from a xenon gas supply (A), a flow image and a lambda image are created and stored in a flow image memory (90) and a lambda image memory (92). The flow and lambda images, as well as a standard CT image, are displayed in quadrants of a video monitor (102). A joystick (106) enables the operator to designate a region of interest on the displayed image representations. Corresponding flow and lambda values are retrieved from the flow and lambda image memories for each spatial location or pixel within the region of interest. The flow and lambda values are loaded into a flow vs. lambda image memory (112) which is addressed in one direction by the flow values and in another by the lambda values to create a count of the flow and lambda value pairs. The information in the flow vs. lambda image memory is displayed as a histogram in one quadrant of the video display.