Sagittal 18FDG tumor uptake images for three mice (A, B, C) receiving two injections of 18FDG, one while the animal was breathing air and the second after the animal was switched to carbogen breathing, or vice-versa (animals maintained at 37°C throughout the study). The first column shows the relative tumor 18F-FDG uptake for air breathing. The second column represents tumor FDG uptake for the animal after switching to carbogen breathing (residual 18F-FDG from initial injection subtracted). The first and second columns are shown with the same relative color scale. The third column shows the pixels in which air uptake exceeded carbogen uptake, expressed as 100*(AirImage - CarbogenImage)/(mean counts in Air tumor VOI). Ideally the denominator would have been the air image, but due to image noise considerations the normalization described was done instead. Negative values are not displayed in column 3. Instead the fourth column shows the (many fewer) pixels in which carbogen exceeded air (i.e. the negative of the equation used for column 3. Corrections were made for injected activity and for residual activity. The numeric values shown on the color bar apply only to columns 3 and 4.