Effects of variable smoking (number of cigarettes) and venous plasma nicotine levels on radiotracer displacement. A, Percentage of displacement of total radioactivity (Ct) in regions of interest for the range of smoking levels from 0 to 2.8 cigarettes, based on the ratio of displaced Ct by each cigarette dose to prebreak Ct, without correction for the imperfect steady state of the radiotracer found in control scans. B and C, Displacement of specific binding (SB) (a measure that takes into account nondisplaceable radioactivity derived from the asymptotic portion of the saturation curve in part A) by the varying smoking levels without and with correction for the imperfect steady state, respectively. D, Specific binding displacement as a function of venous plasma nicotine levels, corrected.