Quantitative computed tomography (CT) applications based on statistical iterative reconstruction algorithms require accurate models of the CT acquisition process, with a key component being the x-ray fan beam intensity. We present a method to experimentally determine the extra-focal radiation profile incident on individual CT detectors. Using a tungsten cylinder as a knife edge, a super-sampled signal was created from sinogram data, which traced the "occlusion" of the x-ray source as seen by a detector. By differentiating this signal and correcting for finite detector size and motion blur, the effective source profile can be recovered. Extra-focal scatter was found to be on the order of 1-3 percent of the focal beam intensity, with lower relative magnitude at the isocenter and increasing towards the edge of the fan beam, with its profile becoming asymmetric at large angles. The implications for reconstruction algorithms and QCT applications will be discussed.