Abstract: A helical scan is performed with a fan beam or cone beam CT system to acquire a first set of sinogram data sets. The subject is injected with a contrast agent and the identical helical scan is performed to acquire a second set of sinogram data sets. Corresponding projection views are subtracted in the two data sets and a number of different images are reconstructed from the difference data set. One image is a tomographic image produced using a filtered backprojection method and a second image is a topograph produced by selecting and displaying projection views acquired at the same view angle in successive sinogram data sets.