A scan converter is disclosed for use in a sector scan type ultrasound imaging system in which ultrasound echo signals return from targets along a plurality of radial scan lines distributed in a sector field with a substantially equal tangential angular increment with respect to a reference line. The scan converter comprises a sampling circuit which samples the echo signals at a speed inversely proportional to the cosine of the angle of deflection of the radial scan lines with respect to the reference line so that the sampled points align themselves in lateral lines perpendicular to the reference line. The sampled echo signals are converted into digital echo data and stored in a random access memory in the direction of its columns. The stored data are retrieved serially in the direction of the rows and fed into an interpolator for making interpolations between the successively retrieved echo data. The interpolation data are read into a line memory in succession with the data retrieved from the RAM during each raster scan interval, and then retrieved at a constant rate from preselected storage locations so that the data on the raster scan lines have approximately equal density. The retrieved data from the line memory are converted in analog signals and displayed on a viewing screen.