Image transmission over low-bandwidth channels can be speeded up if the image coding mechanism supports regions of interest (RoIs). By such a scheme, image parts not belonging to RoIs can be encoded at a lower bitrate. This paper describes a flexible dynamic RoI scheme which supports the definition of arbitrarilyshaped RoIs before the start of the transmission process and the refinement of an already transmitted image by the definition of new RoIs at any time during image transmission. The overhead to represent RoIs is kept as small as possible by exploiting a polygonbased RoI representation. Span arithmetic on a multiresolution grid is used to support the redundancy-free transmission of overlapping RoIs.