A system for selecting a region of interest in an image is provided. A user interface (1) is applied for receiving user input indicative of a region of interest (504) in an image (502). A slab selector (2) is provided for selecting a position and a thickness of a slab (503) of the image (502), based on a position and a size of the region of interest (504), the slab (503) comprising at least part of the region of interest (504). A visualization subsystem (3) is provided for visualizing the slab (503). The slab selector (2) is arranged for selecting the position and thickness of the slab (503) such that the slab (503) comprises the region of interest (504) and a thickness of the slab (503) corresponds to a size of the region of interest (504) in a thickness direction of the slab (503). The image comprises a dynamic image. The projection image (505) is obtained by processing voxel values of the dynamic image both along a ray and temporally.