Abstract: A method and system for providing a portal image of an object utilizing a radiotheraphy system is disclosed. The radiotherapy system includes a system for providing at least one opening over the object. The at least one opening allows radiation therethrough. The method and system comprises acquiring a plurality of images through the at least one opening while moving the at least one opening over the object. Each of the plurality of images includes regions within the opening and regions outside the opening. The method and system includes obtaining a resulting portal image from the plurality of images. Radiation scattered to regions outside the opening is minimized in the resulting portal image to improve image quality. A method and system in accordance with the present invention utilizes a virtual grid to provide a resulting portal image with more contrast. The virtual grid is provided through an opening or a slit. The slit is preferably provided by a plate/jaw system in the radiotherapy system.