The present invention relates to a cathode for an X-ray tube, an X-ray tube, a system for X-ray imaging, and a method for an assembly of a cathode for an X-ray tube. In order to provide a cathode with an improved and facilitated assembly, a cathode (10) for an X-ray tube is provided, comprising a filament (12), a support structure (14), a body structure (16), and a filament frame structure (18). The filament is provided to emit electrons towards an anode in an electron emitting direction (24), and the filament at least partially comprises a helical structure (26). Further, the filament is held by the support structure, which is fixedly connected to the body structure. The filament frame structure is provided for electron-optical focussing of the emitted electrons, and the filament frame structure is provided adjacent to the outer boundaries of the filament. The filament frame structure comprises frame surface portions arranged transverse to the emitting direction, and the filament frame structure is held by the support structure.