1. A computed tomography unit comprising: a radiation source which emits a pyramid-shaped X-ray beam adapted to penetrate an examination subject; a radiation detector on which said X-ray beam is incident after passing through said examination subject, said X-ray source and said radiation detector thereby scanning said examination subject with a portion of a surface of said examination subject, facing said X-ray source, having said pyramid shaped X-ray beam penetrate therethrough; and an optical recording device which, during said scanning, records an optical image of said portion of said surface.