An image guiding device includes a gantry, an imaging source, an imager, and an image server. The imaging source, the imager and the image server are all mounted on the gantry. The imager is connected to the image server. The imaging source is arranged to face the imager. The imaging source emits X-rays. The imager receives X-rays passing through an affected part of a patient, generates projection data based on the received X-rays, and sends the projection data to the image server. The image server processes the projection data to obtain an image of the affected part.