| 摘要 |
A computed tomography has a frame carrying an x-ray source and an x-ray detector which are rotated by the frame around an examination subject to obtain a series of exposures from different angular positions. The x-ray source emits a fan beam from a focus and includes a focus deflecting unit for adjusting the position of the focus in the event that, due to mechanical wear or play, the frame is caused to rotate around a point which differs from the intended rotational center. The intended rotational center is a point through which a straight line extending between the focus and the center of the radiation detector always proceeds for every angular position. Two sensors are disposed on the rotating center frame in addition to the x-ray source and the radiation detector, these two sensors respectively defining two measuring points and supplying output signals corresponding to the respective positions of these measuring points relative to a reference point on a stationary part of the apparatus. The output signals are supplied to correction electronics, which also receives an output signal from a focus control unit, and effects a deflection of the focus by means of the focus deflecting unit as needed to maintain rotation of the frame around the intended rotational center. |
