关键词: 根尖片, 相对近距离, 口外投照法

Abstract:

Objective    To investigate the feasibility of the relatively close anode distance projection method in situations where patients， who cannot accommodate or tolerate intraoral film placement，usually because of gagging or trismus，need a finely defined image of upper premolars and（or）molars. Methods    A new projection method，based on the theory that the image of the object closer to the X-ray source is blur，while that of the object closer to the film is clearer，was used. The relatively close anode distance projection uses an intraoral dental machine combined with 5 × 7 inch films. Results    Images showing all premolars and molars in one quadrant simultaneously were obtained. Upper premolars and molars were in the center of the image，below the ipsilateral maxillary sinus，above lower molars，posteroinferior to the magnified and blur contralateral posterior part of the ramus of mandible，and anterior to the cervical vertebra. Conclusion    It is usually，but not always，possible to achieve a finely defined image of the upper premolars and molars with minimal distortion and no overlying bony shadows，which is especially suitable for those with limitation of mouth opening. In addition，the relatively close anode distance technique is easy to master and convenient to be generalized.

Key words: periapical radiograph, relatively close anode distance, extraoral projection